React Interview Questions & Answers

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Table of Contents

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, 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.

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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.

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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.

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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 <h1> tag is returned as JavaScript function to the render function.

   export default function App() {
     return <h1 className="greeting">{"Hello, this is a JSX Code!"}</h1>;
   }
   

   If you don't use JSX syntax then the respective JavaScript code should be written as below,

   import { createElement } from "react";
   
   export default function App() {
     return createElement(
       "h1",
       { className: "greeting" },
       "Hello, this is a JSX Code!"
     );
   }
   

   See Class

   class App extends React.Component {
     render() {
       return <h1 className="greeting">{"Hello, this is a JSX Code!"}</h1>;
     }
   }
   

   Note: JSX is stricter than HTML

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5. What is the difference between an Element and a Component?

   Element:

   • A React Element is a plain JavaScript object that describes what you want to see on the UI. It represents a DOM node or a component at a specific point in time.

   • Elements are immutable: once created, you cannot change their properties. Instead, you create new elements to reflect updates.

   • Elements can be nested within other elements through their props.

   • Creating an element is a fast, lightweight operation—it does not create any actual DOM nodes or render anything to the screen directly.

     Example (without JSX):

     const element = React.createElement("button", { id: "login-btn" }, "Login");
     

     Equivalent JSX syntax:

     <button id="login-btn">Login</button>
     

     The object returned by React.createElement:

     {
       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):

     const Button = ({ handleLogin }) => (
       <button id="login-btn" onClick={handleLogin}>
         Login
       </button>
     );
     

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

     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.

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6. How to create components in React?

   Components are the building blocks of creating User Interfaces(UI) in React. There are two possible ways to create a component.

   1. Function Components: This is the simplest way to create a component. Those are pure JavaScript functions that accept props object as the one and only one parameter and return React elements to render the output:

      function Greeting({ message }) {
        return <h1>{`Hello, ${message}`}</h1>;
      }
      

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

      class Greeting extends React.Component {
        render() {
          return <h1>{`Hello, ${this.props.message}`}</h1>;
        }
      }
      

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7. When to use a Class Component over a Function Component?

   After the addition of Hooks(i.e. React 16.8 onwards) it is always recommended to use Function components over Class components in React. Because you could use state, lifecycle methods and other features that were only available in class component present in function component too.

   But even there are two reasons to use Class components over Function components.

   1. If you need a React functionality whose Function component equivalent is not present yet, like Error Boundaries.
   2. In older versions, If the component needs state or lifecycle methods then you need to use class component.

   So the summary to this question is as follows:

   Use Function Components:

   • If you don't need state or lifecycle methods, and your component is purely presentational.
   • For simplicity, readability, and modern code practices, especially with the use of React Hooks for state and side effects.

   Use Class Components:

   • If you need to manage state or use lifecycle methods.
   • In scenarios where backward compatibility or integration with older code is necessary.

   Note: You can also use reusable react error boundary third-party component without writing any class. i.e, No need to use class components for Error boundaries.

   The usage of Error boundaries from the above library is quite straight forward.

   Note when using react-error-boundary: ErrorBoundary is a client component. You can only pass props to it that are serializable or use it in files that have a "use client"; directive.

   "use client";
   
   import { ErrorBoundary } from "react-error-boundary";
   
   <ErrorBoundary fallback={<div>Something went wrong</div>}>
     <ExampleApplication />
   </ErrorBoundary>;
   

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8. What are Pure Components?

   Pure components are the components which render the same output for the same state and props. In function components, you can achieve these pure components through memoized React.memo() API wrapping around the component. This API prevents unnecessary re-renders by comparing the previous props and new props using shallow comparison. So it will be helpful for performance optimizations.

   But at the same time, it won't compare the previous state with the current state because function component itself prevents the unnecessary rendering by default when you set the same state again.

   The syntactic representation of memoized components looks like below,

   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).

   import { memo, useState } from "react";
   
   const EmployeeProfile = memo(function EmployeeProfile({ name, email }) {
     return (
       <>
         <p>Name:{name}</p>
         <p>Email: {email}</p>
       </>
     );
   });
   export default function EmployeeRegForm() {
     const [name, setName] = useState("");
     const [email, setEmail] = useState("");
     return (
       <>
         <label>
           Name:{" "}
           <input value={name} onChange={(e) => setName(e.target.value)} />
         </label>
         <label>
           Email:{" "}
           <input value={email} onChange={(e) => setEmail(e.target.value)} />
         </label>
         <hr />
         <EmployeeProfile name={name} />
       </>
     );
   }
   

   In the above code, the email prop has not been passed to child component. So there won't be any re-renders for email prop change.

   In class components, the components extending React.PureComponent instead of React.Component become the pure components. When props or state changes, PureComponent will do a shallow comparison on both props and state by invoking shouldComponentUpdate() lifecycle method.

   Note: React.memo() is a higher-order component.

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9. What is state in React?

   State of a component is an object that holds some information that may change over the lifetime of the component. The important point is whenever the state object changes, the component re-renders. It is always recommended to make our state as simple as possible and minimize the number of stateful components.

   

   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.

   import { useState } from "react";
   
   function User() {
     const [message, setMessage] = useState("Welcome to React world");
   
     return (
       <div>
         <h1>{message}</h1>
       </div>
     );
   }
   

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

   See Class

   import React from "react";
   class User extends React.Component {
     constructor(props) {
       super(props);
   
       this.state = {
         message: "Welcome to React world",
       };
     }
   
     render() {
       return (
         <div>
           <h1>{this.state.message}</h1>
         </div>
       );
     }
   }
   

   State is similar to props, but it is private and fully controlled by the component ,i.e., it is not accessible to any other component till the owner component decides to pass it.

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10. What are props in React?

    Props are inputs to components. They are single values or objects containing a set of values that are passed to components on creation similar to HTML-tag attributes. Here, the data is passed down from a parent component to a child component.

    The primary purpose of props in React is to provide following component functionality:

    1. Pass custom data to your component.
    2. Trigger state changes.
    3. Use via this.props.reactProp inside component's render() method.

    For example, let us create an element with reactProp property:

    <Element reactProp={"1"} />
    

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

    props.reactProp;
    

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

    import React from "react";
    import ReactDOM from "react-dom";
    
    const ChildComponent = (props) => {
      return (
        <div>
          <p>{props.name}</p>
          <p>{props.age}</p>
          <p>{props.gender}</p>
        </div>
      );
    };
    
    const ParentComponent = () => {
      return (
        <div>
          <ChildComponent name="John" age="30" gender="male" />
          <ChildComponent name="Mary" age="25" geneder="female" />
        </div>
      );
    };
    

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

const ChildComponent = ({ name, age, gender = "male" }) => {
  return (
    <div>
      <p>{name}</p>
      <p>{age}</p>
      <p>{gender}</p>
    </div>
  );
};

Note: The default value won't be used if you pass null or 0 value. i.e, default value is only used if the prop value is missed or undefined value has been passed.

import React from "react";
import ReactDOM from "react-dom";

class ChildComponent extends React.Component {
  render() {
    return (
      <div>
        <p>{this.props.name}</p>
        <p>{this.props.age}</p>
        <p>{this.props.gender}</p>
      </div>
    );
  }
}

class ParentComponent extends React.Component {
  render() {
    return (
      <div>
        <ChildComponent name="John" age="30" gender="male" />
        <ChildComponent name="Mary" age="25" gender="female" />
      </div>
    );
  }
}

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11. What is the difference between state and props?

