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Most important React.js interview questions for busy Front End Engineers (updated for 2025)
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Curated top React.js interview questions with high quality answers for acing your front end interviews.
React is a JavaScript library created by Facebook for building user interfaces, primarily for single-page applications. It allows developers to create reusable components that manage their own state. Key benefits of React include a component-based architecture for modular code, the virtual DOM for efficient updates, a declarative UI for more readable code, one-way data binding for predictable data flow, and a strong community and ecosystem with abundant resources and tools.
Key characteristics of React:
- Declarative: You describe the desired state of your UI based on data, and React handles updating the actual DOM efficiently.
- Component-based: Build reusable and modular UI elements (components) that manage their own state and logic.
- Virtual DOM: React uses a lightweight in-memory representation of the actual DOM, allowing it to perform updates selectively and efficiently.
- JSX: While not mandatory, JSX provides a syntax extension that allows you to write HTML-like structures within your JavaScript code, making UI development more intuitive.
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A React Node is any renderable unit in React, such as an element, string, number, or
null. A React Element is an immutable object describing what to render, created using JSX orReact.createElement. A React Component is a function or class that returns React Elements, enabling the creation of reusable UI pieces.Read thedetailed answer onGreatFrontEnd which allows progress tracking, contains more code samples, and useful resources.
JSX stands for JavaScript XML. It is a syntax extension for JavaScript that allows you to write HTML-like code within JavaScript. JSX makes it easier to create React components by allowing you to write what looks like HTML directly in your JavaScript code. Under the hood, JSX is transformed into JavaScript function calls, typically using a tool like Babel. For example,
<div>Hello, world!</div>in JSX is transformed intoReact.createElement('div', null, 'Hello, world!').Read thedetailed answer onGreatFrontEnd which allows progress tracking, contains more code samples, and useful resources.
In React,
stateis a local data storage that is managed within a component and can change over time, whilepropsare read-only attributes passed from a parent component to a child component. State is used for data that changes within a component, whereas props are used to pass data and event handlers to child components.Read thedetailed answer onGreatFrontEnd which allows progress tracking, contains more code samples, and useful resources.
The
keyprop in React is used to uniquely identify elements in a list. It helps React optimize rendering by efficiently updating and reordering elements. Without a uniquekey, React may re-render elements unnecessarily, leading to performance issues and bugs.{items.map((item)=><ListItemkey={item.id}value={item.value}/>);}
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Using array indices as the value for
keys in React can lead to performance issues and bugs. When the order of items changes, React may not correctly identify which items have changed, leading to unnecessary re-renders or incorrect component updates. It's better to use unique identifiers forkeys to ensure React can efficiently manage and update the DOM.Read thedetailed answer onGreatFrontEnd which allows progress tracking, contains more code samples, and useful resources.
Controlled components in React are those where the form data is handled by the component's state. The state is the single source of truth, and any changes to the form input are managed through event handlers. Uncontrolled components, on the other hand, store their own state internally and rely on refs to access the form values. Controlled components offer more control and are easier to test, while uncontrolled components can be simpler to implement for basic use cases.
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Using context in React can lead to performance issues if not managed properly. It can cause unnecessary re-renders of components that consume the context, even if the part of the context they use hasn't changed. Additionally, overusing context for state management can make the code harder to understand and maintain. It's important to use context sparingly and consider other state management solutions like Redux or Zustand for more complex state needs.
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Hooks in React allow you to use state and other React features without writing a class. They make it easier to reuse stateful logic between components, improve code readability, and simplify the codebase by reducing the need for lifecycle methods. Hooks like
useStateanduseEffectare commonly used to manage state and side effects in functional components.Read thedetailed answer onGreatFrontEnd which allows progress tracking, contains more code samples, and useful resources.
React hooks have a few essential rules to ensure they work correctly. Always call hooks at the top level of your React function, never inside loops, conditions, or nested functions. Only call hooks from React function components or custom hooks. These rules ensure that hooks maintain the correct state and lifecycle behavior.
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useEffectanduseLayoutEffectare React hooks used to handle side effects in functional components, but they differ in timing and use cases:useEffect: Runs asynchronously after the DOM has been painted. It is suitable for tasks like data fetching, subscriptions, or logging.useLayoutEffect: Runs synchronously after DOM mutations but before the browser paints. Use it for tasks like measuring DOM elements or synchronizing the UI with the DOM.
