# Technical Documentation: sudheerj/reactjs-interview-questions > ℹ️ **Provenance:** Multi-Tier Ingestion · [CodeWiki Reference](https://codewiki.google/github.com/sudheerj/reactjs-interview-questions) ### Overview (`README.md`) # React Interview Questions & Answers > Click :star: if you like the project. Pull Requests are highly appreciated. Follow me [@SudheerJonna](https://twitter.com/SudheerJonna) for technical updates. ---

> Practice 280+ JavaScript coding interview questions in-browser. Built by ex-FAANG interviewers. No AI-generated fluff. No fake reviews. [Try GreatFrontEnd →](https://www.greatfrontend.com/questions/react-interview-questions?utm_source=github&utm_medium=referral&utm_campaign=sudheerj-react&fpr=sudheerj&gnrs=sudheerj) 💡

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--- **Note:** This repository is specific to ReactJS. Please check [JavaScript Interview Questions](https://github.com/sudheerj/javascript-interview-questions) for core JavaScript questions and [Data Structures and Algorithms](https://github.com/sudheerj/datastructures-algorithms) for DSA-related questions or problems. ### Table of Contents

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| No. | Questions | | --- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | **Core React** | | 1 | [What is React?](#what-is-react) | | 2 | [What is the history behind React’s evolution?](#what-is-the-history-behind-react-evolution) | | 3 | [What are the major features of React?](#what-are-the-major-features-of-react) | | 4 | [What is JSX?](#what-is-jsx) | | 5 | [What is the difference between an Element and a Component?](#what-is-the-difference-between-an-element-and-a-component) | | 6 | [How do you create components in React?](#how-to-create-components-in-react) | | 7 | [When should you use a Class Component over a Function Component?](#when-to-use-a-class-component-over-a-function-component) | | 8 | [What are Pure Components?](#what-are-pure-components) | | 9 | [What is state in React?](#what-is-state-in-react) | | 10 | [What are props in React?](#what-are-props-in-react) | | 11 | [What is the difference between state and props?](#what-is-the-difference-between-state-and-props) | | 12 | [What is the difference between HTML and React event handling?](#what-is-the-difference-between-html-and-react-event-handling) | | 13 | [What are synthetic events in React?](#what-are-synthetic-events-in-react) | | 14 | [What are inline conditional expressions?](#what-are-inline-conditional-expressions) | | 15 | [What is the "key" prop and what is its benefit when used in arrays of elements?](#what-is-key-prop-and-what-is-the-benefit-of-using-it-in-arrays-of-elements) | | 16 | [What is the Virtual DOM?](#what-is-virtual-dom) | | 17 | [How does the Virtual DOM work?](#how-virtual-dom-works) | | 18 | [What is the difference between Shadow DOM and Virtual DOM?](#what-is-the-difference-between-shadow-dom-and-virtual-dom) | | 19 | [What is React Fiber?](#what-is-react-fiber) | | 20 | [What is the main goal of React Fiber?](#what-is-the-main-goal-of-react-fiber) | | 21 | [What are controlled components?](#what-are-controlled-components) | | 22 | [What are uncontrolled components?](#what-are-uncontrolled-components) | | 23 | [What is the difference between createElement and cloneElement?](#what-is-the-difference-between-createelement-and-cloneelement) | | 24 | [What is Lifting State Up in React?](#what-is-lifting-state-up-in-react) | | 25 | [What are Higher-Order Components?](#what-are-higher-order-components) | | 26 | [What is the children prop?](#what-is-children-prop) | | 27 | [How do you write comments in React?](#how-to-write-comments-in-react) | | 28 | [What is reconciliation?](#what-is-reconciliation) | | 29 | [Does the lazy function support named exports?](#does-the-lazy-function-support-named-exports) | | 30 | [Why does React use className instead of the class attribute?](#why-react-uses-classname-over-class-attribute) | | 31 | [What are Fragments?](#what-are-fragments) | | 32 | [Why are Fragments better than container divs?](#why-fragments-are-better-than-container-divs) | | 33 | [What are portals in React?](#what-are-portals-in-react) | | 34 | [What are stateless components?](#what-are-stateless-components) | | 35 | [What are stateful components?](#what-are-stateful-components) | | 36 | [How do you apply validation to props in React?](#how-to-apply-validation-on-props-in-react) | | 37 | [What are the advantages of React?](#what-are-the-advantages-of-react) | | 38 | [What are the limitations of React?](#what-are-the-limitations-of-react) | | 39 | [What are the recommended ways for static type checking?](#what-are-the-recommended-ways-for-static-type-checking) | | 40 | [What is the use of the react-dom package?](#what-is-the-use-of-react-dom-package) | | 41 | [What is ReactDOMServer?](#what-is-reactdomserver) | | 42 | [How do you use innerHTML in React?](#how-to-use-innerhtml-in-react) | | 43 | [How do you apply styles in React?](#how-to-use-styles-in-react) | | 44 | [How are events different in React?](#how-events-are-different-in-react) | | 45 | [What is the impact of using indexes as keys?](#what-is-the-impact-of-indexes-as-keys) | | 46 | [How do you conditionally render components?](#how-do-you-conditionally-render-components) | | 47 | [Why do we need to be careful when spreading props on DOM elements?](#why-we-need-to-be-careful-when-spreading-props-on-dom-elements) | | 48 | [How do you memoize a component?](#how-do-you-memoize-a-component) | | 49 | [How do you implement Server-Side Rendering (SSR)?](#how-you-implement-server-side-rendering-or-ssr) | | 50 | [How do you enable production mode in React?](#how-to-enable-production-mode-in-react) | | 51 | [Do Hooks replace