<details><summary><b>See Class</b></summary>
     <p>

</p>
     </details>
    Note: JSX is stricter than HTML
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5.  ### What is the difference between an Element and a Component?
      Element:
      - A React Element is a plain JavaScript object that describes what you want to see on the UI. It represents a DOM node or a component at a specific point in time. 
      - Elements are immutable: once created, you cannot change their properties. Instead, you create new elements to reflect updates.
      - Elements can be nested within other elements through their props.
      - Creating an element is a fast, lightweight operation—it does not create any actual DOM nodes or render anything to the screen directly.
        Example (without JSX):

Equivalent JSX syntax:

The object returned by React.createElement:

Elements are then passed to the React DOM renderer (e.g., ReactDOM.render()), which translates them to actual DOM nodes.
        ---
      Component:
      - A Component is a function or class that returns an element (or a tree of elements) to describe part of the UI. Components can accept inputs (called props) and manage their own state (in case of class or function components with hooks).
      - Components allow you to split the UI into independent, reusable pieces, each isolated and composable.
      - You can define a component using a function or a class:
        Example (Function Component with JSX):

When JSX is compiled, it's transformed into a tree of React.createElement calls:

---
      In summary:
      - Elements are the smallest building blocks in React—objects that describe what you want to see.
      - Components are functions or classes that return elements and encapsulate logic, structure, and behavior for parts of your UI.
       > Think of elements as the instructions for creating UI, and components as reusable blueprints that combine logic and structure to generate those instructions.
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6.  ### How to create components in React?
    Components are the building blocks of creating User Interfaces(UI) in React. There are two possible ways to create a component.
    1. Function Components: This is the simplest way to create a component. Those are pure JavaScript functions that accept props object as the one and only one parameter and return React elements to render the output:

2. Class Components: You can also use ES6 class to define a component. The above function component can be written as a class component:

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7.  ### When to use a Class Component over a Function Component?
    After the addition of Hooks(i.e. React 16.8 onwards) it is always recommended to use Function components over Class components in React. Because you could use state, lifecycle methods and other features that were only available in class component present in function component too.
    But even there are two reasons to use Class components over Function components.
    1. If you need a React functionality whose Function component equivalent is not present yet, like Error Boundaries.
    2. In older versions, If the component needs _state or lifecycle methods_ then you need to use class component.
    So the summary to this question is as follows:
    Use Function Components:
    - If you don't need state or lifecycle methods, and your component is purely presentational.
    - For simplicity, readability, and modern code practices, especially with the use of React Hooks for state and side effects.
    Use Class Components:
    - If you need to manage state or use lifecycle methods.
    - In scenarios where backward compatibility or integration with older code is necessary.
    Note: You can also use reusable react error boundary third-party component without writing any class. i.e, No need to use class components for Error boundaries.
    The usage of Error boundaries from the above library is quite straight forward.
    > _Note when using react-error-boundary:_ ErrorBoundary is a client component. You can only pass props to it that are serializable or use it in files that have a "use client"; directive.

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8.  ### What are Pure Components?
    Pure components are the components which render the same output for the same state and props. In function components, you can achieve these pure components through memoized React.memo() API wrapping around the component. This API prevents unnecessary re-renders by comparing the previous props and new props using shallow comparison. So it will be helpful for performance optimizations.
    But at the same time, it won't compare the previous state with the current state because function component itself prevents the unnecessary rendering by default when you set the same state again.
    The syntactic representation of memoized components looks like below,

Below is the example of how child component(i.e., EmployeeProfile) prevents re-renders for the same props passed by parent component(i.e.,EmployeeRegForm).

In the above code, the email prop has not been passed to child component. So there won't be any re-renders for email prop change.
    In class components, the components extending _React.PureComponent_ instead of _React.Component_ become the pure components. When props or state changes, _PureComponent_ will do a shallow comparison on both props and state by invoking shouldComponentUpdate() lifecycle method.
    Note: React.memo() is a higher-order component.
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9.  ### What is state in React?
    _State_ of a component is an object that holds some information that may change over the lifetime of the component. The important point is whenever the state object changes, the component re-renders. It is always recommended to make our state as simple as possible and minimize the number of stateful components.
    !state
    Let's take an example of User component with message state. Here, useState hook has been used to add state to the User component and it returns an array with current state and function to update it.

Whenever React calls your component or access useState hook, it gives you a snapshot of the state for that particular render.
    <details><summary><b>See Class</b></summary>
    <p>

</p>
    </details>
    State is similar to props, but it is private and fully controlled by the component ,i.e., it is not accessible to any other component till the owner component decides to pass it.
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10. ### What are props in React?
    _Props_ are inputs to components. They are single values or objects containing a set of values that are passed to components on creation similar to HTML-tag attributes. Here, the data is passed down from a parent component to a child component.
    The primary purpose of props in React is to provide following component functionality:
    1. Pass custom data to your component.
    2. Trigger state changes.
    3. Use via this.props.reactProp inside component's render() method.
    For example, let us create an element with reactProp property:

This reactProp (or whatever you came up with) attribute name then becomes a property attached to React's native props object which originally already exists on all components created using React library.

For example, the usage of props in function component looks like below:

The properties from props object can be accessed directly using destructing feature from ES6 (ECMAScript 2015). It is also possible to fallback to default value when the prop value is not specified. The above child component can be simplified like below.

Note: The default value won't be used if you pass null or 0 value. i.e, default value is only used if the prop value is missed or undefined value has been passed.
  <details><summary><b>See Class</b></summary>
     The Props accessed in Class Based Component as below

</details>
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11. ### What is the difference between state and props?
    In React, both state and props are plain JavaScript objects, but they serve different purposes and have distinct behaviors:
    ### State
    - Definition:  
      State is a data structure that is managed within a component. It represents information that can change over the lifetime of the component.
    - Mutability:  
      State is mutable, meaning it can be changed using the setter function (setState in class components or the updater function from useState in functional components).
    - Scope:  
      State is local to the component where it is defined. Only that component can modify its own state.
    - Usage:  
      State is typically used for data that needs to change in response to user actions, network responses, or other dynamic events.
    - Re-rendering:  
      Updating the state triggers a re-render of the component and its descendants.
    ### Props
    - Definition:  
      Props (short for “properties”) are inputs to a component, provided by its parent component.
    - Mutability:  
      Props are read-only. A component cannot modify its own props; they are immutable from the component’s perspective.
    - Scope:  
      Props are used to pass data and event handlers down the component tree, enabling parent components to configure or communicate with their children.
    - Usage:  
      Props are commonly used to make components reusable and configurable. They allow the same component to be rendered with different data or behavior.
    - Analogy:  
      Think of props as arguments to a function, whereas state is like variables declared inside the function.
    ### Summary Table
    | Feature   | State                               | Props                             |
    |-----------|-------------------------------------|-----------------------------------|
    | Managed by| The component itself                | Parent component                  |
    | Mutable   | Yes                                 | No (read-only)                    |
    | Scope     | Local to the component              | Passed from parent to child       |
    | Usage     | Manage dynamic data and UI changes  | Configure and customize component |
    | Update    | Using setState/useState             | Cannot be updated by the component|
    ---
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12. ### What is the difference between HTML and React event handling?
    Below are some of the main differences between HTML and React event handling,
    1. In HTML, the event name usually represents in _lowercase_ as a convention:

Whereas in React it follows _camelCase_ convention:

2. In HTML, you can return false to prevent default behavior:

Whereas in React you must call preventDefault() explicitly:

3. In HTML, you need to invoke the function by appending ()
       Whereas in react you should not append () with the function name. (refer "activateLasers" function in the first point for example)
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13. ### What are synthetic events in React?
    SyntheticEvent is a cross-browser wrapper around the browser's native event. Its API is same as the browser's native event, including stopPropagation() and preventDefault(), except the events work identically across all browsers. The native events can be accessed directly from synthetic events using nativeEvent attribute.
    Let's take an example of BookStore title search component with the ability to get all native event properties

List of common synthetic events are:
    *   onClick
    *   onChange
    *   onSubmit
    *   onKeyDown, onKeyUp
    *   onFocus, onBlur
    *   onMouseEnter, onMouseLeave
    *   onTouchStart, onTouchEnd
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14. ### What are inline conditional expressions?
    You can use either _if statements_ or _ternary expressions_ which are available in JS(and JSX in React) to conditionally execute or render expressions. Apart from these approaches, you can also embed any expressions in JSX by wrapping them in curly braces and then followed by JS logical operator &&. It is helpful to render elements conditionally within a single line and commonly used for concise logic, especially in JSX rendering.

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15. ### What is "key" prop and what is the benefit of using it in arrays of elements?
    A key is a special attribute you should include when mapping over arrays to render data. _Key_ prop helps React identify which items have changed, are added, or are removed.
    Keys should be unique among its siblings. Most often we use ID from our data as _key_:

When you don't have stable IDs for rendered items, you may use the item _index_ as a _key_ as a last resort:

Benefits of key:
      *   Enables React to efficiently update and re-render components.
      *   Prevents unnecessary re-renders by reusing components when possible.
      *   Helps maintain internal state of list items correctly.
    Note:
    1. Using _indexes_ for _keys_ is not recommended if the order of items may change. This can negatively impact performance and may cause issues with component state.
    2. If you extract list item as separate component then apply _keys_ on list component instead of li tag.
    3. There will be a warning message in the console if the key prop is not present on list items.
    4. The key attribute accepts either string or number and internally convert it as string type.
    5. Don't generate the key on the fly something like key={Math.random()}. Because the keys will never match up between re-renders and DOM created everytime.
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16. ### What is Virtual DOM?
    The _Virtual DOM_ (VDOM) is a lightweight, in-memory representation of _Real DOM_ used by libraries like React to optimize UI rendering. The representation of a UI is kept in memory and synced with the "real" DOM. It's a step that happens between the render function being called and the displaying of elements on the screen. This entire process is called _reconciliation_.
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17. ### How Virtual DOM works?
    The _Virtual DOM_ works in five simple steps.
    1. Initial Render  
        When a UI component renders for the first time, it returns JSX. React uses this structure to create a Virtual DOM tree, which is a lightweight copy of the actual DOM. This Virtual DOM is then used to build and render the Real DOM in the browser.
    2. State or Props Change  
        When the component's state or props change, React creates a new Virtual DOM reflecting the updated UI. However, it doesn't immediately update the Real DOM; instead, it works in memory to prepare for an efficient update.
               
      !vdom
    3. Diffing Algorithm  
        React then compares the new Virtual DOM with the previous one using a process called diffing. It determines what has changed between the two versions and identifies the minimal set of updates needed.
       
       !vdom2  
    4. Reconciliation  
        Based on the diffing results, React decides which parts of the Real DOM should be updated. It avoids re-rendering the entire DOM and instead updates only the elements that actually changed.
        