Code example:
importReact,{useEffect,useLayoutEffect,useRef}from'react';functionExample(){constref=useRef();useEffect(()=>{console.log('useEffect: Runs after DOM paint');});useLayoutEffect(()=>{console.log('useLayoutEffect: Runs before DOM paint');console.log('Element width:',ref.current.offsetWidth);});return<divref={ref}>Hello</div>;}
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What is the purpose of callback function argument format of
setState()in React and when should it be used?The callback function argument format of
setState()in React is used to ensure that state updates are based on the most recent state and props. This is particularly important when the new state depends on the previous state. Instead of passing an object directly tosetState(), you pass a function that takes the previous state and props as arguments and returns the new state.this.setState((prevState,props)=>({counter:prevState.counter+props.increment,}));
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The dependency array of
useEffectdetermines when the effect should re-run. If the array is empty, the effect runs only once after the initial render. If it contains variables, the effect runs whenever any of those variables change. If omitted, the effect runs after every render.Read thedetailed answer onGreatFrontEnd which allows progress tracking, contains more code samples, and useful resources.
The
useRefhook in React is used to create a mutable object that persists across renders. It can be used to access and manipulate DOM elements directly, store mutable values that do not cause re-renders when updated, and keep a reference to a value without triggering a re-render. For example, you can useuseRefto focus an input element:importReact,{useRef,useEffect}from'react';functionTextInputWithFocusButton(){constinputEl=useRef(null);useEffect(()=>{inputEl.current.focus();},[]);return<inputref={inputEl}type="text"/>;}
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The
useCallbackhook in React is used to memoize functions, preventing them from being recreated on every render. This is particularly useful when passing callbacks to optimized child components that rely on reference equality to prevent unnecessary renders. You should useuseCallbackwhen you have a function that is passed as a prop to a child component and you want to avoid the child component re-rendering unnecessarily.constmemoizedCallback=useCallback(()=>{doSomething(a,b);},[a,b]);
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The
useMemohook in React is used to memoize expensive calculations so that they are only recomputed when one of the dependencies has changed. This can improve performance by avoiding unnecessary recalculations. You should useuseMemowhen you have a computationally expensive function that doesn't need to run on every render.constmemoizedValue=useMemo(()=>computeExpensiveValue(a,b),[a,b]);
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The
useReducerhook in React is used for managing complex state logic in functional components. It is an alternative touseStateand is particularly useful when the state has multiple sub-values or when the next state depends on the previous one. It takes a reducer function and an initial state as arguments and returns the current state and a dispatch function.const[state,dispatch]=useReducer(reducer,initialState);
Use
useReducerwhen you have complex state logic that involves multiple sub-values or when the next state depends on the previous state.Read thedetailed answer onGreatFrontEnd which allows progress tracking, contains more code samples, and useful resources.
The
useIdhook in React is used to generate unique IDs for elements within a component. This is particularly useful for accessibility purposes, such as linking form inputs with their labels. It ensures that IDs are unique across the entire application, even if the component is rendered multiple times.import{useId}from'react';functionMyComponent(){constid=useId();return(<div><labelhtmlFor={id}>Name:</label><inputid={id}type="text"/></div>);}
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Re-rendering in React refers to the process where a component updates its output to the DOM in response to changes in state or props. When a component's state or props change, React triggers a re-render to ensure the UI reflects the latest data. This process involves calling the component's render method again to produce a new virtual DOM, which is then compared to the previous virtual DOM to determine the minimal set of changes needed to update the actual DOM.
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React Fragments are used to group multiple elements without adding extra nodes to the DOM. This is useful when you want to return multiple elements from a component's render method without wrapping them in an additional HTML element. You can use the shorthand syntax
<>...</>or theReact.Fragmentsyntax.return(<><ChildComponent1/><ChildComponent2/></>);
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forwardRef()in React is used to pass a ref through a component to one of its child components. This is useful when you need to access a DOM element or a child component's instance directly from a parent component. You wrap your functional component withforwardRef()and use therefparameter to forward the ref to the desired child element.importReact,{forwardRef}from'react';constMyComponent=forwardRef((props,ref)=><inputref={ref}{...props}/>);
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To reset a component's state in React, you can set the state back to its initial value. This can be done by defining an initial state and then using the
setStatefunction to reset it. For example, if you have a state object like this:const[state,setState]=useState(initialState);
You can reset it by calling:
setState(initialState);
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React recommends against mutating state because it can lead to unexpected behavior and bugs. React relies on state immutability to efficiently determine when to re-render components. When state is mutated directly, React may not detect the changes, leading to stale or incorrect UI updates. Instead, always create a new state object using methods like
setStateor theuseStatehook.Read thedetailed answer onGreatFrontEnd which allows progress tracking, contains more code samples, and useful resources.
Error boundaries in React are components that catch JavaScript errors anywhere in their child component tree, log those errors, and display a fallback UI instead of crashing the whole application. They are implemented using the
componentDidCatchlifecycle method and thestatic getDerivedStateFromErrormethod. Error boundaries do not catch errors inside event handlers, asynchronous code, or server-side rendering.Read thedetailed answer onGreatFrontEnd which allows progress tracking, contains more code samples, and useful resources.