render props and higher-order components?](#do-hooks-replace-render-props-and-higher-order-components) | | 52 | [What is a switching component?](#what-is-a-switching-component) | | 54 | [What are the pointer events supported in React?](#what-are-the-pointer-events-supported-in-react) | | 55 | [Why should component names start with a capital letter?](#why-should-component-names-start-with-capital-letter) | | 56 | [Are custom DOM attributes supported in React v16?](#are-custom-dom-attributes-supported-in-react-v16) | | 57 | [How do you loop inside JSX?](#how-to-loop-inside-jsx) | | 58 | [How do you access props within attribute quotes?](#how-do-you-access-props-in-attribute-quotes) | | 59 | [What is a React PropType array with shape?](#what-is-react-proptype-array-with-shape) | | 60 | [How do you conditionally apply class attributes?](#how-to-conditionally-apply-class-attributes) | | 61 | [What is the difference between React and ReactDOM?](#what-is-the-difference-between-react-and-reactdom) | | 62 | [Why is ReactDOM separated from React?](#why-reactdom-is-separated-from-react) | | 63 | [How do you use the React label element?](#how-to-use-react-label-element) | | 64 | [How do you combine multiple inline style objects?](#how-to-combine-multiple-inline-style-objects) | | 65 | [How do you re-render the view when the browser is resized?](#how-to-re-render-the-view-when-the-browser-is-resized) | | 66 | [How do you pretty-print JSON with React?](#how-to-pretty-print-json-with-react) | | 67 | [Why can’t you update props in React?](#why-you-cant-update-props-in-react) | | 68 | [How do you focus an input element on page load?](#how-to-focus-an-input-element-on-page-load) | | 69 | [How can you find the version of React at runtime in the browser?](#how-can-we-find-the-version-of-react-at-runtime-in-the-browser) | | 70 | [How do you add Google Analytics for React Router?](#how-to-add-google-analytics-for-react-router) | | 71 | [How do you apply vendor prefixes to inline styles in React?](#how-do-you-apply-vendor-prefixes-to-inline-styles-in-react) | | 72 | [How do you import and export components using React and ES6?](#how-to-import-and-export-components-using-react-and-es6) | | 73 | [What are the exceptions to React component naming?](#what-are-the-exceptions-on-react-component-naming) | | 74 | [Is it possible to use async/await in plain React?](#is-it-possible-to-use-asyncawait-in-plain-react) | | 75 | [What are common folder structures for React?](#what-are-the-common-folder-structures-for-react) | | 76 | [What are popular packages for animation?](#what-are-the-popular-packages-for-animation) | | 77 | [What are the benefits of style modules?](#what-is-the-benefit-of-styles-modules) | | 78 | [What are popular React-specific linters?](#what-are-the-popular-react-specific-linters) | | | **React Router** | | 79 | [What is React Router?](#what-is-react-router) | | 80 | [How is React Router different from the history library?](#how-react-router-is-different-from-history-library) | | 81 | [What are the components of React Router v6?](#what-are-the-router-components-of-react-router-v6) | | 82 | [What is the purpose of the push and replace methods of history?](#what-is-the-purpose-of-push-and-replace-methods-of-history) | | 83 | [How do you programmatically navigate using React Router v4?](#how-do-you-programmatically-navigate-using-react-router-v4) | | 84 | [How do you get query parameters in React Router v4?](#how-to-get-query-parameters-in-react-router-v4) | | 85 | [Why do you get a "Router may have only one child element" warning?](#why-you-get-router-may-have-only-one-child-element-warning) | | 86 | [How do you pass params to the history.push method in React Router v4?](#how-to-pass-params-to-historypush-method-in-react-router-v4) | | 87 | [How do you implement a default or NotFound page?](#how-to-implement-default-or-notfound-page) | | 88 | [How do you get history in React Router v4?](#how-to-get-history-on-react-router-v4) | | 89 | [How do you perform an automatic redirect after login?](#how-to-perform-automatic-redirect-after-login) | | **React Internationalization** | | 90 | [What is React Intl?](#what-is-react-intl) | | 91 | [What are the main features of React Intl?](#what-are-the-main-features-of-react-intl) | | 92 | [What are the two ways of formatting in React Intl?](#what-are-the-two-ways-of-formatting-in-react-intl) | | 93 | [How do you use FormattedMessage as a placeholder with React Intl?](#how-to-use-formattedmessage-as-placeholder-using-react-intl) | | 94 | [How do you access the current locale with React Intl?](#how-to-access-current-locale-with-react-intl) | | 95 | [How do you format a date using React Intl?](#how-to-format-date-using-react-intl) | | | **React Testing** | | 96 | [What is the Shallow Renderer in React testing?](#what-is-shallow-renderer-in-react-testing) | | 97 | [What is the TestRenderer package in React?](#what-is-testrenderer-package-in-react) | | 98 | [What is the purpose of the ReactTestUtils package?](#what-is-the-purpose-of-reacttestutils-package) | | 99 | [What is Jest?](#what-is-jest) | | 100 | [What are the advantages of Jest over Jasmine?](#what-are-the-advantages-of-jest-over-jasmine) | | 101 | [Can you give a simple example of a Jest test case?](#give-a-simple-example-of-jest-test-case) | | | **React Redux** | | 102 | [What is Flux?](#what-is-flux) | | 103 | [What is Redux?](#what-is-redux) | | 104 | [What are the core principles of Redux?](#what-are-the-core-principles-of-redux) | | 105 | [What are the downsides of Redux compared to Flux?](#what-are-the-downsides-of-redux-compared-to-flux) | | 106 | [What is the difference between mapStateToProps() and mapDispatchToProps()?](#what-is-the-difference-between-mapstatetoprops-and-mapdispatchtoprops) | | 