    In React, both state and props are plain JavaScript objects, but they serve different purposes and have distinct behaviors:

    State

    • Definition:
      State is a data structure that is managed within a component. It represents information that can change over the lifetime of the component.
    • Mutability:
      State is mutable, meaning it can be changed using the setter function (setState in class components or the updater function from useState in functional components).
    • Scope:
      State is local to the component where it is defined. Only that component can modify its own state.
    • Usage:
      State is typically used for data that needs to change in response to user actions, network responses, or other dynamic events.
    • Re-rendering:
      Updating the state triggers a re-render of the component and its descendants.

    Props

    • Definition:
      Props (short for “properties”) are inputs to a component, provided by its parent component.
    • Mutability:
      Props are read-only. A component cannot modify its own props; they are immutable from the component’s perspective.
    • Scope:
      Props are used to pass data and event handlers down the component tree, enabling parent components to configure or communicate with their children.
    • Usage:
      Props are commonly used to make components reusable and configurable. They allow the same component to be rendered with different data or behavior.
    • Analogy:
      Think of props as arguments to a function, whereas state is like variables declared inside the function.

    Summary Table

    Feature	State	Props
    Managed by	The component itself	Parent component
    Mutable	Yes	No (read-only)
    Scope	Local to the component	Passed from parent to child
    Usage	Manage dynamic data and UI changes	Configure and customize component
    Update	Using setState/useState	Cannot be updated by the component

    ---

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12. What is the difference between HTML and React event handling?

    Below are some of the main differences between HTML and React event handling,

    1. In HTML, the event name usually represents in lowercase as a convention:

       <button onclick="activateLasers()"></button>
       

       Whereas in React it follows camelCase convention:

       <button onClick={activateLasers}>
       

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

       <a
         href="#"
         onclick='console.log("The link was clicked."); return false;'
       />
       

       Whereas in React you must call preventDefault() explicitly:

       function handleClick(event) {
         event.preventDefault();
         console.log("The link was clicked.");
       }
       

    3. In HTML, you need to invoke the function by appending () Whereas in react you should not append () with the function name. (refer "activateLasers" function in the first point for example)

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13. What are synthetic events in React?

    SyntheticEvent is a cross-browser wrapper around the browser's native event. Its API is same as the browser's native event, including stopPropagation() and preventDefault(), except the events work identically across all browsers. The native events can be accessed directly from synthetic events using nativeEvent attribute.

    Let's take an example of BookStore title search component with the ability to get all native event properties

    function BookStore() {
      function handleTitleChange(e) {
        console.log("The new title is:", e.target.value);
        console.log('Synthetic event:', e); // React SyntheticEvent
        console.log('Native event:', e.nativeEvent); // Browser native event
        e.stopPropagation();
        e.preventDefault();
      }
    
      return <input name="title" onChange={handleTitleChange} />;
    }
    

    List of common synthetic events are:

    • onClick
    • onChange
    • onSubmit
    • onKeyDown, onKeyUp
    • onFocus, onBlur
    • onMouseEnter, onMouseLeave
    • onTouchStart, onTouchEnd

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14. What are inline conditional expressions?

    You can use either if statements or ternary expressions which are available in JS(and JSX in React) to conditionally execute or render expressions. Apart from these approaches, you can also embed any expressions in JSX by wrapping them in curly braces and then followed by JS logical operator &&. It is helpful to render elements conditionally within a single line and commonly used for concise logic, especially in JSX rendering.

    <h1>Hello!</h1>;
    {
      messages.length > 0 && !isLogin ? (
        <h2>You have {messages.length} unread messages.</h2>
      ) : (
        <h2>You don't have unread messages.</h2>
      );
    }
    

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15. What is "key" prop and what is the benefit of using it in arrays of elements?

    A key is a special attribute you should include when mapping over arrays to render data. Key prop helps React identify which items have changed, are added, or are removed.

    Keys should be unique among its siblings. Most often we use ID from our data as key:

    const todoItems = todos.map((todo) => <li key={todo.id}>{todo.text}</li>);
    

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

    const todoItems = todos.map((todo, index) => (
      <li key={index}>{todo.text}</li>
    ));
    

    Benefits of key:

    • Enables React to efficiently update and re-render components.
    • Prevents unnecessary re-renders by reusing components when possible.
    • Helps maintain internal state of list items correctly.

    Note:

    1. Using indexes for keys is not recommended if the order of items may change. This can negatively impact performance and may cause issues with component state.
    2. If you extract list item as separate component then apply keys on list component instead of li tag.
    3. There will be a warning message in the console if the key prop is not present on list items.
    4. The key attribute accepts either string or number and internally convert it as string type.
    5. Don't generate the key on the fly something like key={Math.random()}. Because the keys will never match up between re-renders and DOM created everytime.

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16. What is Virtual DOM?

    The Virtual DOM (VDOM) is a lightweight, in-memory representation of Real DOM used by libraries like React to optimize UI rendering. The representation of a UI is kept in memory and synced with the "real" DOM. It's a step that happens between the render function being called and the displaying of elements on the screen. This entire process is called reconciliation.

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17. How Virtual DOM works?

    The Virtual DOM works in five simple steps.

    1. Initial Render
    When a UI component renders for the first time, it returns JSX. React uses this structure to create a Virtual DOM tree, which is a lightweight copy of the actual DOM. This Virtual DOM is then used to build and render the Real DOM in the browser.

    2. State or Props Change
    When the component's state or props change, React creates a new Virtual DOM reflecting the updated UI. However, it doesn't immediately update the Real DOM; instead, it works in memory to prepare for an efficient update.

    

    3. Diffing Algorithm
    React then compares the new Virtual DOM with the previous one using a process called diffing. It determines what has changed between the two versions and identifies the minimal set of updates needed.

    

    4. Reconciliation
    Based on the diffing results, React decides which parts of the Real DOM should be updated. It avoids re-rendering the entire DOM and instead updates only the elements that actually changed.

    

    5. Efficient DOM Updates
    This entire process—working with the Virtual DOM, diffing, and selective updating—makes the UI rendering much faster and more efficient than manipulating the Real DOM directly.

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18. What is the difference between Shadow DOM and Virtual DOM?

    The Shadow DOM is a browser technology designed primarily for scoping variables and CSS in web components. The Virtual DOM is a concept implemented by libraries in JavaScript on top of browser APIs.

    The key differences in a table format shown below:

    Feature	Shadow DOM	Virtual DOM
    Purpose	Encapsulation for Web Components	Efficient UI rendering
    Managed by	Browser	JS frameworks (e.g., React)
    DOM Type	Part of real DOM (scoped)	In-memory representation
    Encapsulation	Yes	No
    Use Case	Web Components, scoped styling	UI diffing and minimal DOM updates

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19. What is React Fiber?