       !vdom3
    5. Efficient DOM Updates  
        This entire process—working with the Virtual DOM, diffing, and selective updating—makes the UI rendering much faster and more efficient than manipulating the Real DOM directly.
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18. ### What is the difference between Shadow DOM and Virtual DOM?
    The _Shadow DOM_ is a browser technology designed primarily for scoping variables and CSS in _web components_. The _Virtual DOM_ is a concept implemented by libraries in JavaScript on top of browser APIs.
    The key differences in a table format shown below:
    | Feature | Shadow DOM | Virtual DOM |
    | --- | --- | --- |
    | Purpose | Encapsulation for Web Components | Efficient UI rendering |
    | Managed by | Browser | JS frameworks (e.g., React) |
    | DOM Type | Part of real DOM (scoped) | In-memory representation |
    | Encapsulation | Yes | No |
    | Use Case | Web Components, scoped styling | UI diffing and minimal DOM updates |
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19. ### What is React Fiber?
    React Fiber is the new reconciliation engine in React, introduced in React 16. It’s a complete rewrite of React’s core algorithm(old stack-based algorithm) for rendering and updating the UI. Fiber enhances React’s ability to handle asynchronous rendering, prioritized updates(assign priority to different types of updates), and interruption(ability to pause, abort, or reuse work) of rendering work, enabling smoother and more responsive user interfaces.
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20. ### What is the main goal of React Fiber?
    The goal of _React Fiber_ is to increase its suitability for areas like animation, layout, and gestures. Its headline feature is incremental rendering: the ability to split rendering work into chunks and spread it out over multiple frames.
    Its main goals are:
    *   Incremental Rendering – Breaks work into chunks for smoother updates.
    *   Interruptible Rendering – Pauses and resumes rendering to keep the UI responsive.
    *   Prioritization – Handles high-priority updates (e.g. animations) before low-priority ones.
    *   Concurrency Support – Enables working on multiple UI versions simultaneously.
    *   Better Error Handling – Supports component-level error boundaries.
    *   Suspense Support – Allows waiting for async data before rendering.
    *   Improved DevTools – Enables better debugging and performance tracking.
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21. ### What are controlled components?
    A controlled component is a React component that fully manages the form element's state(e.g, elements like <input>, <textarea>, or <select>))  using React's internal state mechanism. i.e, The component does not manage its own internal state — instead, React acts as the single source of truth for form data.
    The controlled components will be implemented using the below steps,
    1. Initialize the state using useState hooks in function components or inside constructor for class components.
    2. Set the value of the form element to the respective state variable.
    3. Create an event handler(onChange) to handle the user input changes through useState's updater function or setState from class component.
    4. Attach the above event handler to form element's change or click events
    Note: React re-renders the component every time the input value changes.
   For example, the name input field updates the username using handleChange event handler as below,

In these components, DOM does not hold the actual data instead React does.
   
   Benefits:
   *   Easy to implement validation, conditional formatting, or live feedback.
   *   Full control over form data.
   *   Easier to test and debug because the data is centralized in the component’s state.
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22. ### What are uncontrolled components?
    The Uncontrolled components are form elements (like <input>, <textarea>, or <select>) that manage their own state internally via the DOM, rather than through React state.
    You can query the DOM using a ref to find its current value when you need it. This is a bit more like traditional HTML.
    The uncontrolled components will be implemented using the below steps,
    1. Create a ref using useRef react hook in function component or React.createRef() in class based component.
    2. Attach this ref to the form element.
    3. The form element value can be accessed directly through ref in event handlers or componentDidMount for class components
    In the below UserProfile component, the username input is accessed using ref.

Note: Here, DOM is in charge of the value. React only accesses the value when needed (via ref).
    Benefits:
     *   Less boilerplate — no need for useState and onChange.
     *   Useful for quick form setups or when integrating with non-React code.
     *   Slightly better performance in very large forms (fewer re-renders).
    In most cases, it's recommend to use controlled components to implement forms. In a controlled component, form data is handled by a React component. The alternative is uncontrolled components, where form data is handled by the DOM itself.
    <details><summary><b>See Class</b></summary>
    <p>

</p>
    </details>
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23. ### What is the difference between createElement and cloneElement?
    Both React.createElement and React.cloneElement are used to work with React elements, but they serve different purposes.
    #### createElement: 
    Creates a new React element from scratch. JSX elements will be transpiled to React.createElement() functions to create React elements which are going to be used for the object representation of UI.
    Syntax:

Example:

#### cloneElement:
     The cloneElement method is used to clone an existing React element and optionally adds or overrides props.
    Syntax:

Example:

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24. ### What is Lifting State Up in React?
    When several components need to share the same changing data then it is recommended to _lift the shared state up_ to their closest common ancestor. That means if two child components share the same data from its parent, then move the state to parent instead of maintaining local state in both of the child components.
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25. ### What are Higher-Order Components?
    A _higher-order component_ (_HOC_) is a function that takes a component and returns a new enhanced component with additional props, behavior, or data. It’s a design pattern based on React’s compositional nature, allowing you to reuse logic across multiple components without modifying their internals.
    We consider HOCs pure components because they don’t mutate or copy behavior from the original component—they simply wrap it, enhance it, and pass through the necessary props. The wrapped component remains decoupled and reusable.

Let's take an example of a withAuth higher-order component (HOC) in React. This HOC will check if a user is authenticated and either render the wrapped component if authenticated or redirect (or show a message) if not.
    withAuth HOC Example:

Usage

HOC can be used for many use cases:
    1. Code reuse, logic and bootstrap abstraction (e.g., fetching data, permissions, theming).
    2. Render hijacking (e.g., conditional rendering or layout changes).
    3. State abstraction and manipulation(e.g., handling form logic).
    4. Props manipulation(e.g., injecting additional props or filtering them).
    
    Some of the real-world examples of HOCs in react eco-system:
    1. connect() from react-redux
    2. withRouter() from React Router v5
    3. withTranslation() from react-i18next
    4. withApollo() from Apollo client
    5. withFormik from Formik library
    6. withTheme from styled components
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26. ### What is children prop?
    The children prop is a special prop in React used to pass elements between the opening and closing tags of a component. It is commonly used in layout and wrapper componnents. 
    A simple usage of children prop looks as below,

Here, everything inside <MyDiv>...</MyDiv> is passed as children to the custom div component.
    The children can be text, JSX elements, fragments, arrays and functions(for advance use case like render props).
    <details><summary><b>See Class</b></summary>
    <p>

</p>
    </details>
    Note: There are several methods available in the legacy React API to work with this prop. These include React.Children.map, React.Children.forEach, React.Children.count, React.Children.only, React.Children.toArray.
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27. ### How to write comments in React?
    The comments in React/JSX are similar to JavaScript Multiline comments but are wrapped in curly braces.
    Single-line comments:

Multi-line comments:

You can use // and / / in JS logic, hooks, and functions.
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28. ### What is reconciliation?
    Reconciliation is the process through which React updates the Browser DOM and makes React work faster. React use a diffing algorithm so that component updates are predictable and faster. React would first calculate the difference between the real DOM and the copy of DOM (Virtual DOM) when there's an update of components.
    React stores a copy of Browser DOM which is called Virtual DOM. When we make changes or add data, React creates a new Virtual DOM and compares it with the previous one. This comparison is done by Diffing Algorithm.
    Now React compares the Virtual DOM with Real DOM. It finds out the changed nodes and updates only the changed nodes in Real DOM leaving the rest nodes as it is. This process is called _Reconciliation_.
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29. ### Does the lazy function support named exports?
    No, currently React.lazy function supports default exports only. If you would like to import modules which are named exports, you can create an intermediate module that reexports it as the default. It also ensures that tree shaking keeps working and don’t pull unused components.
    Let's take a component file which exports multiple named components,

and reexport MoreComponents.js components in an intermediate file IntermediateComponent.js

Now you can import the module using lazy function as below,

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30. ### Why React uses className over class attribute?
    React uses className instead of class because of a JavaScript naming conflict with the class keyword.
    1. class is a reserved keyword in JavaScript
        In JavaScript, class is used to define ES6 classes:

If you try to use class as a variable or property name, it will throw a syntax error. Since JSX is just JavaScript with XML-like syntax, using class directly in JSX would break the parser.
    2. JSX Is JavaScript
    
        When you write JSX like this:

It will be compiled to:

But class is invalid in this object literal context (since it clashes with the JS keyword), hence React instead uses className.

which compiles to:

React then translates className to class in the final HTML DOM.
    3. Aligns with DOM APIs
        In vanilla JavaScript, you interact with element classes using:

React follows this convention, staying consistent with the DOM API's property name rather than HTML’s attribute.
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31. ### What are fragments?
    It's a common pattern or practice in React for a component to return multiple elements. _Fragments_ let you group a list of children without adding extra nodes to the DOM.
    You need to use either <Fragment> or a shorter syntax having empty tag (<></>).
    Below is the example of how to use fragment inside _Story_ component.

It is also possible to render list of fragments inside a loop with the mandatory key attribute supplied.

Usually, you don't need to use <Fragment> until there is a need of _key_ attribute. The usage of shorter syntax looks like below.

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32. ### Why fragments are better than container divs?
    Below are the list of reasons to prefer fragments over container DOM elements,
    1. Fragments are a bit faster and use less memory by not creating an extra DOM node. This only has a real benefit on very large and deep trees.
    2. Some CSS mechanisms like _Flexbox_ and _CSS Grid_ have a special parent-child relationships, and adding divs in the middle makes it hard to keep the desired layout.
    3. The DOM Inspector is less cluttered.
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33. ### What are portals in React?
    A Portal is a React feature that enables rendering children into a DOM node that exists outside the parent component's DOM hierarchy, while still preserving the React component hierarchy. Portals help avoid CSS stacking issues—for example, elements with position: fixed may not behave as expected inside a parent with transform. Portals solve this by rendering content (like modals or tooltips) outside such constrained DOM contexts.

*   child: Any valid React node (e.g., JSX, string, fragment).
    *   container: A real DOM node (e.g., document.getElementById('modal-root')).
    Even though the content renders elsewhere in the DOM, it still behaves like a normal child in React. It has access to context, state, and event handling.
    Example:- Modal:

The above code will render the modal content into the body element in the HTML, not inside the component's usual location.
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34. ### What are stateless components?
    If the behaviour of a component is independent of its state then it can be a stateless component. You can use either a function or a class for creating stateless components. But unless you need to use a lifecycle hook in your components, you should go for function components. There are a lot of benefits if you decide to use function components here; they are easy to write, understand, and test, a little faster, and you can avoid the this keyword altogether.
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35. ### What are stateful components?
    If the behaviour of a component is dependent on the _state_ of the component then it can be termed as stateful component. These _stateful components_ are either function components with hooks or _class components_.
    Let's take an example of function stateful component which update the state based on click event,

<details><summary><b>See Class</b></summary>
    <p>
    The equivalent class stateful component with a state that gets initialized in the constructor.

</p>
    </details>
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36. ### How to apply validation on props in React?
    When the application is running in _development mode_, React will automatically check all props that we set on components to make sure they have _correct type_. If the type is incorrect, React will generate warning messages in the console. It's disabled in _production mode_ due to performance impact. The mandatory props are defined with isRequired.
    The set of predefined prop types:
    1. PropTypes.number
    2. PropTypes.string
    3. PropTypes.array
    4. PropTypes.object
    5. PropTypes.func
    6. PropTypes.node
    7. PropTypes.element
    8. PropTypes.bool
    9. PropTypes.symbol
    10. PropTypes.any
    We can define propTypes for User component as below:

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42. ### How to use innerHTML in React?
    The dangerouslySetInnerHTML attribute is React's replacement for using innerHTML in the browser DOM. Just like innerHTML, it is risky to use this attribute considering cross-site scripting (XSS) attacks. You just need to pass a __html object as key and HTML text as value.
    In this example MyComponent uses dangerouslySetInnerHTML attribute for setting HTML markup:

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43. ### How to use styles in React?
    The style attribute accepts a JavaScript object with camelCased properties rather than a CSS string. This is consistent with the DOM style JavaScript property, is more efficient, and prevents XSS security holes.