107 | [Can you dispatch an action in a reducer?](#can-i-dispatch-an-action-in-reducer) | | 108 | [How do you access the Redux store outside a component?](#how-to-access-redux-store-outside-a-component) | | 109 | [What are the drawbacks of the MVW pattern?](#what-are-the-drawbacks-of-mvw-pattern) | | 110 | [Are there any similarities between Redux and RxJS?](#are-there-any-similarities-between-redux-and-rxjs) | | 111 | [How do you reset state in Redux?](#how-to-reset-state-in-redux) | | 112 | [What is the difference between React Context and React Redux?](#what-is-the-difference-between-react-context-and-react-redux) | | 113 | [Why are Redux state functions called reducers?](#why-are-redux-state-functions-called-reducers) | | 114 | [How do you make an AJAX request in Redux?](#how-to-make-ajax-request-in-redux) | | 115 | [Should you keep all component states in the Redux store?](#should-i-keep-all-components-state-in-redux-store) | | 116 | [What is the proper way to access the Redux store?](#what-is-the-proper-way-to-access-redux-store) | | 117 | [What is the difference between a component and a container in React Redux?](#what-is-the-difference-between-component-and-container-in-react-redux) | | 118 | [What is the purpose of constants in Redux?](#what-is-the-purpose-of-the-constants-in-redux) | | 119 | [What are the different ways to write mapDispatchToProps()?](#what-are-the-different-ways-to-write-mapdispatchtoprops) | | 120 | [What is the use of the ownProps parameter in mapStateToProps() and mapDispatchToProps()?](#what-is-the-use-of-the-ownprops-parameter-in-mapstatetoprops-and-mapdispatchtoprops) | | 121 | [How do you structure Redux top-level directories?](#how-to-structure-redux-top-level-directories) | | 122 | [What is Redux Saga?](#what-is-redux-saga) | | 123 | [What is the mental model of Redux Saga?](#what-is-the-mental-model-of-redux-saga) | | 124 | [What are the differences between call and put in Redux Saga?](#what-are-the-differences-between-call-and-put-in-redux-saga) | | 125 | [What is Redux Thunk?](#what-is-redux-thunk) | | 126 | [What are the differences between Redux Saga and Redux Thunk?](#what-are-the-differences-between-redux-saga-and-redux-thunk) | | 127 | [What is Redux DevTools?](#what-is-redux-devtools) | | 128 | [What are the features of Redux DevTools?](#what-are-the-features-of-redux-devtools) | | 129 | [What are Redux selectors and why should you use them?](#what-are-redux-selectors-and-why-use-them) | | 130 | [What is Redux Form?](#what-is-redux-form) | | 131 | [What are the main features of Redux Form?](#what-are-the-main-features-of-redux-form) | | 132 | [How do you add multiple middlewares to Redux?](#how-to-add-multiple-middlewares-to-redux) | | 133 | [How do you set the initial state in Redux?](#how-to-set-initial-state-in-redux) | | 134 | [How is Relay different from Redux?](#how-relay-is-different-from-redux) | | 135 | [What is an action in Redux?](#what-is-an-action-in-redux) | | | **React Native** | | 136 | [What is the difference between React Native and React?](#what-is-the-difference-between-react-native-and-react) | | 137 | [How do you test React Native apps?](#how-to-test-react-native-apps) | | 138 | [How do you log in React Native?](#how-to-do-logging-in-react-native) | | 139 | [How do you debug React Native apps?](#how-to-debug-your-react-native) | | | **React Supported Libraries and Integration** | | 140 | [What is Reselect and how does it work?](#what-is-reselect-and-how-it-works) | | 141 | [What is Flow?](#what-is-flow) | | 142 | [What is the difference between Flow and PropTypes?](#what-is-the-difference-between-flow-and-proptypes) | | 143 | [How do you use Font Awesome icons in React?](#how-to-use-font-awesome-icons-in-react) | | 144 | [What is React DevTools?](#what-is-react-dev-tools) | | 145 | [Why does DevTools not load in Chrome for local files?](#why-is-devtools-not-loading-in-chrome-for-local-files) | | 146 | [How do you use Polymer in React?](#how-to-use-polymer-in-react) | | 147 | [What are the advantages of React over Vue.js?](#what-are-the-advantages-of-react-over-vuejs) | | 148 | [What is the difference between React and Angular?](#what-is-the-difference-between-react-and-angular) | | 149 | [Why is the React tab not showing up in DevTools?](#why-react-tab-is-not-showing-up-in-devtools) | | 150 | [What are styled-components?](#what-are-styled-components) | | 151 | [Can you give an example of styled-components?](#give-an-example-of-styled-components) | | 152 | [What is Relay?](#what-is-relay) | | | **Miscellaneous** | | 153 | [What are the main features of the Reselect library?](#what-are-the-main-features-of-reselect-library) | | 154 | [Can you give an example of Reselect usage?](#give-an-example-of-reselect-usage) | | 155 | [Can Redux only be used with React?](#can-redux-only-be-used-with-react) | | 156 | [Do you need a specific build tool to use Redux?](#do-you-need-to-have-a-particular-build-tool-to-use-redux) | | 157 | [How do Redux Form initial values get updated from state?](#how-redux-form-initialvalues-get-updated-from-state) | | 158 | [How do React PropTypes allow different types for one prop?](#how-react-proptypes-allow-different-types-for-one-prop) | | 159 | [Can you import an SVG file as a React component?](#can-i-import-an-svg-file-as-react-component) | | 160 | [What is render hijacking in React?](#what-is-render-hijacking-in-react) | | 161 | [How do you pass numbers to a React component?](#how-to-pass-numbers-to-react-component) | | 162 | [Do you need to keep all state in Redux? Should you ever use React’s internal state?](#do-i-need-to-keep-all-my-state-into-redux-should-i-ever-use-react-internal-state) | | 163 | [What is the purpose of