    React Fiber is the new reconciliation engine in React, introduced in React 16. It’s a complete rewrite of React’s core algorithm(old stack-based algorithm) for rendering and updating the UI. Fiber enhances React’s ability to handle asynchronous rendering, prioritized updates(assign priority to different types of updates), and interruption(ability to pause, abort, or reuse work) of rendering work, enabling smoother and more responsive user interfaces.

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20. What is the main goal of React Fiber?

    The goal of React Fiber is to increase its suitability for areas like animation, layout, and gestures. Its headline feature is incremental rendering: the ability to split rendering work into chunks and spread it out over multiple frames.

    Its main goals are:

    • Incremental Rendering – Breaks work into chunks for smoother updates.
    • Interruptible Rendering – Pauses and resumes rendering to keep the UI responsive.
    • Prioritization – Handles high-priority updates (e.g. animations) before low-priority ones.
    • Concurrency Support – Enables working on multiple UI versions simultaneously.
    • Better Error Handling – Supports component-level error boundaries.
    • Suspense Support – Allows waiting for async data before rendering.
    • Improved DevTools – Enables better debugging and performance tracking.

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21. What are controlled components?

    A controlled component is a React component that fully manages the form element's state(e.g, elements like <input>, <textarea>, or <select>)) using React's internal state mechanism. i.e, The component does not manage its own internal state — instead, React acts as the single source of truth for form data.

    The controlled components will be implemented using the below steps,

    1. Initialize the state using useState hooks in function components or inside constructor for class components.
    2. Set the value of the form element to the respective state variable.
    3. Create an event handler(onChange) to handle the user input changes through useState's updater function or setState from class component.
    4. Attach the above event handler to form element's change or click events

    Note: React re-renders the component every time the input value changes.

For example, the name input field updates the username using handleChange event handler as below,

import React, { useState } from "react";

function UserProfile() {
  const [username, setUsername] = useState("");

  const handleChange = (e) => {
    setUsername(e.target.value);
  };

  return (
    <form>
      <label>
        Name:
        <input type="text" value={username} onChange={handleChange} />
      </label>
    </form>
  );
}

In these components, DOM does not hold the actual data instead React does.

Benefits:

• Easy to implement validation, conditional formatting, or live feedback.
• Full control over form data.
• Easier to test and debug because the data is centralized in the component’s state.

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22. What are uncontrolled components?

    The Uncontrolled components are form elements (like <input>, <textarea>, or <select>) that manage their own state internally via the DOM, rather than through React state. You can query the DOM using a ref to find its current value when you need it. This is a bit more like traditional HTML.

    The uncontrolled components will be implemented using the below steps,

    1. Create a ref using useRef react hook in function component or React.createRef() in class based component.
    2. Attach this ref to the form element.
    3. The form element value can be accessed directly through ref in event handlers or componentDidMount for class components

    In the below UserProfile component, the username input is accessed using ref.

    import React, { useRef } from "react";
    
    function UserProfile() {
      const usernameRef = useRef(null);
    
      const handleSubmit = (event) => {
        event.preventDefault();
        console.log("The submitted username is: " + usernameRef.current.value);
      };
    
      return (
        <form onSubmit={handleSubmit}>
          <label>
            Username:
            <input type="text" ref={usernameRef} />
          </label>
          <button type="submit">Submit</button>
        </form>
      );
    }
    

    Note: Here, DOM is in charge of the value. React only accesses the value when needed (via ref).

    Benefits:

    • Less boilerplate — no need for useState and onChange.
    • Useful for quick form setups or when integrating with non-React code.
    • Slightly better performance in very large forms (fewer re-renders).

    In most cases, it's recommend to use controlled components to implement forms. In a controlled component, form data is handled by a React component. The alternative is uncontrolled components, where form data is handled by the DOM itself.

    See Class

    class UserProfile extends React.Component {
      constructor(props) {
        super(props);
        this.handleSubmit = this.handleSubmit.bind(this);
        this.input = React.createRef();
      }
    
      handleSubmit(event) {
        alert("A name was submitted: " + this.input.current.value);
        event.preventDefault();
      }
    
      render() {
        return (
          <form onSubmit={this.handleSubmit}>
            <label>
              {"Name:"}
              <input type="text" ref={this.input} />
            </label>
            <input type="submit" value="Submit" />
          </form>
        );
      }
    }
    

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23. What is the difference between createElement and cloneElement?

    Both React.createElement and React.cloneElement are used to work with React elements, but they serve different purposes.

    createElement:

    Creates a new React element from scratch. JSX elements will be transpiled to React.createElement() functions to create React elements which are going to be used for the object representation of UI. Syntax:

    React.createElement(type, props, ...children)
    

    Example:

    React.createElement('button', { className: 'btn' }, 'Click Me')
    

    cloneElement:

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

    Syntax:

    React.cloneElement(element, newProps, ...children)
    

    Example:

    const button = <button className="btn">Click Me</button>;
    const cloned = React.cloneElement(button, { className: 'btn-primary' });
    // Result: <button className="btn-primary">Click Me</button>
    

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24. What is Lifting State Up in React?

    When several components need to share the same changing data then it is recommended to lift the shared state up to their closest common ancestor. That means if two child components share the same data from its parent, then move the state to parent instead of maintaining local state in both of the child components.

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25. What are Higher-Order Components?

    A higher-order component (HOC) is a function that takes a component and returns a new enhanced component with additional props, behavior, or data. It’s a design pattern based on React’s compositional nature, allowing you to reuse logic across multiple components without modifying their internals.

    We consider HOCs pure components because they don’t mutate or copy behavior from the original component—they simply wrap it, enhance it, and pass through the necessary props. The wrapped component remains decoupled and reusable.

    const EnhancedComponent = higherOrderComponent(WrappedComponent);
    

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

    withAuth HOC Example:

    import React from 'react';
    import { Navigate } from 'react-router-dom'; // For redirection (assuming React Router v6)
    
    const isAuthenticated = () => {
      // e.g., check for a valid token in localStorage or context
      return !!localStorage.getItem('authToken');
    };
    
    function withAuth(WrappedComponent) {
      return function AuthenticatedComponent(props) {
        if (!isAuthenticated()) {
          // User is NOT authenticated, redirect to login page
          return <Navigate to="/login" replace />;
        }
    
        // User is authenticated, render the wrapped component
        return <WrappedComponent {...props} />;
      };
    }
    
    export default withAuth;
    

    Usage

    import React from 'react';
    import withAuth from './withAuth';
    
    function Dashboard() {
      return <h1>Welcome to the Dashboard!</h1>;
    }
    
    // Wrap Dashboard with withAuth HOC
    export default withAuth(Dashboard);
    

    HOC can be used for many use cases:

    1. Code reuse, logic and bootstrap abstraction (e.g., fetching data, permissions, theming).
    2. Render hijacking (e.g., conditional rendering or layout changes).
    3. State abstraction and manipulation(e.g., handling form logic).
    4. Props manipulation(e.g., injecting additional props or filtering them).

    Some of the real-world examples of HOCs in react eco-system:

    1. connect() from react-redux
    2. withRouter() from React Router v5
    3. withTranslation() from react-i18next
    4. withApollo() from Apollo client
    5. withFormik from Formik library
    6. withTheme from styled components

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26. What is children prop?