To test React applications, you can use tools like Jest and React Testing Library. Jest is a JavaScript testing framework that works well with React, and React Testing Library provides utilities to test React components in a way that resembles how users interact with them. You can write unit tests for individual components, integration tests for component interactions, and end-to-end tests using tools like Cypress.
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React hydration is the process of attaching event listeners and making a server-rendered HTML page interactive on the client side. When a React application is server-side rendered, the HTML is sent to the client, and React takes over to make it dynamic by attaching event handlers and initializing state. This process is called hydration.
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React Portals are used to render children into a DOM node that exists outside the hierarchy of the parent component. This is useful for scenarios like modals, tooltips, and dropdowns where you need to break out of the parent component's overflow or z-index constraints. You can create a portal using
ReactDOM.createPortal(child, container).Read thedetailed answer onGreatFrontEnd which allows progress tracking, contains more code samples, and useful resources.
To debug React applications, you can use the React Developer Tools browser extension to inspect component hierarchies and state. Additionally, you can use
console.logstatements to log data and errors, and leverage breakpoints in your code using browser developer tools. For more advanced debugging, you can use error boundaries to catch and handle errors in your components.Read thedetailed answer onGreatFrontEnd which allows progress tracking, contains more code samples, and useful resources.
React strict mode is a development tool that helps identify potential problems in an application. It activates additional checks and warnings for its descendants. It doesn't render any visible UI and doesn't affect the production build. The benefits include identifying unsafe lifecycle methods, warning about legacy string ref API usage, detecting unexpected side effects, and ensuring that components are resilient to future changes.
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To localize a React application, you typically use a library like
react-i18nextorreact-intl. First, you set up your translation files for different languages. Then, you configure the localization library in your React app. Finally, you use the provided hooks or components to display localized text in your components.// Example using react-i18nextimport{useTranslation}from'react-i18next';constMyComponent=()=>{const{ t}=useTranslation();return<p>{t('welcome_message')}</p>;};
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Code splitting in a React application is a technique used to improve performance by splitting the code into smaller chunks that can be loaded on demand. This helps in reducing the initial load time of the application. You can achieve code splitting using dynamic
import()statements or React'sReact.lazyandSuspense.// Using React.lazy and SuspenseconstLazyComponent=React.lazy(()=>import('./LazyComponent'));functionApp(){return(<React.Suspensefallback={<div>Loading...</div>}><LazyComponent/></React.Suspense>);}
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To optimize the performance of React contexts and reduce rerenders, you can use techniques such as memoizing context values, splitting contexts, and using selectors. Memoizing context values with
useMemoensures that the context value only changes when its dependencies change. Splitting contexts allows you to isolate state changes to specific parts of your application. Using selectors with libraries likeuse-context-selectorcan help you only rerender components that actually need the updated context value.constvalue=useMemo(()=>({ state, dispatch}),[state,dispatch]);
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Higher order components (HOCs) in React are functions that take a component and return a new component with additional props or behavior. They are used to reuse component logic. For example, if you have a component
MyComponent, you can create an HOC like this:constwithExtraProps=(WrappedComponent)=>{return(props)=><WrappedComponent{...props}extraProp="value"/>;};constEnhancedComponent=withExtraProps(MyComponent);
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The Flux pattern is an architectural design used for managing state in applications, particularly in React ecosystems. It enforces a unidirectional data flow, making it easier to manage and debug application state.
- Core components:
- Dispatcher: Manages actions and dispatches them to stores.
- Stores: Hold the state and logic of the application.
- Actions: Payloads of information sent from the application to the dispatcher.
- View: React components that re-render when stores update.
- Benefits:
- Predictable state management due to unidirectional data flow.
- Improved debugging and testing.
- Clear separation of concerns.
Example flow:
- User interacts with theView.
- Actions are triggered and dispatched by theDispatcher.
- Stores process the actions and update their state.
- View re-renders based on the updated state.
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- Core components:
In React, one-way data flow means that data in a React application flows in a single direction, from parent components to child components. This makes the data flow predictable and easier to debug. The main benefits include improved maintainability, easier debugging, and better performance.
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In React applications, asynchronous data loading is typically handled using
useEffectanduseStatehooks. You initiate the data fetch insideuseEffectand update the state with the fetched data. This ensures that the component re-renders with the new data. Here's a simple example:importReact,{useState,useEffect}from'react';functionDataFetchingComponent(){const[data,setData]=useState(null);const[loading,setLoading]=useState(true);useEffect(()=>{asyncfunctionfetchData(){constresponse=awaitfetch('https://api.example.com/data');constresult=awaitresponse.json();setData(result);setLoading(false);}fetchData();},[]);if(loading){return<div>Loading...</div>;}return<div>{JSON.stringify(data)}</div>;}
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Server-side rendering (SSR) in React involves rendering React components on the server and sending the fully rendered HTML to the client. This approach improves initial load times and SEO. The server handles the initial rendering, and the client takes over with React's hydration process. Benefits include faster initial page loads, better SEO, and improved performance on slower devices.