registerServiceWorker in React?](#what-is-the-purpose-of-registerserviceworker-in-react) | | 164 | [What is the React.memo function?](#what-is-react-memo-function) | | 165 | [What is the React.lazy function?](#what-is-react-lazy-function) | | 166 | [How do you prevent unnecessary updates using setState?](#how-to-prevent-unnecessary-updates-using-setstate) | | 167 | [How do you render arrays, strings, and numbers in React v16?](#how-do-you-render-array-strings-and-numbers-in-react-16-version) | | 168 | [What are Hooks?](#what-are-hooks) | | 169 | [What rules must be followed for Hooks?](#what-rules-need-to-be-followed-for-hooks) | | 170 | [How do you ensure Hooks follow the rules in your project?](#how-to-ensure-hooks-followed-the-rules-in-your-project) | | 171 | [What are the differences between Flux and Redux?](#what-are-the-differences-between-flux-and-redux) | | 172 | [What are the benefits of React Router v4?](#what-are-the-benefits-of-react-router-v4) | | 173 | [Can you describe the componentDidCatch lifecycle method signature?](#can-you-describe-about-componentdidcatch-lifecycle-method-signature) | | 174 | [In which scenarios do error boundaries not catch errors?](#in-which-scenarios-do-error-boundaries-not-catch-errors) | | 175 | [What is the behavior of uncaught errors in React v16?](#what-is-the-behavior-of-uncaught-errors-in-react-16) | | 176 | [What is the proper placement for error boundaries?](#what-is-the-proper-placement-for-error-boundaries) | | 177 | [What is the benefit of a component stack trace from an error boundary?](#what-is-the-benefit-of-component-stack-trace-from-error-boundary) | | 178 | [What are default props?](#what-are-default-props) | | 179 | [What is the purpose of the displayName class property?](#what-is-the-purpose-of-displayname-class-property) | | 180 | [What is the browser support for React applications?](#what-is-the-browser-support-for-react-applications) | | 181 | [What is code-splitting?](#what-is-code-splitting) | | 182 | [What are keyed Fragments?](#what-are-keyed-fragments) | | 183 | [Does React support all HTML attributes?](#does-react-support-all-html-attributes) | | 184 | [When do component props default to true?](#when-component-props-defaults-to-true) | | 185 | [What is Next.js and what are its major features?](#what-is-nextjs-and-major-features-of-it) | | 186 | [How do you pass an event handler to a component?](#how-do-you-pass-an-event-handler-to-a-component) | | 187 | [How do you prevent a function from being called multiple times?](#how-to-prevent-a-function-from-being-called-multiple-times) | | 188 | [How does JSX prevent injection attacks?](#how-jsx-prevents-injection-attacks) | | 189 | [How do you update rendered elements?](#how-do-you-update-rendered-elements) | | 190 | [How do you indicate that props are read-only?](#how-do-you-say-that-props-are-read-only) | | 191 | [What are the conditions for safely using an index as a key?](#what-are-the-conditions-to-safely-use-the-index-as-a-key) | | 192 | [Do keys need to be globally unique?](#is-it-keys-should-be-globally-unique) | | 193 | [What is the popular choice for form handling?](#what-is-the-popular-choice-for-form-handling) | | 194 | [What are the advantages of Formik over the Redux Form library?](#what-are-the-advantages-of-formik-over-redux-form-library) | | 195 | [Why are you not required to use inheritance?](#why-do-you-not-required-to-use-inheritance) | | 196 | [Can you use web components in a React application?](#can-i-use-web-components-in-react-application) | | 197 | [What is a dynamic import?](#what-is-dynamic-import) | | 198 | [What are loadable components?](#what-are-loadable-components) | | 199 | [What is a Suspense component?](#what-is-suspense-component) | | 200 | [What is route-based code splitting?](#what-is-route-based-code-splitting) | | 201 | [What is the purpose of the default value in Context?](#what-is-the-purpose-of-default-value-in-context) | | 202 | [What is the diffing algorithm?](#what-is-diffing-algorithm) | | 203 | [What rules are covered by the diffing algorithm?](#what-are-the-rules-covered-by-diffing-algorithm) | | 204 | [When do you need to use refs?](#when-do-you-need-to-use-refs) | | 205 | [Must a prop be named "render" for render props?](#is-it-prop-must-be-named-as-render-for-render-props) | | 206 | [What are the problems with using render props with Pure Components?](#what-are-the-problems-of-using-render-props-with-pure-components) | | 207 | [What is the windowing technique?](#what-is-windowing-technique) | | 208 | [How do you print falsy values in JSX?](#how-do-you-print-falsy-values-in-jsx) | | 209 | [What is the typical use case for portals?](#what-is-the-typical-use-case-of-portals) | | 210 | [How do you set a default value for an uncontrolled component?](#how-do-you-set-default-value-for-uncontrolled-component) | | 211 | [What is your favorite React stack?](#what-is-your-favorite-react-stack) | | 212 | [What is the difference between the real DOM and the Virtual DOM?](#what-is-the-difference-between-real-dom-and-virtual-dom) | | 213 | [How do you add Bootstrap to a React application?](#how-to-add-bootstrap-to-a-react-application) | | 214 | [Can you list the top websites or applications using React as a front-end framework?](#can-you-list-down-top-websites-or-applications-using-react-as-front-end-framework) | | 215 | [Is it recommended to use the CSS-in-JS technique in React?](#is-it-recommended-to-use-css-in-js-technique-in-react) | | 216 | [Do you need to rewrite all class components with Hooks?](#do-i-need-to-rewrite-all-my-class-components-with-hooks) | | 217 | [How do you fetch data with React Hooks?](#how-to-fetch-data-with-react-hooks) | | 218 | [Do Hooks cover all use cases for