    The children prop is a special prop in React used to pass elements between the opening and closing tags of a component. It is commonly used in layout and wrapper componnents.

    A simple usage of children prop looks as below,

    function MyDiv({ children }){
        return (
          <div>
            {children}
          </div>;
        );
    }
    
    export default function Greeting() {
      return (
        <MyDiv>
          <span>{"Hello"}</span>
          <span>{"World"}</span>
        </MyDiv>
      );
    }
    

    Here, everything inside <MyDiv>...</MyDiv> is passed as children to the custom div component.

    The children can be text, JSX elements, fragments, arrays and functions(for advance use case like render props).

    See Class

    const MyDiv = React.createClass({
      render: function () {
        return <div>{this.props.children}</div>;
      },
    });
    
    ReactDOM.render(
      <MyDiv>
        <span>{"Hello"}</span>
        <span>{"World"}</span>
      </MyDiv>,
      node
    );
    

    Note: There are several methods available in the legacy React API to work with this prop. These include React.Children.map, React.Children.forEach, React.Children.count, React.Children.only, React.Children.toArray.

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27. How to write comments in React?

    The comments in React/JSX are similar to JavaScript Multiline comments but are wrapped in curly braces.

    Single-line comments:

    <div>
      {/* Single-line comments(In vanilla JavaScript, the single-line comments are represented by double slash(//)) */}
      {`Welcome ${user}, let's play React`}
    </div>
    

    Multi-line comments:

    <div>
      {/* Multi-line comments for more than
       one line */}
      {`Welcome ${user}, let's play React`}
    </div>
    

    You can use // and /* */ in JS logic, hooks, and functions.

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28. What is reconciliation?

    Reconciliation is the process through which React updates the Browser DOM and makes React work faster. React use a diffing algorithm so that component updates are predictable and faster. React would first calculate the difference between the real DOM and the copy of DOM (Virtual DOM) when there's an update of components. React stores a copy of Browser DOM which is called Virtual DOM. When we make changes or add data, React creates a new Virtual DOM and compares it with the previous one. This comparison is done by Diffing Algorithm. Now React compares the Virtual DOM with Real DOM. It finds out the changed nodes and updates only the changed nodes in Real DOM leaving the rest nodes as it is. This process is called Reconciliation.

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29. Does the lazy function support named exports?

    No, currently React.lazy function supports default exports only. If you would like to import modules which are named exports, you can create an intermediate module that reexports it as the default. It also ensures that tree shaking keeps working and don’t pull unused components. Let's take a component file which exports multiple named components,

    // MoreComponents.js
    export const SomeComponent = /* ... */;
    export const UnusedComponent = /* ... */;
    

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

    // IntermediateComponent.js
    export { SomeComponent as default } from "./MoreComponents.js";
    

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

    import React, { lazy } from "react";
    const SomeComponent = lazy(() => import("./IntermediateComponent.js"));
    

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30. Why React uses className over class attribute?

    React uses className instead of class because of a JavaScript naming conflict with the class keyword.

    1. class is a reserved keyword in JavaScript In JavaScript, class is used to define ES6 classes:

       class Person {
         constructor(name) {
           this.name = name;
         }
       }
       

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

    2. JSX Is JavaScript

       When you write JSX like this:

       <div class="btn">Click</div>
       

       It will be compiled to:

       React.createElement('div', { class: 'btn' }, 'Click');
       

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

       <div className="btn">Click</div>
       

       which compiles to:

       React.createElement('div', { className: 'btn' }, 'Click');
       

       React then translates className to class in the final HTML DOM.

    3. Aligns with DOM APIs In vanilla JavaScript, you interact with element classes using:

       element.className = 'my-class';
       

       React follows this convention, staying consistent with the DOM API's property name rather than HTML’s attribute.

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31. What are fragments?

    It's a common pattern or practice in React for a component to return multiple elements. Fragments let you group a list of children without adding extra nodes to the DOM. You need to use either <Fragment> or a shorter syntax having empty tag (<></>).

    Below is the example of how to use fragment inside Story component.

    function Story({ title, description, date }) {
      return (
        <Fragment>
          <h2>{title}</h2>
          <p>{description}</p>
          <p>{date}</p>
        </Fragment>
      );
    }
    

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

    function StoryBook() {
      return stories.map((story) => (
        <Fragment key={story.id}>
          <h2>{story.title}</h2>
          <p>{story.description}</p>
          <p>{story.date}</p>
        </Fragment>
      ));
    }
    

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

    function Story({ title, description, date }) {
      return (
        <>
          <h2>{title}</h2>
          <p>{description}</p>
          <p>{date}</p>
        </>
      );
    }
    

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32. Why fragments are better than container divs?

    Below are the list of reasons to prefer fragments over container DOM elements,

    1. Fragments are a bit faster and use less memory by not creating an extra DOM node. This only has a real benefit on very large and deep trees.
    2. Some CSS mechanisms like Flexbox and CSS Grid have a special parent-child relationships, and adding divs in the middle makes it hard to keep the desired layout.
    3. The DOM Inspector is less cluttered.

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33. What are portals in React?

    A Portal is a React feature that enables rendering children into a DOM node that exists outside the parent component's DOM hierarchy, while still preserving the React component hierarchy. Portals help avoid CSS stacking issues—for example, elements with position: fixed may not behave as expected inside a parent with transform. Portals solve this by rendering content (like modals or tooltips) outside such constrained DOM contexts.

    ReactDOM.createPortal(child, container);
    

    • child: Any valid React node (e.g., JSX, string, fragment).
    • container: A real DOM node (e.g., document.getElementById('modal-root')).

    Even though the content renders elsewhere in the DOM, it still behaves like a normal child in React. It has access to context, state, and event handling.

    Example:- Modal:

    function Modal({ children }) {
      return ReactDOM.createPortal(
        <div className="modal">{children}</div>,
        document.body)
      );
    }
    

    The above code will render the modal content into the body element in the HTML, not inside the component's usual location.

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34. What are stateless components?

    If the behaviour of a component is independent of its state then it can be a stateless component. You can use either a function or a class for creating stateless components. But unless you need to use a lifecycle hook in your components, you should go for function components. There are a lot of benefits if you decide to use function components here; they are easy to write, understand, and test, a little faster, and you can avoid the this keyword altogether.

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35. What are stateful components?

    If the behaviour of a component is dependent on the state of the component then it can be termed as stateful component. These stateful components are either function components with hooks or class components.

    Let's take an example of function stateful component which update the state based on click event,

    import React, {useState} from 'react';
    
    const App = (props) => {
    const [count, setCount] = useState(0);
    handleIncrement() {
      setCount(count+1);
    }
    
    return (
      <>
        <button onClick={handleIncrement}>Increment</button>
        <span>Counter: {count}</span>
      </>
      )
    }
    

    See Class

    The equivalent class stateful component with a state that gets initialized in the `constructor`.

    class App extends Component {
      constructor(props) {
        super(props);
        this.state = { count: 0 };
      }
    
      handleIncrement() {
        setState({ count: this.state.count + 1 });
      }
    
      render() {
        <>
          <button onClick={() => this.handleIncrement}>Increment</button>
          <span>Count: {count}</span>
        </>;
      }
    }
    

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36. How to apply validation on props in React?