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Static generation in React applications involves pre-rendering the HTML at build time, rather than on each request. This results in faster load times and better performance since the HTML is already generated and can be served directly from a CDN. It also improves SEO and can reduce server load. Tools like Next.js facilitate static generation by allowing developers to generate static pages easily.
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In React, the presentational vs container component pattern is a design approach where presentational components focus on how things look and container components focus on how things work. Presentational components are concerned with rendering HTML and CSS, while container components handle the logic and state management. This separation helps in maintaining a clean and organized codebase.
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Common pitfalls when doing data fetching in React include not handling loading and error states, causing memory leaks by not cleaning up subscriptions, and not using the right lifecycle methods or hooks. Always ensure you handle these states properly, clean up after your components, and use
useEffectfor side effects in functional components.Read thedetailed answer onGreatFrontEnd which allows progress tracking, contains more code samples, and useful resources.
Render props in React are a technique for sharing code between components using a prop whose value is a function. This function returns a React element and allows you to pass data to the child component. It helps in reusing component logic without using higher-order components or hooks.
classDataFetcherextendsReact.Component{state={data:null};componentDidMount(){fetch(this.props.url).then((response)=>response.json()).then((data)=>this.setState({ data}));}render(){returnthis.props.render(this.state.data);}}// Usage<DataFetcherurl="/api/data"render={(data)=><div>{data ?data.name :'Loading...'}</div>}/>;
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React anti-patterns are practices that can lead to inefficient, hard-to-maintain, or buggy code. Some common anti-patterns include:
- Mutating state directly instead of using
setState - Using
componentWillMountfor data fetching - Overusing
componentWillReceiveProps - Not using keys in lists
- Overusing inline functions in render
- Deeply nested state
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- Mutating state directly instead of using
Choosing between React state, context, and external state managers depends on the complexity and scope of your application's state management needs. Use React state for local component state, React context for global state that needs to be shared across multiple components, and external state managers like Redux or MobX for complex state management that requires advanced features like middleware, time-travel debugging, or when the state needs to be shared across a large application.
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The composition pattern in React is a way to build components by combining smaller, reusable components. Instead of using inheritance, React encourages composition to create complex UIs. You can pass components as children or props to other components to achieve this. For example:
functionWelcomeDialog(){return(<Dialog><h1>Welcome</h1><p>Thank you for visiting our spacecraft!</p></Dialog>);}functionDialog(props){return<divclassName="dialog">{props.children}</div>;}
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The virtual DOM in React is a lightweight copy of the actual DOM. It allows React to efficiently update the UI by comparing the virtual DOM with the real DOM and only making necessary changes. This process is called reconciliation. By using the virtual DOM, React minimizes direct manipulation of the real DOM, which can be slow and inefficient.
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The virtual DOM in React is a lightweight copy of the actual DOM. When the state of a component changes, React creates a new virtual DOM tree and compares it with the previous one using a process called "reconciliation." Only the differences are then updated in the actual DOM, making updates more efficient. The benefits include improved performance and a more declarative way to manage UI. However, it can add complexity and may not be as performant for very simple applications.
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React Fiber is a complete rewrite of React's reconciliation algorithm, introduced in React 16. It improves the rendering process by breaking down rendering work into smaller units, allowing React to pause and resume work, which makes the UI more responsive. This approach enables features like time slicing and suspense, which were not possible with the previous stack-based algorithm.
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Reconciliation in React is the process through which React updates the DOM to match the virtual DOM. When a component's state or props change, React creates a new virtual DOM tree and compares it with the previous one. This comparison process is called "diffing." React then updates only the parts of the actual DOM that have changed, making the updates efficient and fast.
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React Suspense is a feature that allows you to handle asynchronous operations in your React components more gracefully. It enables you to show fallback content while waiting for something to load, such as data fetching or code splitting. You can use it with
React.lazyfor code splitting and with libraries likereact-queryfor data fetching.constLazyComponent=React.lazy(()=>import('./LazyComponent'));functionMyComponent(){return(<React.Suspensefallback={<div>Loading...</div>}><LazyComponent/></React.Suspense>);}
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When the setter function returned by the
useStatehook is called in React, it schedules an update to the component's state value. React then queues a re-render of the component with the new state. This process is typically asynchronous, and React batches multiple state updates together for performance.Read thedetailed answer onGreatFrontEnd which allows progress tracking, contains more code samples, and useful resources.
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