classes?](#is-hooks-cover-all-use-cases-for-classes) | | 219 | [What is the stable release for Hooks support?](#what-is-the-stable-release-for-hooks-support) | | 220 | [Why do we use array destructuring (square bracket notation) in useState?](#why-do-we-use-array-destructuring-square-brackets-notation-in-usestate) | | 221 | [What sources were used for introducing Hooks?](#what-are-the-sources-used-for-introducing-hooks) | | 222 | [How do you access the imperative API of web components?](#how-do-you-access-imperative-api-of-web-components) | | 223 | [What is Formik?](#what-is-formik) | | 224 | [What are typical middleware choices for handling asynchronous calls in Redux?](#what-are-typical-middleware-choices-for-handling-asynchronous-calls-in-redux) | | 225 | [Do browsers understand JSX code?](#do-browsers-understand-jsx-code) | | 226 | [Can you describe data flow in React?](#describe-about-data-flow-in-react) | | 227 | [What is MobX?](#what-is-mobx) | | 228 | [What are the differences between Redux and MobX?](#what-are-the-differences-between-redux-and-mobx) | | 229 | [Should you learn ES6 before learning ReactJS?](#should-i-learn-es6-before-learning-reactjs) | | 230 | [Explain concurrent rendering with an example](#explain-concurrent-rendering-with-an-example) | | 231 | [What is the difference between async mode and concurrent mode?](#what-is-the-difference-between-async-mode-and-concurrent-mode) | | 233 | [What is the purpose of the ESLint plugin for Hooks?](#what-is-the-purpose-of-eslint-plugin-for-hooks) | | 234 | [What is the difference between imperative and declarative programming in React?](#what-is-the-difference-between-imperative-and-declarative-in-react) | | 235 | [What are the benefits of using TypeScript with ReactJS?](#what-are-the-benefits-of-using-typescript-with-reactjs) | | 236 | [How do you ensure a user remains authenticated on page refresh while using Context API state management?](#how-do-you-make-sure-that-user-remains-authenticated-on-page-refresh-while-using-context-api-state-management) | | 237 | [What are the benefits of the new JSX transform?](#what-are-the-benefits-of-new-jsx-transform) | | 238 | [How is the new JSX transform different from the old transform?](#how-is-the-new-jsx-transform-different-from-old-transform) | | 239 | [What are React Server Components?](#what-are-react-server-components) | | 240 | [What is prop drilling?](#what-is-prop-drilling) | | 241 | [What is the difference between the useState and useRef Hooks?](#what-is-the-difference-between-usestate-and-useref-hook) | | 242 | [What is a wrapper component?](#what-is-a-wrapper-component) | | 243 | [What are the differences between the useEffect and useLayoutEffect Hooks?](#what-are-the-differences-between-useeffect-and-uselayouteffect-hooks) | | 244 | [What are the differences between functional and class components?](#what-are-the-differences-between-functional-and-class-components) | | 245 | [What is Strict Mode in React?](#what-is-strict-mode-in-react) | | 246 | [What is the benefit of Strict Mode?](#what-is-the-benefit-of-strict-mode) | | 247 | [Why does Strict Mode render twice in React?](#why-does-strict-mode-render-twice-in-react) | | 248 | [What are the rules of JSX?](#what-are-the-rules-of-jsx) | | 249 | [What is the reason multiple JSX tags must be wrapped?](#what-is-the-reason-behind-multiple-jsx-tags-to-be-wrapped) | | 250 | [How do you prevent mutating array variables?](#how-do-you-prevent-mutating-array-variables) | | 251 | [What are capture phase events?](#what-are-capture-phase-events) | | 252 | [How does React update the screen in an application?](#how-does-react-updates-screen-in-an-application) | | 253 | [How does React batch multiple state updates?](#how-does-react-batch-multiple-state-updates) | | 254 | [Is it possible to prevent automatic batching?](#is-it-possible-to-prevent-automatic-batching) | | 255 | [What is React hydration?](#what-is-react-hydration) | | 256 | [How do you update objects inside state?](#how-do-you-update-objects-inside-state) | | 257 | [How do you update nested objects inside state?](#How-do-you-update-nested-objects-inside-state) | | 258 | [How do you update arrays inside state?](#how-do-you-update-arrays-inside-state) | | 259 | [How do you use the Immer library for state updates?](#how-do-you-use-immer-library-for-state-updates) | | 260 | [What are the benefits of preventing direct state mutations?](#what-are-the-benefits-of-preventing-the-direct-state-mutations) | | 261 | [What are the preferred and non-preferred array operations for updating state?](#what-are-the-preferred-and-non-preferred-array-operations-for-updating-the-state) | | 262 | [What will happen when defining nested function components?](#what-will-happen-by-defining-nested-function-components) | | 263 | [Can I use keys for non-list items?](#can-i-use-keys-for-non-list-items) | | 264 | [What are the guidelines to follow for writing reducers?](#what-are-the-guidelines-to-be-followed-for-writing-reducers) | | | **Hooks** | | 265 | [What is useReducer hook? Can you describe its usage?](#what-is-use-reducer-hook-can-you-describe-its-usage) | | 266 | [How do you compare useState and useReducer?](#how-do-you-compare-use-state-and-use-reducer) | | 267 | [How does Context work with the useContext hook?](#how-does-context-works-using-usecontext-hook) | | 268 | [What are the use cases of the useContext hook?](#what-are-the-use-cases-of-usecontext-hook) | | 269 | [When should you use client and server components?](#when-to-use-client-and-server-components) | | 270 | [What are the differences between the Page Router and App Router in Next.js?](#what-are-the-differences-between-page-router-and-app-router-in-nextjs) | | 271 | [Can you describe the