Style keys are camelCased in order to be consistent with accessing the properties on DOM nodes in JavaScript (e.g. node.style.backgroundImage).
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44. ### How events are different in React?
    Handling events in React elements has some syntactic differences:
    1. React event handlers are named using camelCase, rather than lowercase.
    2. With JSX you pass a function as the event handler, rather than a string.
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45. ### What is the impact of indexes as keys?
    Keys should be stable, predictable, and unique so that React can keep track of elements.
    In the below code snippet each element's key will be based on ordering, rather than tied to the data that is being represented. This limits the optimizations that React can do and creates confusing bugs in the application.

If you use element data for unique key, assuming todo.id is unique to this list and stable, React would be able to reorder elements without needing to reevaluate them as much.

Note: If you don't specify key prop at all, React will use index as a key's value while iterating over an array of data.
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46. ### How do you conditionally render components?
    In some cases you want to render different components depending on some state. JSX does not render false or undefined, so you can use conditional _short-circuiting_ to render a given part of your component only if a certain condition is true.

If you need an if-else condition then use _ternary operator_.

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47. ### Why we need to be careful when spreading props on DOM elements?
    When we _spread props_ we run into the risk of adding unknown HTML attributes, which is a bad practice. Instead we can use prop destructuring with ...rest operator, so it will add only required props.
    For example,

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48. ### How do you memoize a component?
    There are memoize libraries available which can be used on function components.
    For example moize library can memoize the component in another component.

Update: Since React v16.6.0, we have a React.memo. It provides a higher order component which memoizes component unless the props change. To use it, simply wrap the component using React.memo before you use it.

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49. ### How you implement Server Side Rendering or SSR?
    React is already equipped to handle rendering on Node servers. A special version of the DOM renderer is available, which follows the same pattern as on the client side.

This method will output the regular HTML as a string, which can be then placed inside a page body as part of the server response. On the client side, React detects the pre-rendered content and seamlessly picks up where it left off.
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50. ### How to enable production mode in React?
    You should use Webpack's DefinePlugin method to set NODE_ENV to production, by which it strip out things like propType validation and extra warnings. Apart from this, if you minify the code, for example, Uglify's dead-code elimination to strip out development only code and comments, it will drastically reduce the size of your bundle.
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51. ### Do Hooks replace render props and higher order components?
    Both render props and higher-order components render only a single child but in most of the cases Hooks are a simpler way to serve this by reducing nesting in your tree.
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52. ### What is a switching component?
    A _switching component_ is a component that renders one of many components. We need to use object to map prop values to components.
    For example, a switching component to display different pages based on page prop:

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53. ### What are React Mixins?
    _Mixins_ are a way to totally separate components to have a common functionality. Mixins should not be used and can be replaced with _higher-order components_ or _decorators_.
    One of the most commonly used mixins is PureRenderMixin. You might be using it in some components to prevent unnecessary re-renders when the props and state are shallowly equal to the previous props and state:

<!-- TODO: mixins are deprecated -->
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54. ### What are the Pointer Events supported in React?
    _Pointer Events_ provide a unified way of handling all input events. In the old days we had a mouse and respective event listeners to handle them but nowadays we have many devices which don't correlate to having a mouse, like phones with touch surface or pens. We need to remember that these events will only work in browsers that support the _Pointer Events_ specification.
    The following event types are now available in _React DOM_:
    1. onPointerDown
    2. onPointerMove
    3. onPointerUp
    4. onPointerCancel
    5. onGotPointerCapture
    6. onLostPointerCapture
    7. onPointerEnter
    8. onPointerLeave
    9. onPointerOver
    10. onPointerOut
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55. ### Why should component names start with capital letter?
    If you are rendering your component using JSX, the name of that component has to begin with a capital letter otherwise React will throw an error as an unrecognized tag. This convention is because only HTML elements and SVG tags can begin with a lowercase letter.

You can define function component whose name starts with lowercase letter, but when it's imported it should have a capital letter. Here lowercase is fine:

While when imported in another file it should start with capital letter:

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56. ### Are custom DOM attributes supported in React v16?
    Yes. In the past, React used to ignore unknown DOM attributes. If you wrote JSX with an attribute that React doesn't recognize, React would just skip it.
    For example, let's take a look at the below attribute:

Would render an empty div to the DOM with React v15:

In React v16 any unknown attributes will end up in the DOM:

This is useful for supplying browser-specific non-standard attributes, trying new DOM APIs, and integrating with opinionated third-party libraries.
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57. ### How to loop inside JSX?
    You can simply use Array.prototype.map with ES6 _arrow function_ syntax.
    For example, the items array of objects is mapped into an array of components:

But you can't iterate using for loop:

This is because JSX tags are transpiled into _function calls_, and you can't use statements inside expressions. This may change thanks to do expressions which are _stage 1 proposal_.
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58. ### How do you access props in attribute quotes?
    React (or JSX) doesn't support variable interpolation inside an attribute value. The below representation won't work:

But you can put any JS expression inside curly braces as the entire attribute value. So the below expression works:

Using _template strings_ will also work:

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59. ### What is React proptype array with shape?
    If you want to pass an array of objects to a component with a particular shape then use React.PropTypes.shape() as an argument to React.PropTypes.arrayOf().

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60. ### How to conditionally apply class attributes?
    You shouldn't use curly braces inside quotes because it is going to be evaluated as a string.

Instead you need to move curly braces outside (don't forget to include spaces between class names):

_Template strings_ will also work:

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61. ### What is the difference between React and ReactDOM?
    The react package contains React.createElement(), React.Component, React.Children, and other helpers related to elements and component classes. You can think of these as the isomorphic or universal helpers that you need to build components. The react-dom package contains ReactDOM.render(), and in react-dom/server we have _server-side rendering_ support with ReactDOMServer.renderToString() and ReactDOMServer.renderToStaticMarkup().
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62. ### Why ReactDOM is separated from React?
    The React team worked on extracting all DOM-related features into a separate library called _ReactDOM_. React v0.14 is the first release in which the libraries are split. By looking at some of the packages, react-native, react-art, react-canvas, and react-three, it has become clear that the beauty and essence of React has nothing to do with browsers or the DOM.
    To build more environments that React can render to, React team planned to split the main React package into two: react and react-dom. This paves the way to writing components that can be shared between the web version of React and React Native.
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63. ### How to use React label element?
    If you try to render a <label> element bound to a text input using the standard for attribute, then it produces HTML missing that attribute and prints a warning to the console.

Since for is a reserved keyword in JavaScript, use htmlFor instead.

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64. ### How to combine multiple inline style objects?
    You can use _spread operator_ in regular React:

If you're using React Native then you can use the array notation:

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65. ### How to re-render the view when the browser is resized?
    You can use the useState hook to manage the width and height state variables, and the useEffect hook to add and remove the resize event listener. The [] dependency array passed to useEffect ensures that the effect only runs once (on mount) and not on every re-render.

<details>
    <summary><h4>Using Class Component</h4></summary>
    You can listen to the resize event in componentDidMount() and then update the dimensions (width and height). You should remove the listener in componentWillUnmount() method.

</details>
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66. ### How to pretty print JSON with React?
    We can use <pre> tag so that the formatting of the JSON.stringify() is retained:

<details><summary><b>See Class</b></summary>
      <p>

</p>
      </details>
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67. ### Why can't you update props in React?
    The React philosophy is that props should be _immutable_(read only) and _top-down_. This means that a parent can send any prop values to a child, but the child can't modify received props.
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68. ### How to focus an input element on page load?
    You need to use useEffect hook to set focus on input field during page load time for functional component.

<details><summary><b>See Class</b></summary>
      <p>
      You can do it by creating _ref_ for input element and using it in componentDidMount():

</p>
      </details>
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69. ### How can we find the version of React at runtime in the browser?
    You can use React.version to get the version.

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71. ### How do you apply vendor prefixes to inline styles in React?
    React _does not_ apply _vendor prefixes_ automatically. You need to add vendor prefixes manually.

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72. ### How to import and export components using React and ES6?
    You should use default for exporting the components

<details><summary><b>See Class</b></summary>
    <p>

</p>
    </details>
    With the export specifier, the MyProfile is going to be the member and exported to this module and the same can be imported without mentioning the name in other components.

But If you're not using a bundler like Webpack or Babel, you will need _Babel_ and transform-async-to-generator plugin. However, React Native ships with Babel and a set of transforms.
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75.  ### What are the common folder structures for React?
     There are two common practices for React project file structure.
     1.  Grouping by features or routes:
        One common way to structure projects is locate CSS, JS, and tests together, grouped by feature or route.

2.  Grouping by file type:
        Another popular way to structure projects is to group similar files together.

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76. ### What are the popular packages for animation?
    _React Transition Group_ and _React Motion_ are popular animation packages in React ecosystem.
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77. ### What is the benefit of styles modules?
    It is recommended to avoid hard coding style values in components. Any values that are likely to be used across different UI components should be extracted into their own modules.
    For example, these styles could be extracted into a separate component:

And then imported individually in other components:

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78. ### What are the popular React-specific linters?
    ESLint is a popular JavaScript linter. There are plugins available that analyse specific code styles. One of the most common for React is an npm package called eslint-plugin-react. By default, it will check a number of best practices, with rules checking things from keys in iterators to a complete set of prop types.
    Another popular plugin is eslint-plugin-jsx-a11y, which will help fix common issues with accessibility. As JSX offers slightly different syntax to regular HTML, issues with alt text and tabindex, for example, will not be picked up by regular plugins.
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React Router  Copy Chapter
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79. ### What is React Router?
    React Router is a powerful routing library built on top of React that helps you add new screens and flows to your application incredibly quickly, all while keeping the URL in sync with what's being displayed on the page.
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80. ### How React Router is different from history library?
    React Router is a wrapper around the history library which handles interaction with the browser's window.history with its browser and hash histories. It also provides memory history which is useful for environments that don't have global history, such as mobile app development (React Native) and unit testing with Node.
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81. ### What are the <Router> components of React Router v6?
    React Router v6 provides below 4 <Router> components:
    1.  <BrowserRouter>:Uses the HTML5 history API for standard web apps.
    2.  <HashRouter>:Uses hash-based routing for static servers.
    3.  <MemoryRouter>:Uses in-memory routing for testing and non-browser environments.
    4.  <StaticRouter>:Provides static routing for server-side rendering (SSR).
    The above components will create _browser_, _hash_, _memory_ and _static_ history instances. React Router v6 makes the properties and methods of the history instance associated with your router available through the context in the router object.
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82. ### What is the purpose of push() and replace() methods of history?
    A history instance has two methods for navigation purpose.
    1.  push()
    2.  replace()
    If you think of the history as an array of visited locations, push() will add a new location to the array and replace() will replace the current location in the array with the new one.
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83. ### How do you programmatically navigate using React Router v4?
    There are three different ways to achieve programmatic routing/navigation within components.
    1.  Using the withRouter() higher-order function:
        The withRouter() higher-order function will inject the history object as a prop of the component. This object provides push() and replace() methods to avoid the usage of context.

