React Interview Questions

Welcome to our in-depth exploration of key React concepts designed to prepare you for front end interviews. React, developed and maintained by Facebook, has become a cornerstone technology in modern web development.

In this blog post, we will delve into crucial topics you're likely to encounter during your React interviews.

From JSX and the Virtual DOM to React's component lifecycle, data flow, and common interview questions, we'll cover it all.

Understanding JSX:

JSX (JavaScript XML) is a syntax extension for JavaScript, resembling XML/HTML. It empowers developers to write HTML-like code directly within JavaScript, enhancing the readability and intuitiveness of React components.

1const element = <h1>Hello, World!</h1>;

In this example, <h1>Hello, World!</h1> is JSX code, which will be transformed into JavaScript code behind the scenes.

DOM and VDOM:

DOM (Document Object Model) represents the document as a tree-like structure of elements.

VDOM (Virtual DOM) is a lightweight copy of the actual DOM stored in memory. React utilizes VDOM to minimize direct DOM manipulations, ensuring a more efficient rendering process compared to direct DOM updates.

React Phases and Reconciliation

React Component Lifecycle Phases:

Initialization: During this phase, React component will prepare by setting up the default props and initial state for the upcoming tough journey.

Mounting: Mounting refers to putting the elements into the browser DOM. Since React uses VirtualDOM, the entire browser DOM which has been currently rendered would not be refreshed.

Updating: In this phase, a component will be updated when there is a change in the state or props of a component.

Unmounting: In this last phase of the component lifecycle, the component will be removed from the DOM or will be unmounted from the browser DOM.

Render and Commit Phases:

Render Phase: Components are re-evaluated and potentially re-rendered based on state or prop changes. Importantly, no changes are made directly to the actual DOM during this phase.

Commit Phase: Changes identified in the VDOM during the Render phase are efficiently applied to the actual DOM, minimizing unnecessary reflows and repaints.

Reconciliation Process:

Reconciliation is React's efficient updating mechanism, comparing current and previous elements. Only necessary changes are applied, ensuring optimal performance.

Data Flow and Prop Management

Flux Architecture: Facebook's method for handling complex data in web applications, managing data flow in React applications.

Data Flow in React: Follows a unidirectional pattern, ensuring predictability and maintainability. Data flows from parent components to child components through props, enabling seamless communication and consistent updates.

Prop Drilling: While natural, it can lead to complexity in large applications. Mitigate by using Context API, Redux, or passing only necessary props instead of entire objects. Context API enables clean, maintainable code by allowing components to share data without manual prop passing.

Crafting React Applications

Understanding Synthetic Events:

Synthetic events unify different browsers' native events, ensuring consistency across platforms and browsers.

1import React from 'react';
2
3function ClickComponent() {
4    const handleClick = (event: React.MouseEvent<HTMLButtonElement>) => {
5        // Accessing properties of the synthetic event
6        console.log('Button clicked!');
7        console.log('Event type:', event.type);
8        console.log('Mouse X Coordinate:', event.clientX);
9        console.log('Mouse Y Coordinate:', event.clientY);
10    };
11
12    return <button onClick={handleClick}>Click Me</button>;
13}

Controlled vs. Uncontrolled Components:

Controlled Component: State of the input element is controlled by React, ensuring consistent behavior.

1import { useState } from 'react';
2
3function FormValidation() {
4    let [inputValue, setInputValue] = useState('');
5    const updateInput = (e) => {
6        setInputValue(e.target.value);
7    };
8
9    return (
10        <form>
11            <input type="text" value={inputValue} onChange={updateInput} />
12        </form>
13    );
14}

Uncontrolled Component: Input elements inside uncontrolled components work like normal HTML input form elements, handled by the DOM itself.

1import { useRef } from 'react';
2
3function FormValidation() {
4    const inputValue = useRef();
5    const handleSubmit = (e) => {
6        e.preventDefault();
7        console.log(`Input value: ${inputValue.current.value}`);
8    };
9
10    return (
11        <form onSubmit={handleSubmit}>
12            <input type="text" ref={inputValue} />
13            <button type="submit">Submit</button>
14        </form>
15    );
16}

Importance of Keys in Lists:

Keys identify changed, updated, or deleted items, enabling React to re-render only necessary components, enhancing performance.

1type User = {
2    id: string;
3    name: string;
4};
5
6const listUsers = users.map(({ id, name }: User) => {
7    return <li key={id}>{name}</li>;
8});

Exploring Higher-Order Components (HOCs):

HOCs act as containers, simplifying components and promoting reusability. Ideal for implementing common logic across multiple components.

1import React, { useEffect } from 'react';
2
3// Higher-Order Component function
4const withLogger = (WrappedComponent) => {
5    // Functional component that returns the enhanced component
6    const WithLogger = (props) => {
7        useEffect(() => {
8            console.log(`Component ${WrappedComponent.name} is mounted.`);
9            return () => {
10                console.log(
11                    `Component ${WrappedComponent.name} is about to be unmounted.`
12                );
13            };
14        }, []); // Empty dependency array ensures effect runs once after the initial render
15
16        // Wrapping the component with additional functionality
17        return <WrappedComponent {...props} />;
18    };
19
20    return WithLogger;
21};
22
23// Example usage of the Higher-Order Component
24const MyComponent = () => {
25    return <div>My Component</div>;
26};
27
28// Wrapping MyComponent with the withLogger HOC
29const MyComponentWithLogger = withLogger(MyComponent);

Conclusion

Mastering React goes beyond syntax, it involves understanding core concepts like JSX, VDOM, component lifecycles, data flow, and efficient state management.

By comprehending these fundamental concepts and preparing for common interview questions, you're on your way to becoming a successful React developer.

Remember, practice, exploration, and continuous building are key to strengthening your React skills.

Best of luck with your interviews! 💪