useMemo() Hook?](#can-you-describe-the-usememo-hook) | | 272 | [Can Hooks be used in class components?](#can-hooks-be-used-in-class-components) | | 273 | [What is an updater function? Should an updater function be used in all cases?](#what-is-an-updater-function-should-an-updater-function-be-used-in-all-cases) | | 274 | [Can useState take a function as an initial value?](#can-usestate-take-a-function-as-an-initial-value) | | 275 | [What types of values can useState hold?](#what-types-of-values-can-usestate-hold) | | 276 | [What happens if you call useState conditionally?](#what-happens-if-you-call-usestate-conditionally) | | 277 | [Is useState Synchronous or Asynchronous?](#is-usestate-synchronous-or-asynchronous) | | 278 | [Can you explain how useState works internally?](#can-you-explain-how-usestate-works-internally) | | 279 | [What is useReducer? Why do you use useReducer?](#what-is-usereducer-why-do-you-use-usereducer) | | 280 | [How does useReducer work? Explain with an example](#how-does-usereducer-works-explain-with-an-example) | | 281 | [Can you combine useReducer with useContext?](#can-you-combine-usereducer-with-usecontext) | | 282 | [Can you dispatch multiple actions in a row with useReducer?](#can-you-dispatch-multiple-actions-in-a-row-with-usereducer) | | 283 | [Is dispatch from useReducer asynchronous and does it update state immediately?](#is-dispatch-from-usereducer-asynchronous-and-does-it-update-state-immediately) | | 284 | [How does useContext work? Explain with an example](#how-does-usecontext-works-explain-with-an-example) | | 285 | [Can you use multiple Contexts in one component?](#can-you-use-multiple-contexts-in-one-component) | | 286 | [What's a common pitfall when using useContext with objects?](#whats-a-common-pitfall-when-using-usecontext-with-objects) | | 287 | [What would the context value be for no matching provider?](#what-would-the-context-value-be-for-no-matching-provider) | | 288 | [How do reactive dependencies in the useEffect dependency array affect its execution behavior?](#how-do-reactive-dependencies-in-the-useeffect-dependency-array-affect-its-execution-behavior) | | 289 | [When and how often does React invoke the setup and cleanup functions inside a useEffect hook?](#when-and-how-often-does-react-invoke-the-setup-and-cleanup-functions-inside-a-useeffect-hook) | | 290 | [What happens if you return a Promise from useEffect?](#what-happens-if-you-return-a-promise-from-useeffect) | | 291 | [Can you have multiple useEffect hooks in a single component?](#can-you-have-multiple-useeffect-hooks-in-a-single-component) | | 292 | [How to prevent infinite loops with useEffect?](#how-to-prevent-infinite-loops-with-useeffect) | | 293 | [What are the use cases of useLayoutEffect?](#what-are-the-usecases-of-uselayouteffect) | | 294 | [How does useLayoutEffect work during server-side rendering (SSR)?](#how-does-uselayouteffect-work-during-server-side-rendering-ssr) | | 295 | [What happens if you use useLayoutEffect for non-layout logic?](#what-happens-if-you-use-uselayouteffect-for-non-layout-logic) | | 296 | [How does useLayoutEffect cause layout thrashing?](#how-does-uselayouteffect-cause-layout-thrashing) | | 297 | [How do you use useRef to access a DOM element in React? Give an example](#how-do-you-use-useref-to-access-a-dom-element-in-react-give-an-example) | | 298 | [Can you use useRef to persist values across renders?](#can-you-use-useref-to-persist-values-across-renders) | | 299 | [Can useRef be used to store previous values?](#can-useref-be-used-to-store-previous-values) | | 300 | [Is it possible to access a ref in the render method?](#is-it-possible-to-access-a-ref-in-the-render-method) | | 301 | [What are the common use cases of useRef hook?](#what-are-the-common-usecases-of-useref-hook) | | 302 | [What is useImperativeHandle Hook? Give an example](#what-is-useimperativehandle-hook-give-an-example) | | 303 | [When should you use useImperativeHandle?](#when-should-you-use-useimperativehandle) | | 304 | [Is it possible to use useImperativeHandle without forwardRef?](#is-that-possible-to-use-useimperativehandle-without-forwardref) | | 305 | [How is useMemo different from useCallback?](#how-is-usememo-different-from-usecallback) | | 306 | [Does useMemo prevent re-rendering of child components?](#does-usememo-prevent-re-rendering-of-child-components) | | 307 | [What is useCallback and why is it used?](#what-is-usecallback-and-why-is-it-used) | | 308 | [What are Custom React Hooks, and how can you develop one?](#what-are-custom-react-hooks-and-how-can-you-develop-one) | | 309 | [How does React Fiber work? Explain in detail](#how-does-react-fiber-works-explain-in-detail) | | 310 | [What is the useId hook and when should you use it?](#what-is-the-useid-hook-and-when-should-you-use-it) | | 311 | [What is the useDeferredValue hook?](#what-is-the-usedeferredvalue-hook) | | 312 | [What is the useTransition hook and how does it differ from useDeferredValue?](#what-is-the-usetransition-hook-and-how-does-it-differ-from-usedeferredvalue) | | 313 | [What is the useSyncExternalStore hook?](#what-is-the-usesyncexternalstore-hook) | | 314 | [What is the useInsertionEffect hook?](#what-is-the-useinsertioneffect-hook) | | 315 | [How do you share state logic between components using custom hooks?](#how-do-you-share-state-logic-between-components-using-custom-hooks) | | 316 | [What is the useDebugValue hook?](#what-is-the-usedebugvalue-hook) | | 317 | [How do you handle cleanup in useEffect?](#how-do-you-handle-cleanup-in-useeffect) | | 318 | [What are the differences between useEffect and useEvent (experimental)?](#what-are-the-differences-between-useeffect-and-useevent-experimental) | | 319 | [What are the best practices for using React