2.  Using <Route> component and render props pattern:
        The <Route> component passes the same props as withRouter(), so you will be able to access the history methods through the history prop.

3.  Using context:
        This option is not recommended and treated as unstable API.

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84. ### How to get query parameters in React Router v4?
    The ability to parse query strings was taken out of React Router v4 because there have been user requests over the years to support different implementation. So the decision has been given to users to choose the implementation they like. The recommended approach is to use query strings library.

You can also use URLSearchParams if you want something native:

You should use a _polyfill_ for IE11.
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85. ### Why you get "Router may have only one child element" warning?
    You have to wrap your Route's in a <Switch> block because <Switch> is unique in that it renders a route exclusively.
    At first you need to add Switch to your imports:

Then define the routes within <Switch> block:

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86. ### How to pass params to history.push method in React Router v4?
    While navigating you can pass props to the history object:

The search property is used to pass query params in push() method.
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87. ### How to implement _default_ or _NotFound_ page?
    A <Switch> renders the first child <Route> that matches. A <Route> with no path always matches. So you just need to simply drop path attribute as below

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88. ### How to get history on React Router v4?
    Below are the list of steps to get history object on React Router v4,
    1.  Create a module that exports a history object and import this module across the project.
        For example, create history.js file:

2.  You should use the <Router> component instead of built-in routers. Import the above history.js inside index.js file:

3.  You can also use push method of history object similar to built-in history object:

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89. ### How to perform automatic redirect after login?
    The react-router package provides <Redirect> component in React Router. Rendering a <Redirect> will navigate to a new location. Like server-side redirects, the new location will override the current location in the history stack.

<details><summary><b>See Class</b></summary>
      <p>

</p>
       </details>
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React Internationalization  Copy Chapter
90. ### What is React Intl?
    The _React Intl_ library makes internationalization in React straightforward, with off-the-shelf components and an API that can handle everything from formatting strings, dates, and numbers, to pluralization. React Intl is part of _FormatJS_ which provides bindings to React via its components and API.
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91. ### What are the main features of React Intl?
    Below are the main features of React Intl,
    1.  Display numbers with separators.
    2.  Display dates and times correctly.
    3.  Display dates relative to "now".
    4.  Pluralize labels in strings.
    5.  Support for 150+ languages.
    6.  Runs in the browser and Node.
    7.  Built on standards.
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92. ### What are the two ways of formatting in React Intl?
    The library provides two ways to format strings, numbers, and dates:
    1.  Using react components:

2.  Using an API:

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93. ### How to use <FormattedMessage> as placeholder using React Intl?
    The <Formatted... /> components from react-intl return elements, not plain text, so they can't be used for placeholders, alt text, etc. In that case, you should use lower level API formatMessage(). You can inject the intl object into your component using injectIntl() higher-order component and then format the message using formatMessage() available on that object.

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94. ### How to access current locale with React Intl?
    You can get the current locale in any component of your application using injectIntl():

Then you can assert as follows:

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97. ### What is TestRenderer package in React?
    This package provides a renderer that can be used to render components to pure JavaScript objects, without depending on the DOM or a native mobile environment. This package makes it easy to grab a snapshot of the platform view hierarchy (similar to a DOM tree) rendered by a ReactDOM or React Native without using a browser or jsdom.

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98. ### What is the purpose of ReactTestUtils package?
    _ReactTestUtils_ are provided in the with-addons package and allow you to perform actions against a simulated DOM for the purpose of unit testing.
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99. ### What is Jest?
    _Jest_ is a JavaScript unit testing framework created by Facebook based on Jasmine and provides automated mock creation and a jsdom environment. It's often used for testing components.
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100. ### What are the advantages of Jest over Jasmine?
     There are couple of advantages compared to Jasmine:
     - Automatically finds tests to execute in your source code.
     - Automatically mocks dependencies when running your tests.
     - Allows you to test asynchronous code synchronously.
     - Runs your tests with a fake DOM implementation (via jsdom) so that your tests can be run on the command line.
     - Runs tests in parallel processes so that they finish sooner.
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101. ### Give a simple example of Jest test case
     Let's write a test for a function that adds two numbers in sum.js file:

Create a file named sum.test.js which contains actual test:

And then add the following section to your package.json:

Finally, run yarn test or npm test and Jest will print a result:

React Redux  Copy Chapter
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102. ### What is flux?
       Flux is an application architecture (not a framework or library) designed by Facebook to manage data flow in React applications. It was created as an alternative to the traditional MVC (Model-View-Controller) pattern, and it emphasizes a unidirectional data flow to make state changes more predictable and easier to debug.
       Flux complements React by organizing the way data moves through your application, especially in large-scale or complex projects.
       #### Core Concepts of Flux
       Flux operates using four key components, each with a specific responsibility:
       *   Actions
             *   Plain JavaScript objects or functions that describe _what happened_ (e.g., user interactions or API responses).
             *   Example: { type: 'ADD_TODO', payload: 'Buy milk' }
       *   Dispatcher
             *   A central hub that receives actions and dispatches them to the appropriate stores.
             *   There is only one dispatcher in a Flux application.
       *   Stores
             *   Hold the application state and business logic.
             *   Respond to actions from the dispatcher and update themselves accordingly.
             *   They emit change events that views can listen to.
       *   Views (React Components)
             *   Subscribe to stores and re-render when the data changes.
             *   They can also trigger new actions (e.g., on user input).

       The workflow between dispatcher, stores and views components with distinct inputs and outputs as follows:
       !flux
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103.  ### What is Redux?
       Redux is a predictable state container for JavaScript applications, most commonly used with React. It helps you manage and centralize your application’s state in a single source of truth, enabling easier debugging, testing, and maintenance—especially in large or complex applications. Redux core is tiny library(about 2.5kB gzipped) and has no dependencies.
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104. ### What are the core principles of Redux?
     Redux follows three fundamental principles:
     1. Single source of truth: The state of your whole application is stored in an object tree within a single store. The single state tree makes it easier to keep track of changes over time and debug or inspect the application.

2. State is read-only: The only way to change the state is to emit an action, an object describing what happened. This ensures that neither the views nor the network callbacks will ever write directly to the state.

3. Changes are made with pure functions(Reducers): To specify how the state tree is transformed by actions, you write reducers. Reducers are just pure functions that take the previous state and an action as parameters, and return the next state.

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105. ### What are the downsides of Redux compared to Flux?
     While Redux offers a powerful and predictable state management solution, it comes with a few trade-offs when compared to Flux. These include:
     1.  Immutability is essential  
        Redux enforces a strict immutability model for state updates, which differs from Flux’s more relaxed approach. This means you must avoid mutating state directly. Many Redux-related libraries assume immutability, so your team must be disciplined in writing pure update logic. You can use tools like redux-immutable-state-invariant, Immer, or Immutable.js to help enforce this practice, especially during development.
     2.  Careful selection of complementary packages  
        Redux is more minimal by design and provides extension points such as middleware and store enhancers. This has led to a large ecosystem, but it also means you must thoughtfully choose and configure additional packages for features like undo/redo, persistence, or form handling—something Flux explicitly leaves out but may be simpler to manage in smaller setups.
     3.  Limited static type integration  
        While Flux has mature support for static type checking with tools like Flow, Redux’s type integration is less seamless. Although TypeScript is commonly used with Redux now, early Flow support was limited, and more boilerplate was required for static type safety. This may affect teams that rely heavily on type systems for large codebases.

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106. ### What is the difference between mapStateToProps() and mapDispatchToProps()?
     mapStateToProps() is a utility which helps your component get updated state (which is updated by some other components):

mapDispatchToProps() is a utility which will help your component to fire an action event (dispatching action which may cause change of application state):

It is recommended to always use the “object shorthand” form for the mapDispatchToProps.
     Redux wraps it in another function that looks like (…args) => dispatch(onTodoClick(…args)), and pass that wrapper function as a prop to your component.

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107. ### Can I dispatch an action in reducer?
     Dispatching an action within a reducer is an anti-pattern. Your reducer should be _without side effects_, simply digesting the action payload and returning a new state object. Adding listeners and dispatching actions within the reducer can lead to chained actions and other side effects.
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108. ### How to access Redux store outside a component?
     You just need to export the store from the module where it created with createStore(). Also, it shouldn't pollute the global window object.

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109. ### What are the drawbacks of MVW pattern?
     1. DOM manipulation is very expensive which causes applications to behave slow and inefficient.
     2. Due to circular dependencies, a complicated model was created around models and views.
     3. Lot of data changes happens for collaborative applications(like Google Docs).
     4. No way to do undo (travel back in time) easily without adding so much extra code.
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110. ### Are there any similarities between Redux and RxJS?
     These libraries are very different for very different purposes, but there are some vague similarities.
     Redux is a tool for managing state throughout the application. It is usually used as an architecture for UIs. Think of it as an alternative to (half of) Angular. RxJS is a reactive programming library. It is usually used as a tool to accomplish asynchronous tasks in JavaScript. Think of it as an alternative to Promises. Redux uses the Reactive paradigm because the Store is reactive. The Store observes actions from a distance, and changes itself. RxJS also uses the Reactive paradigm, but instead of being an architecture, it gives you basic building blocks, Observables, to accomplish this pattern.
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111. ### How to reset state in Redux?
     You need to write a _root reducer_ in your application which delegate handling the action to the reducer generated by combineReducers().
     For example, let us take rootReducer() to return the initial state after USER_LOGOUT action. As we know, reducers are supposed to return the initial state when they are called with undefined as the first argument, no matter the action.

In case of using redux-persist, you may also need to clean your storage. redux-persist keeps a copy of your state in a storage engine. First, you need to import the appropriate storage engine and then, to parse the state before setting it to undefined and clean each storage state key.

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115. ### Should I keep all component's state in Redux store?
     Keep your data in the Redux store, and the UI related state internally in the component.
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116. ### What is the proper way to access Redux store?
     The best way to access your store in a component is to use the connect() function, that creates a new component that wraps around your existing one. This pattern is called _Higher-Order Components_, and is generally the preferred way of extending a component's functionality in React. This allows you to map state and action creators to your component, and have them passed in automatically as your store updates.
     Let's take an example of <FilterLink> component using connect:

Due to it having quite a few performance optimizations and generally being less likely to cause bugs, the Redux developers almost always recommend using connect() over accessing the store directly (using context API).

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117. ### What is the difference between component and container in React Redux?
     Component is a class or function component that describes the presentational part of your application.
     Container is an informal term for a component that is connected to a Redux store. Containers _subscribe_ to Redux state updates and _dispatch_ actions, and they usually don't render DOM elements; they delegate rendering to presentational child components.
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118. ### What is the purpose of the constants in Redux?
     Constants allows you to easily find all usages of that specific functionality across the project when you use an IDE. It also prevents you from introducing silly bugs caused by typos – in which case, you will get a ReferenceError immediately.
     Normally we will save them in a single file (constants.js or actionTypes.js).

