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React Interview Questions & Answers

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

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No.	Questions
	Old Q&A
0	What is Concurrent Rendering? (Legacy)
1	Why should we not update the state directly?
2	What is the purpose of callback function as an argument of setState()?
3	How to bind methods or event handlers in JSX callbacks?
4	How to pass a parameter to an event handler or callback?
5	What is the use of refs?
6	How to create refs?
7	What are forward refs?
8	Which is preferred option with in callback refs and findDOMNode()?
9	Why are String Refs legacy?
10	What are the different phases of component lifecycle?
11	What are the lifecycle methods of React?
12	How to create props proxy for HOC component?
13	What is context?
14	What is the purpose of using super constructor with props argument?
15	How to set state with a dynamic key name?
16	What would be the common mistake of function being called every time the component renders?
17	What are error boundaries in React v16
18	How are error boundaries handled in React v15?
19	What is the purpose of render method of react-dom?
20	What will happen if you use setState in constructor?
21	Is it good to use setState() in componentWillMount() method?
22	What will happen if you use props in initial state?
23	How you use decorators in React?
24	What is CRA and its benefits?
25	What is the lifecycle methods order in mounting?
26	What are the lifecycle methods going to be deprecated in React v16?
27	What is the purpose of getDerivedStateFromProps() lifecycle method?
28	What is the purpose of getSnapshotBeforeUpdate() lifecycle method?
29	What is the recommended way for naming components?
30	What is the recommended ordering of methods in component class?
31	Why we need to pass a function to setState()?
32	Why is isMounted() an anti-pattern and what is the proper solution?
33	What is the difference between constructor and getInitialState?
34	Can you force a component to re-render without calling setState?
35	What is the difference between super() and super(props) in React using ES6 classes?
36	What is the difference between setState and replaceState methods?
37	How to listen to state changes?
38	What is the recommended approach of removing an array element in react state?
39	Is it possible to use React without rendering HTML?
40	What are the possible ways of updating objects in state?
41	What are the approaches to include polyfills in your create-react-app?
42	How to use https instead of http in create-react-app?
43	How to avoid using relative path imports in create-react-app?
44	How to update a component every second?
45	Why is a component constructor called only once?
46	How to define constants in React?
47	How to programmatically trigger click event in React?
48	How to make AJAX call and In which component lifecycle methods should I make an AJAX call?
49	What are render props?
50	How to dispatch an action on load?
51	How to use connect from React Redux?
52	Whats the purpose of at symbol in the redux connect decorator?
53	How to use TypeScript in create-react-app application?
54	Does the statics object work with ES6 classes in React?
55	Why are inline ref callbacks or functions not recommended?
56	What are HOC factory implementations?
57	How to use class field declarations syntax in React classes?
58	Why do you not need error boundaries for event handlers?
59	What is the difference between try catch block and error boundaries?
60	What is the required method to be defined for a class component?
61	What are the possible return types of render method?
62	What is the main purpose of constructor?
63	Is it mandatory to define constructor for React component?
64	Why should not call setState in componentWillUnmount?
65	What is the purpose of getDerivedStateFromError?
66	What is the methods order when component re-rendered?
67	What are the methods invoked during error handling?
68	What is the purpose of unmountComponentAtNode method?
69	What are the limitations with HOCs?
70	How to debug forwardRefs in DevTools?
71	Is it good to use arrow functions in render methods?
72	How do you say that state updates are merged?
73	How do you pass arguments to an event handler?
74	How to prevent component from rendering?
75	Give an example on How to use context?
76	How do you use contextType?
77	What is a consumer?
78	How do you solve performance corner cases while using context?
79	What is the purpose of forward ref in HOCs?
80	Is it ref argument available for all functions or class components?
81	Why do you need additional care for component libraries while using forward refs?
82	How to create react class components without ES6?
83	Is it possible to use react without JSX?
84	How do you create HOC using render props?
85	What is react scripts?
86	What are the features of create react app?
87	What is the purpose of renderToNodeStream method?
88	How do you get redux scaffolding using create-react-app?
89	What is state mutation and how to prevent it?
90	What are React Mixins?
91	Can I use javascript urls in react16.9?

Core React

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, 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
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
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 tr