    When the application is running in development mode, React will automatically check all props that we set on components to make sure they have correct type. If the type is incorrect, React will generate warning messages in the console. It's disabled in production mode due to performance impact. The mandatory props are defined with isRequired.

    The set of predefined prop types:

    1. PropTypes.number
    2. PropTypes.string
    3. PropTypes.array
    4. PropTypes.object
    5. PropTypes.func
    6. PropTypes.node
    7. PropTypes.element
    8. PropTypes.bool
    9. PropTypes.symbol
    10. PropTypes.any

    We can define propTypes for User component as below:

    import React from "react";
    import PropTypes from "prop-types";
    
    class User extends React.Component {
      static propTypes = {
        name: PropTypes.string.isRequired,
        age: PropTypes.number.isRequired,
      };
    
      render() {
        return (
          <>
            <h1>{`Welcome, ${this.props.name}`}</h1>
            <h2>{`Age, ${this.props.age}`}</h2>
          </>
        );
      }
    }
    

    Note: In React v15.5 PropTypes were moved from React.PropTypes to prop-types library.

    Modern Recommendation: While PropTypes are still supported, TypeScript is now the industry standard for type checking in React applications. Consider using TypeScript for better type safety, IDE support, and compile-time error detection.

    The Equivalent Functional Component

    import React from "react";
    import PropTypes from "prop-types";
    
    function User({ name, age }) {
      return (
        <>
          <h1>{`Welcome, ${name}`}</h1>
          <h2>{`Age, ${age}`}</h2>
        </>
      );
    }
    
    User.propTypes = {
      name: PropTypes.string.isRequired,
      age: PropTypes.number.isRequired,
    };
    

    Modern TypeScript Version

    import React from "react";
    
    interface UserProps {
      name: string;
      age: number;
    }
    
    function User({ name, age }: UserProps) {
      return (
        <>
          <h1>{`Welcome, ${name}`}</h1>
          <h2>{`Age, ${age}`}</h2>
        </>
      );
    }
    

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37. What are the advantages of React?

    Below are the list of main advantages of React,

    1. Increases the application's performance with Virtual DOM.
    2. JSX makes code easy to read and write.
    3. It renders both on client and server side (SSR).
    4. Easy to integrate with frameworks (Angular, Backbone) since it is only a view library.
    5. Easy to write unit and integration tests with tools such as Jest.

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38. What are the limitations of React?

    Apart from the advantages, there are few limitations of React too,

    1. React is just a view library, not a full framework.
    2. There is a learning curve for beginners who are new to web development.
    3. Integrating React into a traditional MVC framework requires some additional configuration.
    4. The code complexity increases with inline templating and JSX.
    5. Too many smaller components leading to over engineering or boilerplate.

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39. What are the recommended ways for static type checking?

    Modern Recommendation (2026): TypeScript is the industry standard for type checking in React applications.

    While PropTypes (React.PropTypes moved to prop-types package since React v15.5) are still available for runtime type checking, they have significant limitations:

    • Only check types at runtime
    • No compile-time errors
    • Limited IDE autocomplete support
    • No inference for complex types

    TypeScript is now the recommended approach because it provides:

    • Compile-time type checking
    • Excellent IDE support with IntelliSense
    • Type inference and generic types
    • Better refactoring capabilities
    • Growing ecosystem and community support

    Note: Flow (Facebook's type checker) has seen declining adoption and is rarely used in new projects.

    Example with TypeScript:

    interface UserProps {
      name: string;
      age: number;
      isActive?: boolean;
    }
    
    function User({ name, age, isActive = true }: UserProps) {
      return <div>{name} - {age}</div>;
    }
    

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40. What is the use of react-dom package?

    The react-dom package provides DOM-specific methods that can be used at the top level of your app. Most of the components are not required to use this module. Some of the methods of this package are:

    1. render()
    2. hydrate()
    3. unmountComponentAtNode()
    4. findDOMNode()
    5. createPortal()

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41. What is ReactDOMServer?

    The ReactDOMServer object enables you to render components to static markup (typically used on node server). This object is mainly used for server-side rendering (SSR). The following methods can be used in both the server and browser environments:

    1. renderToString()
    2. renderToStaticMarkup()

    For example, you generally run a Node-based web server like Express, Hapi, or Koa, and you call renderToString to render your root component to a string, which you then send as response.

    // using Express
    import { renderToString } from "react-dom/server";
    import MyPage from "./MyPage";
    
    app.get("/", (req, res) => {
      res.write(
        "<!DOCTYPE html><html><head><title>My Page</title></head><body>"
      );
      res.write('<div id="content">');
      res.write(renderToString(<MyPage />));
      res.write("</div></body></html>");
      res.end();
    });
    

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42. How to use innerHTML in React?

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

    In this example MyComponent uses dangerouslySetInnerHTML attribute for setting HTML markup:

    function createMarkup() {
      return { __html: "First · Second" };
    }
    
    function MyComponent() {
      return <div dangerouslySetInnerHTML={createMarkup()} />;
    }
    

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43. How to use styles in React?

    The style attribute accepts a JavaScript object with camelCased properties rather than a CSS string. This is consistent with the DOM style JavaScript property, is more efficient, and prevents XSS security holes.

    const divStyle = {
      color: "blue",
      backgroundImage: "url(" + imgUrl + ")",
    };
    
    function HelloWorldComponent() {
      return <div style={divStyle}>Hello World!</div>;
    }
    

    Style keys are camelCased in order to be consistent with accessing the properties on DOM nodes in JavaScript (e.g. node.style.backgroundImage).

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44. How events are different in React?

    Handling events in React elements has some syntactic differences:

    1. React event handlers are named using camelCase, rather than lowercase.
    2. With JSX you pass a function as the event handler, rather than a string.

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45. What is the impact of indexes as keys?

    Keys should be stable, predictable, and unique so that React can keep track of elements.

    In the below code snippet each element's key will be based on ordering, rather than tied to the data that is being represented. This limits the optimizations that React can do and creates confusing bugs in the application.

    {
      todos.map((todo, index) => <Todo {...todo} key={index} />);
    }
    

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

    {
      todos.map((todo) => <Todo {...todo} key={todo.id} />);
    }
    

    Note: If you don't specify key prop at all, React will use index as a key's value while iterating over an array of data.

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46. How do you conditionally render components?

    In some cases you want to render different components depending on some state. JSX does not render false or undefined, so you can use conditional short-circuiting to render a given part of your component only if a certain condition is true.

    const MyComponent = ({ name, address }) => (
      <div>
        <h2>{name}</h2>
        {address && <p>{address}</p>}
      </div>
    );
    

    If you need an if-else condition then use ternary operator.

    const MyComponent = ({ name, address }) => (
      <div>
        <h2>{name}</h2>
        {address ? <p>{address}</p> : <p>{"Address is not available"}</p>}
      </div>
    );
    

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47. Why we need to be careful when spreading props on DOM elements?

