React.js interview questions and answers for 2023

1. What is React?

React is a popular JavaScript library for building user interfaces. It is helpful in building complex and reusable user interface(UI) components of mobile and web applications as it follows the component-based approach.

1. List some of React.js’ features.

Here are some key features of React.js:

• Virtual DOM: React uses a virtual DOM, which is a lightweight representation of the actual DOM. It allows React to efficiently update and render only the necessary parts of the UI, resulting in improved performance.

• Component-based architecture: React promotes a modular approach to building UIs by breaking them down into reusable components. Each component manages its own state, making it easier to develop and maintain complex UIs.

• Declarative syntax: React uses a declarative syntax, where developers describe how the UI should look based on the current state, rather than manually manipulating the DOM. This simplifies UI development and makes it easier to understand and reason about the code.

• Props: React components can receive data and behavior through props. Props are a way to pass data from parent components to child components, enabling a hierarchical data flow and promoting reusability.

• JSX: React uses JSX (JavaScript XML), which is a syntax extension that allows developers to write HTML-like code directly in JavaScript. JSX makes it easier to define the structure and appearance of UI components.

• Unidirectional data flow: React follows a unidirectional data flow, where data flows in a single direction from parent components to child components. This helps maintain a predictable state and makes it easier to debug and understand how data changes propagate through the UI.

• React Native: React can be used to build mobile applications using React Native. React Native allows developers to write JavaScript code that compiles to native mobile components, offering a way to build cross-platform mobile apps with a shared codebase.

• Lifecycle methods: React provides lifecycle methods that allow developers to hook into different stages of a component's lifecycle, such as component initialization, rendering, updating, and unmounting. These methods enable developers to perform actions at specific points in the component's life.

• Rich ecosystem: React has a large and active community, which has resulted in a rich ecosystem of libraries, tools, and extensions. These resources provide additional functionalities, such as state management (e.g., Redux), routing (e.g., React Router), and UI component libraries (e.g., Material-UI).

1. What is JSX?

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1. How do Lists work in React.js?

In React.js, lists are created by rendering an array of data as a collection of components. This allows you to dynamically generate multiple elements based on the data in the array. React provides a convenient way to iterate over an array and generate a list of components using the map() method.

Here's an example that demonstrates how to render a list of items using React.js:

import React from 'react';

function MyList() {
  const data = ['Item 1', 'Item 2', 'Item 3'];

  return (
    <ul>
      {data.map((item, index) => (
        <li key={index}>{item}</li>
      ))}
    </ul>
  );
}

export default MyList;

In this example, we have an array data containing three items. The map() method iterates over each item in the array and generates an <li> (list item) component for each item. The key prop is set to the index value, which should be unique for each item. The resulting list is rendered inside a <ul> (unordered list) component.

By using the map() method and providing a unique key for each list item, React can efficiently update and re-render only the necessary components when the underlying data changes. This ensures optimal performance when working with dynamic lists in React.js.

1. Why are keys used in React.js Lists?

Keys are used in React.js Lists because:

1. Optimizing rendering: When rendering a list of components in React, each component needs a way to uniquely identify itself. React uses the key prop to keep track of the identity of each component in the list. By assigning a unique key to each component, React can efficiently update and re-render only the components that have changed, instead of re-rendering the entire list. This optimization improves the performance of React applications, especially when dealing with large lists.

2. Tracking identity: The key prop helps React identify which items have been added, removed, or re-ordered in a list. When the underlying data of a list changes, React uses the keys to determine the identity of each item and apply the necessary updates to the UI. Without keys, React may have difficulty distinguishing between different list items and may re-render unnecessary components or fail to update the correct components efficiently.

3. Describe an event in Reactjs?

In React.js, an event is a user action or interaction that triggers a specific function or behavior in the application. Events can be things like clicking a button, submitting a form, hovering over an element, or typing in an input field.

