React JS 100 MCQ

Correct Answer: B. React is primarily used to create dynamic and interactive user interfaces.

Correct Answer: B. React uses a virtual DOM to enhance performance compared to the real DOM.

Correct Answer: B. The virtual DOM is a lightweight version of the real DOM that updates asynchronously.

Correct Answer: B. JSX is a syntax extension that allows developers to write HTML within JavaScript.

Correct Answer: B. JSX stands for JavaScript XML and is a syntax extension for writing HTML-like code inside JavaScript.

Correct Answer: D. JavaScript expressions in JSX are embedded using single curly braces { }.

Correct Answer: C. JSX allows developers to write HTML elements directly inside JavaScript.

Correct Answer: B. A functional component is a JavaScript function that returns HTML elements using JSX.

Correct Answer: B. Class components can use lifecycle methods like componentDidMount, while functional components cannot unless using Hooks.

Correct Answer: C. Props are used to pass data from one component to another in React.

Correct Answer: A. The render method is required in a React class component as it returns the HTML for the component.

Correct Answer: B. State in React is used to manage dynamic data that affects how a component renders.

Correct Answer: B. React uses a synthetic event system that wraps around the browser’s native event system, providing cross-browser compatibility.

Correct Answer: A. Event binding in React class components can be done by using onClick={this.handleClick.bind(this)} to bind the method’s ‘this’ context.

Correct Answer: B. You can prevent the default behavior of an event in React by calling event.preventDefault() inside the event handler.

Correct Answer: B. Event delegation in React refers to handling events on the parent component rather than individual child elements.

Correct Answer: D. The correct syntax for handling events in React is onClick={this.handleClick}.

Correct Answer: B. React handles form inputs using its synthetic event system.

Correct Answer: C. The correct syntax for using the onChange event in React is onChange={this.handleChange}.

Correct Answer: A. You can bind an event handler to the correct ‘this’ context in React class components using this.methodName.bind(this) in the constructor.

Correct Answer: A. State in React is used to store dynamic data that can change over time and affect the rendering of the component.

Correct Answer: A. The componentDidMount lifecycle method is called once after the component is rendered to the DOM.

Correct Answer: A. Functional components use the useState hook to manage state in React.

Correct Answer: B. componentWillUnmount is used to clean up resources like subscriptions or event listeners before the component is removed from the DOM.

Correct Answer: B. In a class component, you update state using this.setState({ value: newValue }).

Correct Answer: B. The useState hook is used to initialize and update state in functional components.

Correct Answer: A. The useEffect hook allows you to handle side effects like data fetching, subscriptions, and timers.

Correct Answer: B. The correct syntax for using the useState hook is const {value, setValue} = useState(0);

Correct Answer: B. The useContext hook allows you to consume context values without passing them through every level of a component tree (prop drilling).

Correct Answer: A. The useReducer hook is used for managing more complex state logic than the useState hook.

Correct Answer: B. The Context API allows you to share global data across multiple components without needing to pass props down through every level.

Correct Answer: B. You create a context in React using React.createContext().

Correct Answer: B. Context API is built-in to React, while Redux is an external library used for complex state management.

Correct Answer: B. You consume values from a Context in functional components using the useContext hook.

Correct Answer: C. A controlled component is one where form data is managed by React using state, rather than the DOM.

Correct Answer: C. You handle form submission in React by using event.preventDefault() to prevent the default browser form submission behavior.

Correct Answer: B. Uncontrolled components manage form data through the DOM, while controlled components manage it through React state.

Correct Answer: B. A controlled component is one where the input’s value is managed by React state, using value and onChange attributes.

Correct Answer: A. Validation in forms is typically handled by writing logic inside the onSubmit handler to validate the form inputs before submission.

Correct Answer: B. Controlled components make it easier to handle form validation and control user input, as the form data is managed by React state.

Correct Answer: B. React Router is used to handle navigation and routing between different components in a React application.

Correct Answer: A. You set up routing in React by using the BrowserRouter, Route, and Switch components from React Router.

Correct Answer: B. The Switch component renders the first child Route that matches the current location, ensuring that only one route is rendered at a time.

Correct Answer: A. Route parameters allow you to pass dynamic values through the URL, which can be used to render specific data for a particular route.

Correct Answer: A. A route with a parameter in React Router is defined using a colon before the parameter name, such as path=”/user/”.

Correct Answer: A. Route parameters in React Router can be accessed through the props object using props.match.params.

Correct Answer: B. The Link component in React Router is used to create navigation links that allow users to navigate between routes without reloading the page.

Correct Answer: B. You can redirect users programmatically in React Router by using the history object and calling history.push().

Correct Answer: B. The useHistory hook gives access to the history object, which can be used for programmatic navigation within a React component.

Correct Answer: B. Nested routes are handled by placing child Route components inside a parent Route component, allowing for more complex navigation structures.

Correct Answer: B. Redux is used to manage global state across the entire React application, making state accessible from any component.

Correct Answer: C. The core concepts of Redux are Actions, Reducers, and Store.

Correct Answer: B. An action in Redux is a plain JavaScript object that describes a change or event in the application state.

Correct Answer: C. A reducer specifies how the application’s state changes in response to an action in Redux.

Correct Answer: A. The Redux store is created by calling createStore and passing in the root reducer.