    When we spread props we run into the risk of adding unknown HTML attributes, which is a bad practice. Instead we can use prop destructuring with ...rest operator, so it will add only required props.

    For example,

    const ComponentA = () => (
      <ComponentB isDisplay={true} className={"componentStyle"} />
    );
    
    const ComponentB = ({ isDisplay, ...domProps }) => (
      <div {...domProps}>{"ComponentB"}</div>
    );
    

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48. How do you memoize a component?

    There are memoize libraries available which can be used on function components.

    For example moize library can memoize the component in another component.

    import moize from "moize";
    import Component from "./components/Component"; // this module exports a non-memoized component
    
    const MemoizedFoo = moize.react(Component);
    
    const Consumer = () => {
      <div>
        {"I will memoize the following entry:"}
        <MemoizedFoo />
      </div>;
    };
    

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

    const MemoComponent = React.memo(function MemoComponent(props) {
      /* render using props */
    });
    OR;
    export default React.memo(MyFunctionComponent);
    

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49. How you implement Server Side Rendering or SSR?

    React is already equipped to handle rendering on Node servers. A special version of the DOM renderer is available, which follows the same pattern as on the client side.

    import ReactDOMServer from "react-dom/server";
    import App from "./App";
    
    ReactDOMServer.renderToString(<App />);
    

    This method will output the regular HTML as a string, which can be then placed inside a page body as part of the server response. On the client side, React detects the pre-rendered content and seamlessly picks up where it left off.

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50. How to enable production mode in React?

    You should use Webpack's DefinePlugin method to set NODE_ENV to production, by which it strip out things like propType validation and extra warnings. Apart from this, if you minify the code, for example, Uglify's dead-code elimination to strip out development only code and comments, it will drastically reduce the size of your bundle.

    ⬆ Back to Top

51. Do Hooks replace render props and higher order components?

    Both render props and higher-order components render only a single child but in most of the cases Hooks are a simpler way to serve this by reducing nesting in your tree.

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52. What is a switching component?

    A switching component is a component that renders one of many components. We need to use object to map prop values to components.

    For example, a switching component to display different pages based on page prop:

    import HomePage from "./HomePage";
    import AboutPage from "./AboutPage";
    import ServicesPage from "./ServicesPage";
    import ContactPage from "./ContactPage";
    
    const PAGES = {
      home: HomePage,
      about: AboutPage,
      services: ServicesPage,
      contact: ContactPage,
    };
    
    const Page = (props) => {
      const Handler = PAGES[props.page] || ContactPage;
    
      return <Handler {...props} />;
    };
    
    // The keys of the PAGES object can be used in the prop types to catch dev-time errors.
    Page.propTypes = {
      page: PropTypes.oneOf(Object.keys(PAGES)).isRequired,
    };
    

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53. What are the Pointer Events supported in React?

    Pointer Events provide a unified way of handling all input events. In the old days we had a mouse and respective event listeners to handle them but nowadays we have many devices which don't correlate to having a mouse, like phones with touch surface or pens. We need to remember that these events will only work in browsers that support the Pointer Events specification.

    The following event types are now available in React DOM:

    1. onPointerDown
    2. onPointerMove
    3. onPointerUp
    4. onPointerCancel
    5. onGotPointerCapture
    6. onLostPointerCapture
    7. onPointerEnter
    8. onPointerLeave
    9. onPointerOver
    10. onPointerOut

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54. Why should component names start with capital letter?

    If you are rendering your component using JSX, the name of that component has to begin with a capital letter otherwise React will throw an error as an unrecognized tag. This convention is because only HTML elements and SVG tags can begin with a lowercase letter.

    function SomeComponent {
      // Code goes here
    }
    

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

    function myComponent {
      render() {
        return <div />;
      }
    }
    
    export default myComponent;
    

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

    import MyComponent from "./myComponent";
    

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55. Are custom DOM attributes supported in React v16?

    Note: This question references React v16, which is outdated. The information below applies to React 16+, including current versions (React 18/19).

    Yes. Starting with React 16, React no longer ignores unknown DOM attributes. If you write JSX with an attribute that React doesn't recognize, React will pass it through to the DOM.

    For example, let's take a look at the below attribute:

    <div mycustomattribute={"something"} />
    

    In React 15 and earlier, this would render an empty div:

    <div />
    

    In React 16 and later (including React 18/19), any unknown attributes will end up in the DOM:

    <div mycustomattribute="something" />
    

    This is useful for supplying browser-specific non-standard attributes, trying new DOM APIs, and integrating with opinionated third-party libraries.

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56. How to loop inside JSX?

    You can simply use Array.prototype.map with ES6 arrow function syntax.

    For example, the items array of objects is mapped into an array of components:

    <tbody>
      {items.map((item) => (
        <SomeComponent key={item.id} name={item.name} />
      ))}
    </tbody>
    

    But you can't iterate using for loop:

    <tbody>
      for (let i = 0; i < items.length; i++) {
        <SomeComponent key={items[i].id} name={items[i].name} />
      }
    </tbody>
    

    This is because JSX tags are transpiled into function calls, and you can't use statements inside expressions. This may change thanks to do expressions which are stage 1 proposal.

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57. How do you access props in attribute quotes?

    React (or JSX) doesn't support variable interpolation inside an attribute value. The below representation won't work:

    <img className="image" src="images/{this.props.image}" />
    

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

    <img className="image" src={"images/" + this.props.image} />
    

    Using template strings will also work:

    <img className="image" src={`images/${this.props.image}`} />
    

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58. What is React proptype array with shape?

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

    ReactComponent.propTypes = {
      arrayWithShape: React.PropTypes.arrayOf(
        React.PropTypes.shape({
          color: React.PropTypes.string.isRequired,
          fontSize: React.PropTypes.number.isRequired,
        })
      ).isRequired,
    };
    

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59. How to conditionally apply class attributes?

    You shouldn't use curly braces inside quotes because it is going to be evaluated as a string.

    <div className="btn-panel {this.props.visible ? 'show' : 'hidden'}">
    

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

    <div className={'btn-panel ' + (this.props.visible ? 'show' : 'hidden')}>
    

    Template strings will also work:

    <div className={`btn-panel ${this.props.visible ? 'show' : 'hidden'}`}>
    

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60. What is the difference between React and ReactDOM?

    The react package contains React.createElement(), React.Component, React.Children, and other helpers related to elements and component classes. You can think of these as the isomorphic or universal helpers that you need to build components. The react-dom package contains ReactDOM.render(), and in react-dom/server we have server-side rendering support with ReactDOMServer.renderToString() and ReactDOMServer.renderToStaticMarkup().

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61. Why ReactDOM is separated from React?

    The React team worked on extracting all DOM-related features into a separate library called ReactDOM. React v0.14 is the first release in which the libraries are split. By looking at some of the packages, react-native, react-art, react-canvas, and react-three, it has become clear that the beauty and essence of React has nothing to do with browsers or the DOM.

    To build more environments that React can render to, React team planned to split the main React package into two: react and react-dom. This paves the way to writing components that can be shared between the web version of React and React Native.

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62. How to use React label element?

    If you try to render a <label> element bound to a text input using the standard for attribute, then it produces HTML missing that attribute and prints a warning to the console.