Here's an example of how an event can be handled in React.js:

import React from 'react';

class MyComponent extends React.Component {
  handleClick() {
    console.log('Button clicked!');
  }
  render() {
    return (
      <button onClick={this.handleClick}>Click me!</button>
    );
  }
}

export default MyComponent;

In this example, we have a MyComponent class component that renders a <button> element. The onClick attribute is used to assign an event handler function, handleClick, to the button's click event. The event handler function, handleClick, is defined within the component and logs a message to the console when the button is clicked.

When the button is clicked, React triggers the handleClick function, and the message "Button clicked!" is logged to the console.

It's important to note that in React, event names are written in camelCase, rather than lowercase, to match the naming conventions of JavaScript. For example, onClick instead of onclick, onChange instead of onchange, and so on.

React provides event handling for various types of events, such as onClick, onChange, onSubmit, onMouseOver, onKeyPress, and many more.

1. What is a higher-order component in React?

In React, a Higher-Order Component (HOC) is a design pattern that allows the reusability and composition of component logic. A HOC is a function that takes a component as input and returns a new component with additional functionality or props.

Here's an example of a Higher-Order Component in React functional components:

import React, { useEffect } from 'react';

const withLogger = (WrappedComponent) => {
  return (props) => {
    useEffect(() => {
      console.log('Component has mounted');
    }, []);

    return <WrappedComponent {...props} />;
  };
};

const MyComponent = () => {
  return <div>My Component</div>;
};

const EnhancedComponent = withLogger(MyComponent);

export default EnhancedComponent;

In this example, withLogger is a Higher-Order Component that takes a WrappedComponent as an argument and returns a new functional component. The returned component uses the useEffect hook to execute the logging logic when the component is mounted. It then renders the WrappedComponent and passes along any props it receives.

The MyComponent is a regular functional component. By wrapping MyComponent with withLogger using const EnhancedComponent = withLogger(MyComponent), we create an enhanced version of MyComponent with the additional logging functionality.

The resulting EnhancedComponent can be used just like any other functional component. When it is mounted, the useEffect hook of the HOC is executed, logging the message to the console. The original functionality of MyComponent is preserved, and any props passed to EnhancedComponent are forwarded to MyComponent.

1. How do browsers read JSX files?

Browsers cannot directly read JSX files because JSX is not valid JavaScript. JSX is a syntax extension used in React to write HTML-like code within JavaScript. To make JSX files readable by browsers, they need to be transformed into plain JavaScript code that browsers can understand.

The process of transforming JSX into JavaScript is typically done through a build step called "transpilation" or "compilation." During this step, a tool like Babel is commonly used to convert JSX syntax into regular JavaScript code that browsers can interpret.

1. What are stateless components?

Stateless components, also known as functional components, are a type of component in React that doesn't have internal state management. They are primarily responsible for rendering UI based on the props they receive. Stateless components are defined as plain JavaScript functions rather than classes and are simpler in structure compared to stateful class components.

Here's an example of a stateless component in React:

import React from 'react';

const Greeting = (props) => {
  return <h1>Hello, {props.name}!</h1>;
};

export default Greeting;

In this example, Greeting is a stateless functional component. It takes props as its input and returns a JSX element representing a greeting. The component doesn't have any internal state or lifecycle methods. It simply receives props as arguments and renders the UI accordingly.

1. Mention one difference between Props and State.

Props (short for properties) are used to pass data from a parent component to its child components. They are immutable and are passed down the component tree. Props are read-only and cannot be modified directly by the component that receives them. Instead, they are used to provide data or behavior to child components, enabling component composition and reusability.

On the other hand, State refers to the internal data that a component can hold and manage. State allows a component to store and update values that can change over time. State allows a component to keep track of and manage its own data. Unlike props, which are passed down from a parent component, state is local to a component and can be modified using the setState method provided by React. When the state is updated, React re-renders the component and its children to reflect the new state.