In Redux, you use them in two places:
     1. During action creation:
        Let's take actions.js:

2. In reducers:
        Let's create reducer.js:

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119. ### What are the different ways to write mapDispatchToProps()?
     There are a few ways of binding _action creators_ to dispatch() in mapDispatchToProps().
     Below are the possible options:

The third option is just a shorthand for the first one.
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120. ### What is the use of the ownProps parameter in mapStateToProps() and mapDispatchToProps()?
     If the ownProps parameter is specified, React Redux will pass the props that were passed to the component into your _connect_ functions. So, if you use a connected component:

The ownProps inside your mapStateToProps() and mapDispatchToProps() functions will be an object:

You can use this object to decide what to return from those functions.
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121. ### How to structure Redux top level directories?
     Most of the applications has several top-level directories as below:
     1. Components: Used for _dumb_ components unaware of Redux.
     2. Containers: Used for _smart_ components connected to Redux.
     3. Actions: Used for all action creators, where file names correspond to part of the app.
     4. Reducers: Used for all reducers, where files name correspond to state key.
     5. Store: Used for store initialization.
     This structure works well for small and medium size apps.
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122. ### What is redux-saga?
     redux-saga is a library that aims to make side effects (asynchronous things like data fetching and impure things like accessing the browser cache) in React/Redux applications easier and better.
     It is available in NPM:

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123. ### What is the mental model of redux-saga?
     _Saga_ is like a separate thread in your application, that's solely responsible for side effects. redux-saga is a redux _middleware_, which means this thread can be started, paused and cancelled from the main application with normal Redux actions, it has access to the full Redux application state and it can dispatch Redux actions as well.
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124. ### What are the differences between call() and put() in redux-saga?
     Both call() and put() are effect creator functions. call() function is used to create effect description, which instructs middleware to call the promise. put() function creates an effect, which instructs middleware to dispatch an action to the store.
     Let's take example of how these effects work for fetching particular user data.

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125. ### What is Redux Thunk?
     _Redux Thunk_ middleware allows you to write action creators that return a function instead of an action. The thunk can be used to delay the dispatch of an action, or to dispatch only if a certain condition is met. The inner function receives the store methods dispatch() and getState() as parameters.
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126. ### What are the differences between redux-saga and redux-thunk?
     Both _Redux Thunk_ and _Redux Saga_ take care of dealing with side effects. In most of the scenarios, Thunk uses _Promises_ to deal with them, whereas Saga uses _Generators_. Thunk is simple to use and Promises are familiar to many developers, Sagas/Generators are more powerful but you will need to learn them. But both middleware can coexist, so you can start with Thunks and introduce Sagas when/if you need them.
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127. ### What is Redux DevTools?
     _Redux DevTools_ is a live-editing time travel environment for Redux with hot reloading, action replay, and customizable UI. If you don't want to bother with installing Redux DevTools and integrating it into your project, consider using Redux DevTools Extension for Chrome and Firefox.
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128. ### What are the features of Redux DevTools?
     Some of the main features of Redux DevTools are below,
     1. Lets you inspect every state and action payload.
     2. Lets you go back in time by _cancelling_ actions.
     3. If you change the reducer code, each _staged_ action will be re-evaluated.
     4. If the reducers throw, you will see during which action this happened, and what the error was.
     5. With persistState() store enhancer, you can persist debug sessions across page reloads.
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129. ### What are Redux selectors and why use them?
     _Selectors_ are functions that take Redux state as an argument and return some data to pass to the component.
     For example, to get user details from the state:

These selectors have two main benefits,
     1. The selector can compute derived data, allowing Redux to store the minimal possible state
     2. The selector is not recomputed unless one of its arguments changes
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130. ### What is Redux Form?
     _Redux Form_ works with React and Redux to enable a form in React to use Redux to store all of its state. Redux Form can be used with raw HTML5 inputs, but it also works very well with common UI frameworks like Material UI, React Widgets and React Bootstrap.
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131. ### What are the main features of Redux Form?
     Some of the main features of Redux Form are:
     1. Field values persistence via Redux store.
     2. Validation (sync/async) and submission.
     3. Formatting, parsing and normalization of field values.
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132. ### How to add multiple middlewares to Redux?
     You can use applyMiddleware().
     For example, you can add redux-thunk and logger passing them as arguments to applyMiddleware():

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133. ### How to set initial state in Redux?
     You need to pass initial state as second argument to createStore:

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134. ### How Relay is different from Redux?
     Relay is similar to Redux in that they both use a single store. The main difference is that relay only manages state originated from the server, and all access to the state is used via _GraphQL_ queries (for reading data) and mutations (for changing data). Relay caches the data for you and optimizes data fetching for you, by fetching only changed data and nothing more.
135. ### What is an action in Redux?
     _Actions_ are plain JavaScript objects or payloads of information that send data from your application to your store. They are the only source of information for the store. Actions must have a type property that indicates the type of action being performed.
     For example, let's take an action which represents adding a new todo item:

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React Native  Copy Chapter
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136. ### What is the difference between React Native and React?
     React is a JavaScript library, supporting both front end web and being run on the server, for building user interfaces and web applications.
     React Native is a mobile framework that compiles to native app components, allowing you to build native mobile applications (iOS, Android, and Windows) in JavaScript that allows you to use React to build your components, and implements React under the hood.
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137. ### How to test React Native apps?
     React Native can be tested only in mobile simulators like iOS and Android. You can run the app in your mobile using expo app (https://expo.io) Where it syncs using QR code, your mobile and computer should be in same wireless network.
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138. ### How to do logging in React Native?
     You can use console.log, console.warn, etc. As of React Native v0.29 you can simply run the following to see logs in the console:

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139. ### How to debug your React Native?
     Follow the below steps to debug React Native app:
     1. Run your application in the iOS simulator.
     2. Press Command + D and a webpage should open up at http://localhost:8081/debugger-ui.
     3. Enable _Pause On Caught Exceptions_ for a better debugging experience.
     4. Press Command + Option + I to open the Chrome Developer tools, or open it via View -> Developer -> Developer Tools.
     5. You should now be able to debug as you normally would.
React supported libraries & Integration  Copy Chapter
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140. ### What is reselect and how it works?
     _Reselect_ is a selector library (for Redux) which uses _memoization_ concept. It was originally written to compute derived data from Redux-like applications state, but it can't be tied to any architecture or library.
     Reselect keeps a copy of the last inputs/outputs of the last call, and recomputes the result only if one of the inputs changes. If the same inputs are provided twice in a row, Reselect returns the cached output. It's memoization and cache are fully customizable.
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141. ### What is Flow?
     _Flow_ is a _static type checker_ designed to find type errors in JavaScript. Flow types can express much more fine-grained distinctions than traditional type systems. For example, Flow helps you catch errors involving null, unlike most type systems.
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142. ### What is the difference between Flow and PropTypes?
     Flow is a _static analysis tool_ (static checker) which uses a superset of the language, allowing you to add type annotations to all of your code and catch an entire class of bugs at compile time.
     PropTypes is a _basic type checker_ (runtime checker) which has been patched onto React. It can't check anything other than the types of the props being passed to a given component. If you want more flexible typechecking for your entire project Flow/TypeScript are appropriate choices.
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143. ### How to use Font Awesome icons in React?
     The below steps followed to include Font Awesome in React:
     1. Install font-awesome:

2. Import font-awesome in your index.js file:

3. Add Font Awesome classes in className:

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144. ### What is React Dev Tools?
     _React Developer Tools_ let you inspect the component hierarchy, including component props and state. It exists both as a browser extension (for Chrome and Firefox), and as a standalone app (works with other environments including Safari, IE, and React Native).
     The official extensions available for different browsers or environments.
     1. Chrome extension
     2. Firefox extension
     3. Standalone app (Safari, React Native, etc)
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145. ### Why is DevTools not loading in Chrome for local files?
     If you opened a local HTML file in your browser (file://...) then you must first open _Chrome Extensions_ and check Allow access to file URLs.
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146. ### How to use Polymer in React?
     You need to follow below steps to use Polymer in React,
     1. Create a Polymer element:

2. Create the Polymer component HTML tag by importing it in a HTML document, e.g. import it in the index.html of your React application:

3. Use that element in the JSX file:

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147. ### What are the advantages of React over Vue.js?
     React has the following advantages over Vue.js:
     1. Gives more flexibility in large apps developing.
     2. Easier to test.
     3. Suitable for mobile apps creating.
     4. More information and solutions available.
Note: The above list of advantages are purely opinionated and it vary based on the professional experience. But they are helpful as base parameters.
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148. ### What is the difference between React and Angular?
     Let's see the difference between React and Angular in a table format.
     | React                                                                                       | Angular                                                                                                                            |
     | ------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------- |
     | React is a library and has only the View layer                                              | Angular is a framework and has complete MVC functionality                                                                          |
     | React handles rendering on the server side                                                  | AngularJS renders only on the client side but Angular 2 and above renders on the server side                                       |
     | React uses JSX that looks like HTML in JS which can be confusing                            | Angular follows the template approach for HTML, which makes code shorter and easy to understand                                    |
     | React Native, which is a React type to build mobile applications are faster and more stable | Ionic, Angular's mobile native app is relatively less stable and slower                                                            |
     | In React, data flows only in one way and hence debugging is easy                            | In Angular, data flows both way i.e it has two-way data binding between children and parent and hence debugging is often difficult |
Note: The above list of differences are purely opinionated and it vary based on the professional experience. But they are helpful as base parameters.
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149. ### Why React tab is not showing up in DevTools?
     When the page loads, _React DevTools_ sets a global named __REACT_DEVTOOLS_GLOBAL_HOOK__, then React communicates with that hook during initialization. If the website is not using React or if React fails to communicate with DevTools then it won't show up the tab.
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150. ### What are Styled Components?
     styled-components is a JavaScript library for styling React applications. It removes the mapping between styles and components, and lets you write actual CSS augmented with JavaScript.
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151. ### Give an example of Styled Components?
     Lets create <Title> and <Wrapper> components with specific styles for each.

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152. ### What is Relay?
     Relay is a JavaScript framework for providing a data layer and client-server communication to web applications using the React view layer.
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Miscellaneous  Copy Chapter
153. ### What are the main features of Reselect library?
     Let's see the main features of Reselect library,
     1. Selectors can compute derived data, allowing Redux to store the minimal possible state.
     2. Selectors are efficient. A selector is not recomputed unless one of its arguments changes.
     3. Selectors are composable. They can be used as input to other selectors.
154. #### Give an example of Reselect usage?
     Let's take calculations and different amounts of a shipment order with the simplified usage of Reselect:

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155. ### Can Redux only be used with React?
     Redux can be used as a data store for any UI layer. The most common usage is with React and React Native, but there are bindings available for Angular, Angular 2, Vue, Mithril, and more. Redux simply provides a subscription mechanism which can be used by any other code.
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156. ### Do you need to have a particular build tool to use Redux?
     Redux is originally written in ES6 and transpiled for production into ES5 with Webpack and Babel. You should be able to use it regardless of your JavaScript build process. Redux also offers a UMD build that can be used directly without any build process at all.
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157. ### How Redux Form initialValues get updated from state?
     You need to add enableReinitialize : true setting.

If your initialValues prop gets updated, your form will update too.
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158. ### How React PropTypes allow different types for one prop?
     You can use oneOfType() method of PropTypes.
     For example, the height property can be defined with either string or number type as below:

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159. ### Can I import an SVG file as react component?
     You can import SVG directly as component instead of loading it as a file. This feature is available with [email protected] and higher.