Hooks?](#what-are-the-best-practices-for-using-react-hooks) | | | **Modern React Features (React 18/19)** | | 320 | [What are the key features introduced in React 18?](#what-are-the-key-features-introduced-in-react-18) | | 321 | [What are the key features introduced in React 19?](#what-are-the-key-features-introduced-in-react-19) | | 322 | [What is the use() hook in React 19?](#what-is-the-use-hook-in-react-19) | | 323 | [What are Server Actions in React 19?](#what-are-server-actions-in-react-19) | | 324 | [What are useFormState and useFormStatus hooks?](#what-are-useformstate-and-useformstatus-hooks) | | 325 | [What is the useOptimistic hook?](#what-is-the-useoptimistic-hook) | | 326 | [What is the React Compiler (React Forget)?](#what-is-the-react-compiler-react-forget) | | 327 | [What is Streaming SSR and how does React 18+ improve it?](#what-is-streaming-ssr-and-how-does-react-18-improve-it) | | 328 | [What is `renderToPipeableStream` and how is it different from renderToString?](#what-is-rendertopipeablestream-and-how-is-it-different-from-rendertostring) | | 329 | [What is the difference between HOCs and Hooks?](#what-is-the-difference-between-hocs-and-hooks) | | 330 | [Does `React.memo` prevent Context consumers from re-rendering?](#does-reactmemo-prevent-context-consumers-from-re-rendering) | | 331 | [How would you create a reusable Context?](#how-would-you-create-a-reusable-context) | | 332 | [Why is Redux Toolkit recommended over Redux?](#why-is-redux-toolkit-recommended-over-redux) | | 333 | [What is the difference between client state and server state?](#what-is-the-difference-between-client-state-and-server-state) | | 334 | [How do you prevent unnecessary Redux re-renders?](#how-do-you-prevent-unnecessary-redux-re-renders) | | 335 | [What is `createSelector`?](#what-is-createselector) | | 336 | [What is normalized Redux state?](#what-is-normalized-redux-state) | | 337 | [Why is normalized state important for performance?](#why-is-normalized-state-important-for-performance) | | 338 | [What is Redux middleware?](#what-is-redux-middleware) | | 339 | [What is the Redux middleware signature?](#what-is-the-redux-middleware-signature) | | 340 | [What happens when you dispatch an action?](#what-happens-when-you-dispatch-an-action) | | 341 | [How would you structure Redux in a large application?](#how-would-you-structure-redux-in-a-large-application) | | 342 | [How do you handle cross-slice communication?](#how-do-you-handle-cross-slice-communication) | | 343 | [What are Redux serializable values?](#what-are-redux-serializable-values) | | 344 | [How do you handle race conditions in Redux?](#how-do-you-handle-race-conditions-in-redux) | | 345 | [How do you persist Redux state?](#how-do-you-persist-redux-state) | | 346 | [How do you debug a Redux performance problem?](#how-do-you-debug-a-redux-performance-problem) | | 347 | [What should be stored in Redux vs derived with selectors?](#what-should-be-stored-in-redux-vs-derived-with-selectors) | | 348 | [Would you put all API data into Redux?](#would-you-put-all-api-data-into-redux) | | 349 | [What is the difference between BrowserRouter, HashRouter, and MemoryRouter?](#what-is-the-difference-between-browserrouter-hashrouter-and-memoryrouter) | | 350 | [What are dynamic routes and what are route parameters?](#what-are-dynamic-routes-and-what-are-route-parameters) | | 351 | [How do you navigate programmatically in React Router?](#how-do-you-navigate-programmatically-in-react-router) | | 352 | [What is the difference between Link, NavLink, and ``?](#what-is-the-difference-between-link-navlink-and-a) | | 353 | [How do you implement protected/private routes?](#how-do-you-implement-protectedprivate-routes) | | 354 | [How do you handle 404 / Not Found routes?](#how-do-you-handle-404--not-found-routes) | | 355 | [What are loaders, actions, and data APIs in modern React Router?](#what-are-loaders-actions-and-data-apis-in-modern-react-router) | | 356 | [How do query parameters work?](#how-do-query-parameters-work) |

### Table of Contents

Hide/Show table of contents

## Core React 1. ### What is React? React (aka React.js or ReactJS) is an **open-source front-end JavaScript library** for building user interfaces based on components. It's used for handling the view layer in web and mobile applications, and allows developers to create reusable UI components and manage the state of those components efficiently. React was created by [Jordan Walke](https://github.com/jordwalke), a software engineer at Facebook (now Meta). It was first deployed on Facebook's News Feed in 2011 and on Instagram in 2012. The library was open-sourced in May 2013 and has since become one of the most popular JavaScript libraries for building modern user interfaces. **[⬆ Back to Top](#table-of-contents)** 2. ### What is the history behind React evolution? The history of ReactJS started in 2010 with the creation of **XHP**. XHP is a PHP extension which improved the syntax of the language such that XML document fragments become valid PHP expressions and the primary purpose was used to create custom and reusable HTML elements. The main principle of this extension was to make front-end code easier to understand and to help avoid cross-site scripting attacks. The project was successful to prevent the malicious content submitted by the scrubbing user. But there was a different problem with XHP in which dynamic web applications require many roundtrips to the server, and XHP did not solve this problem. Also, the whole UI was re-rendered for small change in the application. Later, the initial prototype of React is created with the name **FaxJ** by Jordan inspired from XHP. Finally after sometime React has been introduced as a new library into JavaScript world.