1. What are pure components in ReactJS?

In React, pure components are a specific type of component that optimize rendering performance by reducing unnecessary re-renders. Pure components are designed to be more efficient compared to regular components, especially when dealing with large or complex component trees.

you can achieve the same optimization using the useMemo higher-order component (HOC) in functional components:

import { useState, useMemo } from "react";
import ReactDOM from "react-dom/client";

const App = () => {
  const [count, setCount] = useState(0);
  const [todos, setTodos] = useState([]);
  const calculation = useMemo(() => expensiveCalculation(count), [count]);

  const increment = () => {
    setCount((c) => c + 1);
  };
  const addTodo = () => {
    setTodos((t) => [...t, "New Todo"]);
  };

  return (
    <div>
      <div>
        <h2>My Todos</h2>
        {todos.map((todo, index) => {
          return <p key={index}>{todo}</p>;
        })}
        <button onClick={addTodo}>Add Todo</button>
      </div>
      <hr />
      <div>
        Count: {count}
        <button onClick={increment}>+</button>
        <h2>Expensive Calculation</h2>
        {calculation}
      </div>
    </div>
  );
};

In this example, the useMemo hook is used to memoize the result of the expensiveCalculation function based on the count state. Whenever the count state changes, the expensiveCalculation function is re-evaluated, and the memoized result is stored in the calculation variable.

The component has two state variables: count and todos. The count state is incremented when the "+" button is clicked, and the todos state is updated when the "Add Todo" button is clicked.

The expensiveCalculation function represents a computationally expensive operation that takes the count as input. It performs a loop for a large number of iterations, simulating a time-consuming calculation. By using useMemo, the result of this calculation is memoized and only re-computed when the count state changes.

The rendered JSX displays a list of todos, an "Add Todo" button, the current count, an increment button, and the result of the calculation. Whenever the count state changes, the calculation is re-evaluated and displayed.

Overall, useMemo optimizes the performance of the component by avoiding unnecessary re-evaluations of the expensive calculation when the dependencies don't change, resulting in a smoother user experience.

1. Why will you use empty tags <> </>?

Using empty tags <></> or the shorthand <Fragment></Fragment>, you can group multiple elements together without adding an extra wrapping element. Fragments act as invisible wrapper components and don't render any actual HTML in the DOM.

1. Explain stateful components ?

Stateful components are responsible for maintaining and updating data that can change over time. They have access to the component's state, which is an object that holds various values that can be modified. When the state is updated, React re-renders the component and its children, reflecting the changes in the user interface.

1. What is virtual DOM?

The Virtual DOM (VDOM) is a concept in React that refers to a lightweight, in-memory representation of the actual Document Object Model (DOM) of a web page. It is a technique used by React to efficiently update and render the user interface.

In a traditional web application, when changes are made to the state of the application or user input occurs, the entire DOM tree is recalculated and updated, which can be computationally expensive and slow. The Virtual DOM provides an abstraction layer between the application and the actual browser DOM, allowing React to perform updates more efficiently.

Here's how the Virtual DOM works in React:

1. When a React component's state or props change, React creates a new Virtual DOM tree that represents the updated UI structure.

2. React then compares this new Virtual DOM tree with the previous Virtual DOM tree (generated from the previous state or props) to identify the differences or changes.

3. After identifying the differences, React determines the minimum number of updates needed to synchronize the Virtual DOM with the actual browser DOM.

4. Finally, React applies these optimized updates to the real DOM, updating only the necessary elements, rather than re-rendering the entire UI.

By using the Virtual DOM, React minimizes direct manipulation of the real DOM, which is typically slower and more resource-intensive. Instead of updating the DOM directly for every change, React performs batch updates by applying changes to the Virtual DOM and then updating the real DOM in an efficient manner.

The Virtual DOM allows React to provide a declarative programming model, where you define how the UI should look based on the component's state or props, and React takes care of updating the DOM to reflect the desired UI state. This approach simplifies the development process and helps to optimize performance.

Overall, the Virtual DOM is a key feature of React that helps to make UI updates more efficient, resulting in faster rendering and improved performance compared to directly manipulating the real DOM.

1. Can you outline the differences between Real DOM and Virtual DOM?

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