Note: Don't forget about the curly braces in the import.
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160. ### What is render hijacking in react?
     The concept of render hijacking is the ability to control what a component will output from another component. It means that you decorate your component by wrapping it into a Higher-Order component. By wrapping, you can inject additional props or make other changes, which can cause changing logic of rendering. It does not actually enable hijacking, but by using HOC you make your component behave differently.
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161. ### How to pass numbers to React component?
     We can pass numbers as props to React component using curly braces {} where as strings in double quotes "" or single quotes ''

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162. ### Do I need to keep all my state into Redux? Should I ever use react internal state?
     It is up to the developer's decision, i.e., it is developer's job to determine what kinds of state make up your application, and where each piece of state should live. Some users prefer to keep every single piece of data in Redux, to maintain a fully serializable and controlled version of their application at all times. Others prefer to keep non-critical or UI state, such as “is this dropdown currently open”, inside a component's internal state.
     Below are the rules of thumb to determine what kind of data should be put into Redux
     1. Do other parts of the application care about this data?
     2. Do you need to be able to create further derived data based on this original data?
     3. Is the same data being used to drive multiple components?
     4. Is there value to you in being able to restore this state to a given point in time (ie, time travel debugging)?
     5. Do you want to cache the data (i.e, use what's in state if it's already there instead of re-requesting it)?
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163. ### What is the purpose of registerServiceWorker in React?
     React creates a service worker for you without any configuration by default. The service worker is a web API that helps you cache your assets and other files so that when the user is offline or on a slow network, he/she can still see results on the screen, as such, it helps you build a better user experience, that's what you should know about service worker for now. It's all about adding offline capabilities to your site.

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164. ### What is React memo function?
     Class components can be restricted from re-rendering when their input props are the same using PureComponent or shouldComponentUpdate. Now you can do the same with function components by wrapping them in React.memo.

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165. ### What is React lazy function?
     The React.lazy function lets you render a dynamic import as a regular component. It will automatically load the bundle containing the OtherComponent when the component gets rendered. This must return a Promise which resolves to a module with a default export containing a React component.

Note:
     React.lazy and Suspense is not yet available for server-side rendering. If you want to do code-splitting in a server rendered app, we still recommend React Loadable.
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166. ### How to prevent unnecessary updates using setState?
     You can compare the current value of the state with an existing state value and decide whether to rerender the page or not. If the values are the same then you need to return null to stop re-rendering otherwise return the latest state value.
     For example, the user profile information is conditionally rendered as follows,

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167. ### How do you render Array, Strings and Numbers in React 16 Version?
     Arrays: Unlike older releases, you don't need to make sure render method return a single element in React16. You are able to return multiple sibling elements without a wrapping element by returning an array.
     For example, let us take the below list of developers,

You can also merge this array of items in another array component.

Strings and Numbers: You can also return string and number type from the render method.

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168. ### What are hooks?
     Hooks is a special JavaScript function that allows you use state and other React features without writing a class. This pattern has been introduced as a new feature in React 16.8 and helped to isolate the stateful logic from the components.
     Let's see an example of useState hook:

Note: Hooks can be used inside an existing function component without rewriting the component.
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169. ### What rules need to be followed for hooks?
     You need to follow two rules in order to use hooks,
     1. Call Hooks only at the top level of your react functions: You should always use hooks at the top level of react function before any early returns. i.e, You shouldn’t call Hooks inside loops, conditions, or nested functions. This will ensure that Hooks are called in the same order each time a component renders and it preserves the state of Hooks between multiple re-renders due to useState and useEffect calls.
     Let's see the difference using an example,
     Correct usage::

Incorrect usage::

The useState hook for the country field is being called conditionally within an if block. This can lead to inconsistent state behavior and may cause hooks to be called in a different order on each re-render.
     2. Call Hooks from React Functions only: You shouldn’t call Hooks from regular JavaScript functions or class components. Instead, you should call them from either function components or custom hooks.
     Let's find the difference of correct and incorrect usage with below examples,
     Correct usage::

Incorrect usage::

In the above incorrect usage example, both useState and useEffect are used in non-React functions(normalFunction and fetchData), which is not allowed.
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170.   ### How to ensure hooks followed the rules in your project?
        React team released an ESLint plugin called eslint-plugin-react-hooks that enforces Hook's two rules. It is part of Hooks API. You can add this plugin to your project using the below command,

And apply the below config in your ESLint config file,

This plugin also provide another important rule through react-hooks/exhaustive-deps. It ensures that the dependencies of useEffect, useCallback, and useMemo hooks are correctly listed to avoid potential bugs.

The recommended eslint-config-react-app preset already includes the hooks rules of this plugin.
        For example, the linter enforce proper naming convention for hooks. If you rename your custom hooks which as prefix "use" to something else then linter won't allow you to call built-in hooks such as useState, useEffect etc inside of your custom hook anymore.
        Note: This plugin is intended to use in Create React App by default.
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171. ### What are the differences between Flux and Redux?
     Below are the major differences between Flux and Redux
     | Flux                                           | Redux                                      |
     | ---------------------------------------------- | ------------------------------------------ |
     | State is mutable                               | State is immutable                         |
     | The Store contains both state and change logic | The Store and change logic are separate    |
     | There are multiple stores exist                | There is only one store exist              |
     | All the stores are disconnected and flat       | Single store with hierarchical reducers    |
     | It has a singleton dispatcher                  | There is no concept of dispatcher          |
     | React components subscribe to the store        | Container components uses connect function |
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172. ### What are the benefits of React Router V4?
     Below are the main benefits of React Router V4 module,
     1. In React Router v4(version 4), the API is completely about components. A router can be visualized as a single component(<BrowserRouter>) which wraps specific child router components(<Route>).
     2. You don't need to manually set history. The router module will take care history by wrapping routes with <BrowserRouter> component.
     3. The application size is reduced by adding only the specific router module(Web, core, or native)
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173. ### Can you describe about componentDidCatch lifecycle method signature?
     The componentDidCatch lifecycle method is invoked after an error has been thrown by a descendant component. The method receives two parameters,
     1. error: - The error object which was thrown
     2. info: - An object with a componentStack key contains the information about which component threw the error.
     The method structure would be as follows

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174. ### In which scenarios do error boundaries not catch errors?
     Below are the cases in which error boundaries don't work,
     1. Inside Event handlers
     2. Asynchronous code using setTimeout or requestAnimationFrame callbacks
     3. During Server side rendering
     4. When errors thrown in the error boundary code itself
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175. ### What is the behavior of uncaught errors in react 16?
     In React 16, errors that were not caught by any error boundary will result in unmounting of the whole React component tree. The reason behind this decision is that it is worse to leave corrupted UI in place than to completely remove it. For example, it is worse for a payments app to display a wrong amount than to render nothing.
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176. ### What is the proper placement for error boundaries?
     The granularity of error boundaries usage is up to the developer based on project needs. You can follow either of these approaches,
     1. You can wrap top-level route components to display a generic error message for the entire application.
     2. You can also wrap individual components in an error boundary to protect them from crashing the rest of the application.
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177. ### What is the benefit of component stack trace from error boundary?
     Apart from error messages and javascript stack, React16 will display the component stack trace with file names and line numbers using error boundary concept.
     For example, BuggyCounter component displays the component stack trace as below,
     !stacktrace
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178. ### What are default props?
     The _defaultProps_ can be defined as a property on the component to set the default values for the props. These default props are used when props not supplied(i.e., undefined props), but not for null or 0 as props. That means, If you provide null value then it remains null value. It's the same behavior with 0 as well.
     For example, let us create color default prop for the button component,

If props.color is not provided then it will set the default value to 'red'. i.e, Whenever you try to access the color prop it uses the default value

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179. ### What is the purpose of displayName class property?
     The displayName string is used in debugging messages. Usually, you don’t need to set it explicitly because it’s inferred from the name of the function or class that defines the component. You might want to set it explicitly if you want to display a different name for debugging purposes or when you create a higher-order component.
     For example, To ease debugging, choose a display name that communicates that it’s the result of a withSubscription HOC.

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180. ### What is the browser support for react applications?
     React supports all popular browsers, including Internet Explorer 9 and above, although some polyfills are required for older browsers such as IE 9 and IE 10. If you use es5-shim and es5-sham polyfill then it even support old browsers that doesn't support ES5 methods.
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181. ### What is code-splitting?
     Code-Splitting is a feature supported by bundlers like Webpack and Browserify which can create multiple bundles that can be dynamically loaded at runtime. The react project supports code splitting via dynamic import() feature.
     For example, in the below code snippets, it will make moduleA.js and all its unique dependencies as a separate chunk that only loads after the user clicks the 'Load' button.
     moduleA.js

App.js

<details><summary><b>See Class</b></summary>
    <p>

</p>
</details>
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182. ### What are Keyed Fragments?
     The Fragments declared with the explicit <React.Fragment> syntax may have keys. The general use case is mapping a collection to an array of fragments as below,

Note: key is the only attribute that can be passed to Fragment. In the future, there might be a support for additional attributes, such as event handlers.
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183. ### Does React support all HTML attributes?
     As of React 16, both standard or custom DOM attributes are fully supported. Since React components often take both custom and DOM-related props, React uses the camelCase convention just like the DOM APIs.
     Let us take few props with respect to standard HTML attributes,

These props work similarly to the corresponding HTML attributes, with the exception of the special cases. It also support all SVG attributes.
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184. ### When component props defaults to true?
     If you pass no value for a prop, it defaults to true. This behavior is available so that it matches the behavior of HTML.
     For example, below expressions are equivalent,

Note: It is not recommended to use this approach because it can be confused with the ES6 object shorthand (example, {name} which is short for {name: name})
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185. ### What is NextJS and major features of it?
     Next.js is a popular and lightweight framework for static and server‑rendered applications built with React. It also provides styling and routing solutions. Below are the major features provided by NextJS,
     1. Server-rendered by default
     2. Automatic code splitting for faster page loads
     3. Simple client-side routing (page based)
     4. Webpack-based dev environment which supports (HMR)
     5. Able to implement with Express or any other Node.js HTTP server
     6. Customizable with your own Babel and Webpack configurations
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186. ### How do you pass an event handler to a component?
     You can pass event handlers and other functions as props to child components. The functions can be passed to child component as below,

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187. ### How to prevent a function from being called multiple times?
     If you use an event handler such as onClick or onScroll and want to prevent the callback from being fired too quickly, then you can limit the rate at which callback is executed. This can be achieved in the below possible ways,
     1. Throttling: Changes based on a time based frequency. For example, it can be used using \_.throttle lodash function
     2. Debouncing: Publish changes after a period of inactivity. For example, it can be used using \_.debounce lodash function
     3. RequestAnimationFrame throttling: Changes based on requestAnimationFrame. For example, it can be used using raf-schd lodash function
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188. ### How JSX prevents Injection Attacks?
     React DOM escapes any values embedded in JSX before rendering them. Thus it ensures that you can never inject anything that’s not explicitly written in your application. Everything is converted to a string before being rendered.
     For example, you can embed user input as below,