    <label for={'user'}>{'User'}</label>
    <input type={'text'} id={'user'} />
    

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

    <label htmlFor={'user'}>{'User'}</label>
    <input type={'text'} id={'user'} />
    

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63. How to combine multiple inline style objects?

    You can use spread operator in regular React:

    <button style={{ ...styles.panel.button, ...styles.panel.submitButton }}>
      {"Submit"}
    </button>
    

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

    <button style={[styles.panel.button, styles.panel.submitButton]}>
      {"Submit"}
    </button>
    

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64. How to re-render the view when the browser is resized?

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

    import React, { useState, useEffect } from "react";
    function WindowDimensions() {
      const [dimensions, setDimensions] = useState({
        width: window.innerWidth,
        height: window.innerHeight,
      });
    
      useEffect(() => {
        function handleResize() {
          setDimensions({
            width: window.innerWidth,
            height: window.innerHeight,
          });
        }
        window.addEventListener("resize", handleResize);
        return () => window.removeEventListener("resize", handleResize);
      }, []);
    
      return (
        <span>
          {dimensions.width} x {dimensions.height}
        </span>
      );
    }
    

    Using Class Component

    You can listen to the resize event in componentDidMount() and then update the dimensions (width and height). You should remove the listener in componentWillUnmount() method.

    class WindowDimensions extends React.Component {
      constructor(props) {
        super(props);
        this.updateDimensions = this.updateDimensions.bind(this);
      }
    
      componentWillMount() {
        this.updateDimensions();
      }
    
      componentDidMount() {
        window.addEventListener("resize", this.updateDimensions);
      }
    
      componentWillUnmount() {
        window.removeEventListener("resize", this.updateDimensions);
      }
    
      updateDimensions() {
        this.setState({
          width: window.innerWidth,
          height: window.innerHeight,
        });
      }
    
      render() {
        return (
          <span>
            {this.state.width} x {this.state.height}
          </span>
        );
      }
    }
    

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66. How to pretty print JSON with React?

    We can use <pre> tag so that the formatting of the JSON.stringify() is retained:

    const data = { name: "John", age: 42 };
    
    function User {
        return <pre>{JSON.stringify(data, null, 2)}</pre>;
    }
    
    const container = createRoot(document.getElementById("container"));
    
    container.render(<User />);
    

    See Class

    const data = { name: "John", age: 42 };
    
    class User extends React.Component {
      render() {
        return <pre>{JSON.stringify(data, null, 2)}</pre>;
      }
    }
    
    React.render(<User />, document.getElementById("container"));
    

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67. Why can't you update props in React?

    The React philosophy is that props should be immutable(read only) and top-down. This means that a parent can send any prop values to a child, but the child can't modify received props.

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68. How to focus an input element on page load?

    You need to use useEffect hook to set focus on input field during page load time for functional component.

    import React, { useEffect, useRef } from "react";
    
    const App = () => {
      const inputElRef = useRef(null);
    
      useEffect(() => {
        inputElRef.current.focus();
      }, []);
    
      return (
        <div>
          <input defaultValue={"Won't focus"} />
          <input ref={inputElRef} defaultValue={"Will focus"} />
        </div>
      );
    };
    
    ReactDOM.render(<App />, document.getElementById("app"));
    

    See Class

    You can do it by creating _ref_ for `input` element and using it in `componentDidMount()`:

    class App extends React.Component {
      componentDidMount() {
        this.nameInput.focus();
      }
    
      render() {
        return (
          <div>
            <input defaultValue={"Won't focus"} />
            <input
              ref={(input) => (this.nameInput = input)}
              defaultValue={"Will focus"}
            />
          </div>
        );
      }
    }
    
    ReactDOM.render(<App />, document.getElementById("app"));
    

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69. How can we find the version of React at runtime in the browser?

    You can use React.version to get the version.

    const REACT_VERSION = React.version;
    
    ReactDOM.render(
      <div>{`React version: ${REACT_VERSION}`}</div>,
      document.getElementById("app")
    );
    

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70. How to add Google Analytics for React Router?

    Add a listener on the history object to record each page view:

    history.listen(function (location) {
      window.ga("set", "page", location.pathname + location.search);
      window.ga("send", "pageview", location.pathname + location.search);
    });
    

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71. How do you apply vendor prefixes to inline styles in React?

    React does not apply vendor prefixes automatically. You need to add vendor prefixes manually.

    <div
      style={{
        transform: "rotate(90deg)",
        WebkitTransform: "rotate(90deg)", // note the capital 'W' here
        msTransform: "rotate(90deg)", // 'ms' is the only lowercase vendor prefix
      }}
    />
    

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72. How to import and export components using React and ES6?

    You should use default for exporting the components

    import User from "user";
    
    export default function MyProfile {
        return <User type="customer">//...</User>;
    }
    

    See Class

    ```jsx harmony import React from "react"; import User from "user";

    export default class MyProfile extends React.Component { render() { return //...; } }

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

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73. What are the exceptions on React component naming?

    The component names should start with an uppercase letter but there are few exceptions to this convention. The lowercase tag names with a dot (property accessors) are still considered as valid component names. For example, the below tag can be compiled to a valid component,

         render() {
              return (
                <obj.component/> // `React.createElement(obj.component)`
              )
        }
    

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74. Is it possible to use async/await in plain React?

    Yes, you can use async/await in plain React, as long as your JavaScript environment supports ES2017+. Nowadays most modern browsers and build tools support ES2017+ version. If you're using Create React App, Next.js, Remix, or any modern React setup, async/await is supported out of the box through Babel.

    Example Usage

    import { useEffect, useState } from 'react';
    
    function UserProfile() {
      const [user, setUser] = useState(null);
    
      useEffect(() => {
        const fetchUser = async () => {
          const response = await fetch('/api/user');
          const data = await response.json();
          setUser(data);
        };
    
        fetchUser();
      }, []);
    
      return user ? <div>Hello, {user.name}</div> : <div>Loading...</div>;
    }
    

    But If you're not using a bundler like Webpack or Babel, you will need Babel and transform-async-to-generator plugin. However, React Native ships with Babel and a set of transforms.

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75. What are the common folder structures for React?

    There are two common practices for React project file structure.

    1. Grouping by features or routes:

    One common way to structure projects is locate CSS, JS, and tests together, grouped by feature or route.

    common/
    ├─ Avatar.js
    ├─ Avatar.css
    ├─ APIUtils.js
    └─ APIUtils.test.js
    feed/
    ├─ index.js
    ├─ Feed.js
    ├─ Feed.css
    ├─ FeedStory.js
    ├─ FeedStory.test.js
    └─ FeedAPI.js
    profile/
    ├─ index.js
    ├─ Profile.js
    ├─ ProfileHeader.js
    ├─ ProfileHeader.css
    └─ ProfileAPI.js
    

    2. Grouping by file type:

    Another popular way to structure projects is to group similar files together.

    api/
    ├─ APIUtils.js
    ├─ APIUtils.test.js
    ├─ ProfileAPI.js
    └─ UserAPI.js
    components/
    ├─ Avatar.js
    ├─ Avatar.css
    ├─ Feed.js
    ├─ Feed.css
    ├─ FeedStory.js
    ├─ FeedStory.test.js
    ├─ Profile.js
    ├─ ProfileHeader.js
    └─ ProfileHeader.css
    

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76. What are the popular packages for animation?