Correct Answer: B. React components are connected to the Redux store by using the connect function from react-redux.

Correct Answer: B. The mapStateToProps function maps the state from the Redux store to the props of the connected component.

Correct Answer: B. The mapDispatchToProps function maps dispatchable actions to the props of the component.

Correct Answer: B. Redux middleware is commonly used to handle asynchronous actions such as API calls in the application.

Correct Answer: A. Redux Thunk allows action creators to return functions instead of plain action objects, enabling asynchronous actions like API calls.

Correct Answer: B. Error boundaries are React components that catch JavaScript errors in their child component tree during rendering, preventing the application from crashing.

Correct Answer: A. The componentDidCatch lifecycle method is used in error boundaries to catch errors that occur in the component tree.

Correct Answer: B. Error boundaries do not catch errors in event handlers, which need to be handled with try-catch blocks inside the event handler.

Correct Answer: B. When an error boundary catches an error, it displays a fallback UI, preventing the entire application from crashing.

Correct Answer: B. Async code errors in React should be handled by wrapping the code in a try-catch block.

Correct Answer: B. Errors in Promises can be handled by adding a .catch() block at the end of the Promise chain.

Correct Answer: C. React Developer Tools is a browser extension used specifically for inspecting and debugging React applications.

Correct Answer: B. You can inspect the state and props of a React component using React Developer Tools, which shows the component tree and the current values of props and state.

Correct Answer: B. The profiler feature in React Developer Tools helps identify performance bottlenecks by recording renders and updates of components.

Correct Answer: B. You can view performance optimizations or re-renders in React using the performance tab in React Developer Tools, which shows how components are being rendered.

Correct Answer: B. Memoization in React is used to prevent unnecessary re-renders by caching the results of function calls and reusing them when the inputs haven’t changed.

Correct Answer: C. The useMemo hook is used to memoize values, ensuring that expensive calculations are only performed when necessary.

Correct Answer: A. The useCallback hook memoizes function definitions so that they are not recreated on every render, which can improve performance.

Correct Answer: A. React.memo is a higher-order component that prevents a component from re-rendering unless its props have changed, optimizing performance.

Correct Answer: B. Code splitting is a performance optimization technique that allows React to load only the necessary JavaScript files when needed, improving load times.

Correct Answer: C. React.lazy is used to lazy load components in React, meaning they are only loaded when needed.

Correct Answer: A. The useMemo hook memoizes values, ensuring that expensive recalculations are only performed when necessary, improving performance.

Correct Answer: B. shouldComponentUpdate allows React to decide whether to re-render a component based on changes to its props or state, which can improve performance by avoiding unnecessary renders.

Correct Answer: A. The React Profiler tracks and visualizes the performance of components, helping developers understand rendering times and identify performance bottlenecks.

Correct Answer: A. Memory leaks can be fixed by clearing intervals and timeouts in the componentWillUnmount lifecycle method to ensure resources are properly cleaned up when a component is unmounted.

Correct Answer: B. The fetch method and Axios are commonly used in React to fetch data from APIs.

Correct Answer: B. Data fetching is typically done inside a useEffect hook in functional components to ensure it happens after the component mounts.

Correct Answer: B. Asynchronous data fetching in React is commonly handled using Promises or async/await inside the useEffect hook.

Correct Answer: B. Handling loading states allows you to display a loading indicator while data is being fetched, improving the user experience.

Correct Answer: A. Errors during data fetching are handled using try-catch blocks in async/await functions or .catch() in Promises.

Correct Answer: A. The common HTTP methods for CRUD operations in RESTful APIs are GET, POST, PUT, and DELETE.

Correct Answer: A. A GET request can be performed using Axios with the axios.get(‘/url’) method.

Correct Answer: B. Async/await makes asynchronous code easier to write and read by allowing the use of synchronous-looking syntax for asynchronous operations.

Correct Answer: B. Axios provides automatic JSON handling, simplified syntax, and additional features like request/response interceptors, making it more user-friendly than fetch.

Correct Answer: A. A POST request using the fetch API is done with fetch(‘/url’, { method: ‘POST’, body: JSON.stringify(data) }).

Correct Answer: B. Unit testing in React involves testing individual components in isolation to ensure they work as expected.

Correct Answer: C. Jest and React Testing Library are commonly used for testing React components.

Correct Answer: A. Snapshot testing compares the rendered output of a component to a previously stored snapshot to detect unexpected changes.

Correct Answer: B. Mocking API calls in unit tests is typically done using Jest or other testing libraries to simulate API responses.

Correct Answer: B. Jest provides built-in support for mocking, snapshot testing, and easy configuration for React applications, making it a popular choice for testing React components.

Correct Answer: B. CSS-in-JS refers to writing styles directly in JavaScript files using libraries like styled-components or Emotion.

Correct Answer: B. CSS-in-JS allows for scoped styles within components, which helps avoid class name collisions.

Correct Answer: A. Inline styles are written directly within the component’s JSX, while external stylesheets are stored in separate files and linked to the component.

Correct Answer: B. CSS modules allow developers to write locally scoped CSS classes that only apply to the component where they are used.

Correct Answer: C. It is a best practice to use scoped or modular styles to prevent style conflicts across components and ensure maintainability.