See deep-dive answer

The evolution of React has a fascinating history that spans over a decade: **2010-2011: The Origins** - The journey began with **XHP**, a PHP extension created at Facebook that allowed HTML components to be used in PHP code - XHP improved front-end code readability and helped prevent cross-site scripting (XSS) attacks - However, XHP had limitations with dynamic web applications, requiring frequent server roundtrips and complete UI re-renders for small changes **2011-2012: Early Development** - Jordan Walke created the first prototype called **FaxJS** (later renamed to React), inspired by XHP's component model - The key innovation was bringing XHP's component model to JavaScript with performance improvements - React introduced the Virtual DOM concept to solve the performance issues of full page re-renders - First deployed internally on Facebook's News Feed in 2011 and Instagram in 2012 **2013: Public Release** - React was officially open-sourced at JSConf US in May 2013 - Initial public reception was mixed, with some developers skeptical about the JSX syntax and the approach of mixing markup with JavaScript **2014-2015: Growing Adoption** - React Native was announced in 2015, extending React's paradigm to mobile app development - The ecosystem began to grow with tools like Redux for state management - Companies beyond Facebook began adopting React for production applications **2016-2018: Maturation** - React 16 ("Fiber") was released in 2017 with a complete rewrite of the core architecture - Introduction of new features like Error Boundaries, Portals, and improved server-side rendering - React 16.3 introduced the Context API for easier state management **2019-Present: Modern React** - React Hooks were introduced in React 16.8 (February 2019), revolutionizing state management in functional components - React 17 (October 2020) focused on making React upgrades easier - React 18 (March 2022) introduced concurrent rendering and automatic batching - React continues to evolve with Server Components, the new React compiler (React Forget), and other performance improvements

**Note:** JSX, React's syntax extension, was indeed inspired by XHP's approach of embedding XML-like syntax in code. **[⬆ Back to Top](#table-of-contents)** 3. ### What are the major features of React? React offers a powerful set of features that have made it one of the most popular JavaScript libraries for building user interfaces: **Core Features:** - **Component-Based Architecture**: React applications are built using components - independent, reusable pieces of code that return HTML via a render function. This modular approach enables better code organization, reusability, and maintenance. - **Virtual DOM**: React creates an in-memory data structure cache, computes the resulting differences, and efficiently updates only the changed parts in the browser DOM. This approach significantly improves performance compared to direct DOM manipulation. - **JSX (JavaScript XML)**: A syntax extension that allows writing HTML-like code in JavaScript. JSX makes the code more readable and expressive while providing the full power of JavaScript. - **Unidirectional Data Flow**: React follows a one-way data binding model where data flows from parent to child components. This makes the code more predictable and easier to debug. - **Declarative UI**: React allows you to describe what your UI should look like for a given state, and it handles the DOM updates when the underlying data changes. **Advanced Features:** - **React Hooks**: Introduced in React 16.8, hooks allow using state and other React features in functional components without writing classes. - **Context API**: Provides a way to share values between components without explicitly passing props through every level of the component tree. - **Error Boundaries**: Components that catch JavaScript errors anywhere in their child component tree and display fallback UI instead of crashing. - **Server-Side Rendering (SSR)**: Enables rendering React components on the server before sending HTML to the client, improving performance and SEO. - **Concurrent Mode**: A set of new features (in development) that help React apps stay responsive and gracefully adjust to the user's device capabilities and network speed. - **React Server Components**: A new feature that allows components to be rendered entirely on the server, reducing bundle size and improving performance. - **Suspense**: A feature that lets your components "wait" for something before rendering, supporting code-splitting and data fetching with cleaner code. These features collectively make React powerful for building everything from small widgets to complex, large-scale web applications. **[⬆ Back to Top](#table-of-contents)** 4. ### What is JSX? _JSX_ stands for _JavaScript XML_ and it is an XML-like syntax extension to ECMAScript. Basically it just provides the syntactic sugar for the `React.createElement(type, props, ...children)` function, giving us expressiveness of JavaScript along with HTML like template syntax. In the example below, the text inside `

` tag is returned as JavaScript function to the render function. ```jsx harmony export default function App() { return

{"Hello, this is a JSX Code!"}

; } ``` If you don't use JSX syntax then the respective JavaScript code should be written as below, ```javascript import { createElement } from "react"; export default function App() { return createElement( "h1", { className: "greeting" }, "Hello, this is a JSX Code!" ); } ```

See Class

```jsx harmony class App extends React.Component { render() { return

{"Hello, this is a JSX Code!"}

; } } ```

**Note:** JSX is stricter than HTML **[⬆ Back to Top](#table-of-contents)** 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):** ```js const element = React.createElement("button", { id: "login-btn" }, "Login"); ``` **Equivalent JSX syntax:** ```jsx ``` **The object returned by `React.createElement`:** ```js { type: 'button', props: { id: 'login-btn', children: 'Login' } } ``` 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):** ```jsx const Button = ({ handleLogin }) => ( ); ``` When JSX is compiled, it's transformed into a tree of `React.createElement` calls: ```js const Button = ({ handleLogin }) => React.createElement( "button", { id: "login-btn", onClick: handleLogin }, "Login" ); ``` --- **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. **[⬆ Back to Top](#table-of-contents)** 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: ```jsx harmony function Greeting({ message }) { return

{`Hello, ${message}`}

; } ``` 2. **Class Components:** You can also use ES6 class to define a component. The above function component can be written as a class component: ```jsx harmony class Greeting extends React.Component { render() { return

{`Hello, ${this.props.message}`}

; } } ``` **[⬆ Back to Top](#table-of-contents)** 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](https://github.com/bvaughn/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. ```jsx "use client"; import { ErrorBoundary } from "react-error-boundary"; Something went wrong}> ; ``` **[⬆ Back to Top](#table-of-contents)** 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, ```jsx const MemoizedComponent = memo(SomeComponent, arePropsEqual?); ``` 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). ```jsx import { memo, useState } from "react"; const EmployeeProfile = memo(function EmployeeProfile({ name, email }) { return ( <>

Name:{name}

Email: {email}

); }); export default function EmployeeRegForm() { const [name, setName] = useState(""); const [email, setEmail] = useState(""); return ( <> Name:{" "} setName(e.target.value)} /> Email:{" "} setEmail(e.target.value)} />

); } ``` 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. **[⬆ Back to Top](#table-of-contents)** 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. 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. ```jsx harmony import { useState } from "react"; function User() { const [message, setMessage] = useState("Welcome to React world"); return (

{message}

); } ``` Whenever React calls your component or access `useState` hook, it gives you a snapshot of the state for that particular render.

See Class

```jsx harmony import React from "react"; class User extends React.Component { constructor(props) { super(props); this.state = { message: "Welcome to React world", }; } render() { return (