This way you can prevent XSS(Cross-site-scripting) attacks in the application.
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189. ### How do you update rendered elements?
     You can update UI(represented by rendered element) by passing the newly created element to ReactDOM's render method.
     For example, lets take a ticking clock example, where it updates the time by calling render method multiple times,

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190. ### How do you say that props are readonly?
     When you declare a component as a function or a class, it must never modify its own props.
     Let us take a below capital function,

The above function is called “pure” because it does not attempt to change their inputs, and always return the same result for the same inputs. Hence, React has a single rule saying "All React components must act like pure functions with respect to their props."
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191. ### What are the conditions to safely use the index as a key?
     There are three conditions to make sure, it is safe use the index as a key.
     1. The list and items are static– they are not computed and do not change
     2. The items in the list have no ids
     3. The list is never reordered or filtered.
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192. ### Should keys be globally unique?
     The keys used within arrays should be unique among their siblings but they don’t need to be globally unique. i.e, You can use the same keys with two different arrays.
     For example, the below Book component uses two arrays with different arrays,

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193. ### What is the popular choice for form handling?
     Formik is a form library for react which provides solutions such as validation, keeping track of the visited fields, and handling form submission.
     In detail, You can categorize them as follows,
     1. Getting values in and out of form state
     2. Validation and error messages
     3. Handling form submission
     It is used to create a scalable, performant, form helper with a minimal API to solve annoying stuff.
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194. ### What are the advantages of formik over redux form library?
     Below are the main reasons to recommend formik over redux form library,
     1. The form state is inherently short-term and local, so tracking it in Redux (or any kind of Flux library) is unnecessary.
     2. Redux-Form calls your entire top-level Redux reducer multiple times ON EVERY SINGLE KEYSTROKE. This way it increases input latency for large apps.
     3. Redux-Form is 22.5 kB minified gzipped whereas Formik is 12.7 kB
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195. ### Why are you not required to use inheritance?
     In React, it is recommended to use composition over inheritance to reuse code between components. Both Props and composition give you all the flexibility you need to customize a component’s look and behavior explicitly and safely.
     Whereas, If you want to reuse non-UI functionality between components, it is suggested to extract it into a separate JavaScript module. Later components import it and use that function, object, or class, without extending it.
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196. ### Can I use web components in react application?
     Yes, you can use web components in a react application. Even though many developers won't use this combination, it may require especially if you are using third-party UI components that are written using Web Components.
     For example, let us use Vaadin date picker web component as below,

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197. ### What is dynamic import?
     You can achieve code-splitting in your app using dynamic import.
     Let's take an example of addition,
     1. Normal Import

2. Dynamic Import

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198. ### What are loadable components?
     With the release of React 18, React.lazy and Suspense are now available for server-side rendering. However, prior to React 18, it was recommended to use Loadable Components for code-splitting in a server-side rendered app because React.lazy and Suspense were not available for server-side rendering. Loadable Components lets you render a dynamic import as a regular component. For example, you can use Loadable Components to load the OtherComponent in a separate bundle like this:

Now OtherComponent will be loaded in a separated bundle
     Loadable Components provides additional benefits beyond just code-splitting, such as automatic code reloading, error handling, and preloading. By using Loadable Components, you can ensure that your application loads quickly and efficiently, providing a better user experience for your users.
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199. ### What is suspense component?
     React Suspense is a built-in feature that lets you defer rendering part of your component tree until some condition(asynchronous operation) is met—usually, data or code has finished loading. While waiting, Suspense lets you display a fallback UI like a spinner or placeholder.

     1. Lazy loading components uses suspense feature,

        If the module containing the dynamic import is not yet loaded by the time parent component renders, you must show some fallback content while you’re waiting for it to load using a loading indicator. This can be done using Suspense component.

The above component shows fallback UI instead real component until OtherComponent is fully loaded.
     2. As an another example, suspend until async data(data fetching) is ready

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200. ### What is route based code splitting?
     One of the best place to do code splitting is with routes. The entire page is going to re-render at once so users are unlikely to interact with other elements in the page at the same time. Due to this, the user experience won't be disturbed.
     Let us take an example of route based website using libraries like React Router with React.lazy,

In the above code, the code splitting will happen at each route level.
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201. ### What is the purpose of default value in context?
     The defaultValue argument is only used when a component does not have a matching Provider above it in the tree. This can be helpful for testing components in isolation without wrapping them.
     Below code snippet provides default theme value as Luna.

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202. ### What is diffing algorithm?
     React needs to use algorithms to find out how to efficiently update the UI to match the most recent tree. The diffing algorithms is generating the minimum number of operations to transform one tree into another. However, the algorithms have a complexity in the order of O(n³) where n is the number of elements in the tree.
     In this case, displaying 1000 elements would require in the order of one billion comparisons. This is far too expensive. Instead, React implements a heuristic O(n) algorithm based on two assumptions:
     1. Two elements of different types will produce different trees.
     2. The developer can hint at which child elements may be stable across different renders with a key prop.
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203. ### What are the rules covered by diffing algorithm?
     When diffing two trees, React first compares the two root elements. The behavior is different depending on the types of the root elements. It covers the below rules during reconciliation algorithm,
     1. Elements Of Different Types:
        Whenever the root elements have different types, React will tear down the old tree and build the new tree from scratch. For example, elements <a> to <img>, or from <Article> to <Comment> of different types lead a full rebuild.
     2. DOM Elements Of The Same Type:
        When comparing two React DOM elements of the same type, React looks at the attributes of both, keeps the same underlying DOM node, and only updates the changed attributes. Lets take an example with same DOM elements except className attribute,

3. Component Elements Of The Same Type:
        When a component updates, the instance stays the same, so that state is maintained across renders. React updates the props of the underlying component instance to match the new element, and calls componentWillReceiveProps() and componentWillUpdate() on the underlying instance. After that, the render() method is called and the diff algorithm recurses on the previous result and the new result.
     4. Recursing On Children:
        when recursing on the children of a DOM node, React just iterates over both lists of children at the same time and generates a mutation whenever there’s a difference. For example, when adding an element at the end of the children, converting between these two trees works well.

5. Handling keys:
        React supports a key attribute. When children have keys, React uses the key to match children in the original tree with children in the subsequent tree. For example, adding a key can make the tree conversion efficient,

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204. ### When do you need to use refs?
     There are few use cases to go for refs,
     1. Managing focus, text selection, or media playback.
     2. Triggering imperative animations.
     3. Integrating with third-party DOM libraries.
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205. ### Must prop be named as render for render props?
     Even though the pattern named render props, you don’t have to use a prop named render to use this pattern. i.e, Any prop that is a function that a component uses to know what to render is technically a “render prop”. Lets take an example with the children prop for render props,

Actually children prop doesn’t need to be named in the list of “attributes” in JSX element. Instead, you can keep it directly inside element,

While using this above technique(without any name), explicitly state that children should be a function in your propTypes.

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206. ### What are the problems of using render props with pure components?
     If you create a function inside a render method, it negates the purpose of pure component. Because the shallow prop comparison will always return false for new props, and each render in this case will generate a new value for the render prop. You can solve this issue by defining the render function as instance method.
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207. ### What is windowing technique?
     Windowing is a technique that only renders a small subset of your rows at any given time, and can dramatically reduce the time it takes to re-render the components as well as the number of DOM nodes created. If your application renders long lists of data then this technique is recommended. Both react-window and react-virtualized are popular windowing libraries which provides several reusable components for displaying lists, grids, and tabular data.
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208. ### How do you print falsy values in JSX?
     The falsy values such as false, null, undefined, and true are valid children but they don't render anything. If you still want to display them then you need to convert it to string. Let's take an example on how to convert to a string,

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209. ### What is the typical use case of portals?
     React Portals are primarily used to render UI components such as modals, tooltips, dropdowns, hovercards, and notifications outside of their parent component's DOM tree. This helps avoid common CSS issues caused by parent elements, such as:
     *   overflow: hidden on parent elements clipping or hiding child elements like modals or tooltips,
     *   stacking context and z-index conflicts created by parent containers that prevent child elements from appearing above other content.
     That means, you need to visually “break out” of its container. By rendering these UI elements into a separate DOM node (often directly under <body>), portals ensure they appear above all other content and are not restricted by the parent’s CSS or layout constraints, resulting in correct positioning and visibility regardless of the parent’s styling.
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210.   ### How do you set default value for uncontrolled component?
     In React, the value attribute on form elements will override the value in the DOM. With an uncontrolled component, you might want React to specify the initial value, but leave subsequent updates uncontrolled. To handle this case, you can specify a defaultValue attribute instead of value.

The same applies for select and textArea inputs. But you need to use defaultChecked for checkbox and radio inputs.
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211. ### What is your favorite React stack?
     Even though the tech stack varies from developer to developer, the most popular stack is used in react boilerplate project code. It mainly uses Redux and redux-saga for state management and asynchronous side-effects, react-router for routing purpose, styled-components for styling react components, axios for invoking REST api, and other supported stack such as webpack, reselect, ESNext, Babel.
     You can clone the project https://github.com/react-boilerplate/react-boilerplate and start working on any new react project.
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212. ### What is the difference between Real DOM and Virtual DOM?
     Below are the main differences between Real DOM and Virtual DOM,
     | Real DOM                             | Virtual DOM                          |
     | ------------------------------------ | ------------------------------------ |
     | Updates are slow                     | Updates are fast                     |
     | DOM manipulation is very expensive.  | DOM manipulation is very easy        |
     | You can update HTML directly.        | You Can’t directly update HTML       |
     | It causes too much of memory wastage | There is no memory wastage           |
     | Creates a new DOM if element updates | It updates the JSX if element update |
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213. ### How to add Bootstrap to a react application?
     Bootstrap can be added to your React app in a three possible ways,
     1. Using the Bootstrap CDN:
        This is the easiest way to add bootstrap. Add both bootstrap CSS and JS resources in a head tag.
     2. Bootstrap as Dependency:
        If you are using a build tool or a module bundler such as Webpack, then this is the preferred option for adding Bootstrap to your React application

3. React Bootstrap Package:
        In this case, you can add Bootstrap to our React app is by using a package that has rebuilt Bootstrap components to work particularly as React components. Below packages are popular in this category,
        1. react-bootstrap
        2. reactstrap
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214. ### Can you list down top websites or applications using react as front end framework?
     Below are the top 10 websites using React as their front-end framework,
     1. Facebook
     2. Uber
     3. Instagram
     4. WhatsApp
     5. Khan Academy
     6. Airbnb
     7. Dropbox
     8. Flipboard
     9. Netflix
     10. PayPal
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215. ### Is it recommended to use CSS In JS technique in React?
     React does not have any opinion about how styles are defined but if you are a beginner then good starting point is to define your styles in a separate \*.css file as usual and refer to them using className. This functionality is not part of React but came from third-party libraries. But If you want to try a different approach(CSS-In-JS) then styled-components library is a good option.
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216. ### Do I need to rewrite all my class components with hooks?
     No. But you can try Hooks in a few components(or new components) without rewriting any existing code. Because there are no plans to remove classes in ReactJS.
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217. ### What is useEffect hook? How to fetch data with React Hooks?
     The useEffect hook is a React Hook that lets you perform side effects in function components. Side effects are operations that interact with the outside world or system and aren't directly related to rendering UI — such as fetching data, setting up subscriptions, timers, manually manipulating the DOM, etc.
     In function components, useEffect replaces the class component lifecycle methods(componentDidMount, componentDidUpdate and componentWillUnmount) with a single, unified API.     
     Syntax

This effect hook can be used to fetch data from an API and to set the data in the local state of the component with the useState hook’s update function.
     Here is an example of fetching a list of ReactJS articles from an API using fetch.