    React Transition Group and React Motion are popular animation packages in React ecosystem.

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77. What is the benefit of styles modules?

    It is recommended to avoid hard coding style values in components. Any values that are likely to be used across different UI components should be extracted into their own modules.

    For example, these styles could be extracted into a separate component:

    export const colors = {
      white,
      black,
      blue,
    };
    
    export const space = [0, 8, 16, 32, 64];
    

    And then imported individually in other components:

    import { space, colors } from "./styles";
    

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78. What are the popular React-specific linters?

    ESLint is a popular JavaScript linter. There are plugins available that analyse specific code styles. One of the most common for React is an npm package called eslint-plugin-react. By default, it will check a number of best practices, with rules checking things from keys in iterators to a complete set of prop types.

    Another popular plugin is eslint-plugin-jsx-a11y, which will help fix common issues with accessibility. As JSX offers slightly different syntax to regular HTML, issues with alt text and tabindex, for example, will not be picked up by regular plugins.

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React Router

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79. What is React Router?

    React Router is a powerful routing library built on top of React that helps you add new screens and flows to your application incredibly quickly, all while keeping the URL in sync with what's being displayed on the page.

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80. How React Router is different from history library?

    React Router is a wrapper around the history library which handles interaction with the browser's window.history with its browser and hash histories. It also provides memory history which is useful for environments that don't have global history, such as mobile app development (React Native) and unit testing with Node.

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81. What are the <Router> components of React Router v6?

    React Router v6 provides below 4 <Router> components:

    1. <BrowserRouter>:Uses the HTML5 history API for standard web apps.
    2. <HashRouter>:Uses hash-based routing for static servers.
    3. <MemoryRouter>:Uses in-memory routing for testing and non-browser environments.
    4. <StaticRouter>:Provides static routing for server-side rendering (SSR).

    The above components will create browser, hash, memory and static history instances. React Router v6 makes the properties and methods of the history instance associated with your router available through the context in the router object.

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82. What is the purpose of push() and replace() methods of history?

    A history instance has two methods for navigation purpose.

    1. push()
    2. replace()

    If you think of the history as an array of visited locations, push() will add a new location to the array and replace() will replace the current location in the array with the new one.

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83. How do you programmatically navigate using React Router v4?

    There are three different ways to achieve programmatic routing/navigation within components.

    1. Using the withRouter() higher-order function:

       The withRouter() higher-order function will inject the history object as a prop of the component. This object provides push() and replace() methods to avoid the usage of context.

       import { withRouter } from "react-router-dom"; // this also works with 'react-router-native'
       
       const Button = withRouter(({ history }) => (
         <button
           type="button"
           onClick={() => {
             history.push("/new-location");
           }}
         >
           {"Click Me!"}
         </button>
       ));
       

    2. Using <Route> component and render props pattern:

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

       import { Route } from "react-router-dom";
       
       const Button = () => (
         <Route
           render={({ history }) => (
             <button
               type="button"
               onClick={() => {
                 history.push("/new-location");
               }}
             >
               {"Click Me!"}
             </button>
           )}
         />
       );
       

    3. Using context:

       This option is not recommended and treated as unstable API.

       const Button = (props, context) => (
         <button
           type="button"
           onClick={() => {
             context.history.push("/new-location");
           }}
         >
           {"Click Me!"}
         </button>
       );
       
       Button.contextTypes = {
         history: React.PropTypes.shape({
           push: React.PropTypes.func.isRequired,
         }),
       };
       

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84. How to get query parameters in React Router v4?

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

    const queryString = require("query-string");
    const parsed = queryString.parse(props.location.search);
    

    You can also use URLSearchParams if you want something native:

    const params = new URLSearchParams(props.location.search);
    const foo = params.get("name");
    

    You should use a polyfill for IE11.

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85. Why you get "Router may have only one child element" warning?

    You have to wrap your Route's in a <Switch> block because <Switch> is unique in that it renders a route exclusively.

    At first you need to add Switch to your imports:

    import { Switch, Router, Route } from "react-router";
    

    Then define the routes within <Switch> block:

    <Router>
      <Switch>
        <Route {/* ... */} />
        <Route {/* ... */} />
      </Switch>
    </Router>
    

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86. How to pass params to history.push method in React Router v4?

    While navigating you can pass props to the history object:

    this.props.history.push({
      pathname: "/template",
      search: "?name=sudheer",
      state: { detail: response.data },
    });
    

    The search property is used to pass query params in push() method.

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87. How to implement default or NotFound page?

    A <Switch> renders the first child <Route> that matches. A <Route> with no path always matches. So you just need to simply drop path attribute as below

    <Switch>
      <Route exact path="/" component={Home} />
      <Route path="/user" component={User} />
      <Route component={NotFound} />
    </Switch>
    

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88. How to get history on React Router v4?

    Below are the list of steps to get history object on React Router v4,

    1. Create a module that exports a history object and import this module across the project.

       For example, create history.js file:

       import { createBrowserHistory } from "history";
       
       export default createBrowserHistory({
         /* pass a configuration object here if needed */
       });
       

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

       import { Router } from "react-router-dom";
       import history from "./history";
       import App from "./App";
       
       ReactDOM.render(
         <Router history={history}>
           <App />
         </Router>,
         holder
       );
       

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

       // some-other-file.js
       import history from "./history";
       
       history.push("/go-here");
       

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89. How to perform automatic redirect after login?

    The react-router package provides <Redirect> component in React Router. Rendering a <Redirect> will navigate to a new location. Like server-side redirects, the new location will override the current location in the history stack.

    import { Redirect } from "react-router";
    
    export default function Login {
        if (this.state.isLoggedIn === true) {
          return <Redirect to="/your/redirect/page" />;
        } else {
          return <div>{"Login Please"}</div>;
        }
    }
    

    See Class

    import React, { Component } from "react";
    import { Redirect } from "react-router";
    
    export default class LoginComponent extends Component {
      render() {
        if (this.state.isLoggedIn === true) {
          return <Redirect to="/your/redirect/page" />;
        } else {
          return <div>{"Login Please"}</div>;
        }
      }
    }
    

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React Internationalization

90. What is React Intl?

    The React Intl library makes internationalization in React straightforward, with off-the-shelf components and an API that can handle everything from formatting strings, dates, and numbers, to pluralization. React Intl is part of FormatJS which provides bindings to React via its components and API.

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91. What are the main features of React Intl?

    Below are the main features of React Intl,

    1. Display numbers with separators.
    2. Display dates and times correctly.
    3. Display dates relative to "now".
    4. Pluralize labels in strings.
    5. Support for 150+ languages.
    6. Runs in the browser and Node.
    7. Built on standards.

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92. What are the two ways of formatting in React Intl?

    The library provides two ways to format strings, numbers, and dates:

    1. Using react components:

       <FormattedMessage
         id={"account"}
         defaultMessage={"The amount is less than minimum b