A popular way to simplify this is by using the library axios.
     We provided an empty array as second argument to the useEffect hook to avoid activating it on component updates. This way, it only fetches on component mount.
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218. ### Is Hooks cover all use cases for classes?
     Hooks doesn't cover all use cases of classes but there is a plan to add them soon. Currently there are no Hook equivalents to the uncommon getSnapshotBeforeUpdate and componentDidCatch lifecycles yet.
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219. ### What is the stable release for hooks support?
     React includes a stable implementation of React Hooks in 16.8 release for below packages
     1. React DOM
     2. React DOM Server
     3. React Test Renderer
     4. React Shallow Renderer
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220. ### Why do we use array destructuring (square brackets notation) in useState?
     When we declare a state variable with useState, it returns a pair — an array with two items. The first item is the current value, and the second is a function that updates the value. Using [0] and [1] to access them is a bit confusing because they have a specific meaning. This is why we use array destructuring instead.
     For example, the array index access would look as follows:

Whereas with array destructuring the variables can be accessed as follows:

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221. ### What are the sources used for introducing hooks?
     Hooks got the ideas from several different sources. Below are some of them,
     1. Previous experiments with functional APIs in the react-future repository
     2. Community experiments with render prop APIs such as Reactions Component
     3. State variables and state cells in DisplayScript.
     4. Subscriptions in Rx.
     5. Reducer components in ReasonReact.
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222. ### How do you access imperative API of web components?
     Web Components often expose an imperative API to implement its functions. You will need to use a ref to interact with the DOM node directly if you want to access imperative API of a web component. But if you are using third-party Web Components, the best solution is to write a React component that behaves as a wrapper for your Web Component.
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223. ### What is formik?
     Formik is a small react form library that helps you with the three major problems,
     1. Getting values in and out of form state
     2. Validation and error messages
     3. Handling form submission
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224. ### What are typical middleware choices for handling asynchronous calls in Redux?
     Some of the popular middleware choices for handling asynchronous calls in Redux eco system are Redux Thunk, Redux Promise, Redux Saga.
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225. ### Do browsers understand JSX code?
     No, browsers can't understand JSX code. You need a transpiler to convert your JSX to regular Javascript that browsers can understand. The most widely used transpiler right now is Babel.
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226. ### Describe about data flow in react?
     React implements one-way reactive data flow using props which reduce boilerplate and is easier to understand than traditional two-way data binding.
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227. ### What is MobX?
     MobX is a simple, scalable and battle tested state management solution for applying functional reactive programming (TFRP). For ReactJS application, you need to install below packages,

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228. ### What are the differences between Redux and MobX?
     Below are the main differences between Redux and MobX,
     | Topic         | Redux                                                         | MobX                                                                   |
     | ------------- | ------------------------------------------------------------- | ---------------------------------------------------------------------- |
     | Definition    | It is a javascript library for managing the application state | It is a library for reactively managing the state of your applications |
     | Programming   | It is mainly written in ES6                                   | It is written in JavaScript(ES5)                                       |
     | Data Store    | There is only one large store exist for data storage          | There is more than one store for storage                               |
     | Usage         | Mainly used for large and complex applications                | Used for simple applications                                           |
     | Performance   | Need to be improved                                           | Provides better performance                                            |
     | How it stores | Uses JS Object to store                                       | Uses observable to store the data                                      |
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229. ### Should I learn ES6 before learning ReactJS?
     No, you don’t have to learn es2015/es6 to learn react. But you may find many resources or React ecosystem uses ES6 extensively. Let's see some of the frequently used ES6 features,
     1. Destructuring: To get props and use them in a component

2. Spread operator: Helps in passing props down into a component

3. Arrow functions: Makes compact syntax

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230. ### What is Concurrent Rendering?
     The Concurrent rendering makes React apps to be more responsive by rendering component trees without blocking the main UI thread. It allows React to interrupt a long-running render to handle a high-priority event. i.e, When you enabled concurrent Mode, React will keep an eye on other tasks that need to be done, and if there's something with a higher priority it will pause what it is currently rendering and let the other task finish first. You can enable this in two ways,

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231. ### What is the difference between async mode and concurrent mode?
     Both refers the same thing. Previously concurrent Mode being referred to as "Async Mode" by React team. The name has been changed to highlight React’s ability to perform work on different priority levels. So it avoids the confusion from other approaches to Async Rendering.
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232. ### Can I use javascript urls in react16.9?
     Yes, you can use javascript: URLs but it will log a warning in the console. Because URLs starting with javascript: are dangerous by including unsanitized output in a tag like <a href> and create a security hole.

Remember that the future versions will throw an error for javascript URLs.
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233. ### What is the purpose of eslint plugin for hooks?
     The ESLint plugin enforces rules of Hooks to avoid bugs. It assumes that any function starting with ”use” and a capital letter right after it is a Hook. In particular, the rule enforces that,
     1. Calls to Hooks are either inside a PascalCase function (assumed to be a component) or another useSomething function (assumed to be a custom Hook).
     2. Hooks are called in the same order on every render.
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234. ### What is the difference between Imperative and Declarative in React?
     Imagine a simple UI component, such as a "Like" button. When you tap it, it turns blue if it was previously grey, and grey if it was previously blue.
     The imperative way of doing this would be:

Basically, you have to check what is currently on the screen and handle all the changes necessary to redraw it with the current state, including undoing the changes from the previous state. You can imagine how complex this could be in a real-world scenario.
     In contrast, the declarative approach would be:

Because the declarative approach separates concerns, this part of it only needs to handle how the UI should look in a specific state, and is therefore much simpler to understand.
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235. ### What are the benefits of using TypeScript with ReactJS?
     Below are some of the benefits of using TypeScript with ReactJS,
     1. It is possible to use latest JavaScript features
     2. Use of interfaces for complex type definitions
     3. IDEs such as VS Code was made for TypeScript
     4. Avoid bugs with the ease of readability and Validation
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236. ### How do you make sure that user remains authenticated on page refresh while using Context API State Management?
     When a user logs in and reload, to persist the state generally we add the load user action in the useEffect hooks in the main App.js. While using Redux, loadUser action can be easily accessed.
     App.js

- But while using Context API, to access context in App.js, wrap the AuthState in index.js so that App.js can access the auth context. Now whenever the page reloads, no matter what route you are on, the user will be authenticated as loadUser action will be triggered on each re-render.
     index.js

App.js

loadUser

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237. ### What are the benefits of new JSX transform?
     There are three major benefits of new JSX transform,
     1. It is possible to use JSX without importing React packages
     2. The compiled output might improve the bundle size in a small amount
     3. The future improvements provides the flexibility to reduce the number of concepts to learn React.
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238. ### How is the new JSX transform different from old transform??
     The new JSX transform doesn’t require React to be in scope. i.e, You don't need to import React package for simple scenarios.
     Let's take an example to look at the main differences between the old and the new transform,
     Old Transform:

Now JSX transform convert the above code into regular JavaScript as below,

New Transform:
     The new JSX transform doesn't require any React imports

Under the hood JSX transform compiles to below code

Note: You still need to import React to use Hooks.
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239. ### What are React Server components?
     React Server Component is a way to write React component that gets rendered in the server-side with the purpose of improving React app performance. These components allow us to load components from the backend.
     Note: React Server Components is still under development and not recommended for production yet.
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240. ### What is prop drilling?
     Prop Drilling is the process by which you pass data from one component of the React Component tree to another by going through other components that do not need the data but only help in passing it around.
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241. ### What is the difference between useState and useRef hook?
     1. useState causes components to re-render after state updates whereas useRef doesn’t cause a component to re-render when the value or state changes.
        Essentially, useRef is like a “box” that can hold a mutable value in its (.current) property.
     2. useState allows us to update the state inside components. While useRef allows referencing DOM elements and tracking values.
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242. ### What is a wrapper component?
     A wrapper in React is a component that wraps or surrounds another component or group of components. It can be used for a variety of purposes such as adding additional functionality, styling, or layout to the wrapped components.
     For example, consider a simple component that displays a message:

We can create a wrapper component that will add a border to the message component:

Now we can use the MessageWrapper component instead of the Message component and the message will be displayed with a border:

Wrapper component can also accept its own props and pass them down to the wrapped component, for example, we can create a wrapper component that will add a title to the message component:

Now we can use the MessageWrapperWithTitle component and pass title props:

This way, the wrapper component can add additional functionality, styling, or layout to the wrapped component while keeping the wrapped component simple and reusable.
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243. ### What are the differences between useEffect and useLayoutEffect hooks?
     useEffect and useLayoutEffect are both React hooks that can be used to synchronize a component with an external system, such as a browser API or a third-party library. However, there are some key differences between the two:
     - Timing: useEffect runs after the browser has finished painting, while useLayoutEffect runs synchronously before the browser paints. This means that useLayoutEffect can be used to measure and update layout in a way that feels more synchronous to the user.
     - Browser Paint: useEffect allows browser to paint the changes before running the effect, hence it may cause some visual flicker. useLayoutEffect synchronously runs the effect before browser paints and hence it will avoid visual flicker.
     - Execution Order: The order in which multiple useEffect hooks are executed is determined by React and may not be predictable. However, the order in which multiple useLayoutEffect hooks are executed is determined by the order in which they were called.
     - Error handling: useEffect has a built-in mechanism for handling errors that occur during the execution of the effect, so that it does not crash the entire application. useLayoutEffect does not have this mechanism, and errors that occur during the execution of the effect will crash the entire application.
     In general, it's recommended to use useEffect as much as possible, because it is more performant and less prone to errors. useLayoutEffect should only be used when you need to measure or update layout, and you can't achieve the same result using useEffect.
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244. ### What are the differences between Functional and Class Components?
     There are two different ways to create components in ReactJS. The main differences are listed down as below,
     ## 1. Syntax:
     The class components uses ES6 classes to create the components. It uses render function to display the HTML content in the webpage.
     The syntax for class component looks like as below.

Note: The Pascal Case is the recommended approach to provide naming to a component.
     Functional component has been improved over the years with some added features like Hooks. Here is a syntax for functional component.

## 2. State:
     State contains information or data about a component which may change over time.
     In class component, you can update the state when a user interacts with it or server updates the data using the setState() method. The initial state is going to be assigned in the Constructor() method using the this.state object and it is possible to assign different data types such as string, boolean, numbers, etc.
     A simple example showing how we use the setState() and constructor():

You didn't use state in functional components because it was only supported in class components. But over the years hooks have been implemented in functional components which enables to use state too.
     The useState() hook can used to implement state in functional components. It returns an array with two items: the first item is current state and the next one is a function (setState) that updates the value of the current state.
     Let's see an example to demonstrate the state in functional components,

## 3. Props:
     Props are referred to as "properties". The props are passed into React component just like arguments passed to a function. In other words, they are similar to HTML attributes.
     The props are accessible in child class component using this.props as shown in below example,

Props in functional components are similar to that of the class components but the difference is the absence of 'this' keyword.