React JS Interviews: Top Questions & Expert Tips to Ace Them

React JS has become one of the most popular JavaScript libraries for building user interfaces. As a result, the demand for skilled React developers is high. If you're preparing for a React JS interview, you'll want to be ready for the toughest questions. In this article, we'll explore some of the most challenging React JS interview questions and provide expert tips on how to handle them.

1. What is the Virtual DOM and how does it work in React?

Answer: The Virtual DOM (Document Object Model) is a lightweight, in-memory representation of the actual DOM. It is a key feature in React that improves performance by minimizing the number of direct updates made to the real DOM.

When a change occurs in a React component, React first updates the Virtual DOM. It then compares the new Virtual DOM with the old one, using a process called "diffing". React identifies the minimal set of updates required to bring the actual DOM in sync with the new Virtual DOM. Lastly, it patches the real DOM with those updates, a process known as "reconciliation".

2. What are React Hooks and why are they beneficial?

Answer: React Hooks are functions introduced in React 16.8 that allow you to use state and other React features in functional components without needing to convert them into class components.

Hooks make it easier to reuse stateful logic between components. They also simplify component code by eliminating the need for lifecycle methods and complex patterns, such as higher-order components and render props.

Some common hooks are useState, useEffect, and useContext.

3. Explain the difference between state and props in React.

Answer: In React, both state and props are used to manage and pass data in components. However, they serve different purposes.

  • State: It represents the local state of a component. State is mutable and is managed within the component. When the state is updated, the component re-renders.
  • Props: Short for "properties", props are immutable data passed from a parent component to a child component. Props allow components to communicate and share data with each other.

4. What are controlled and uncontrolled components?

Answer: Controlled and uncontrolled components refer to the way form data is managed in React components.

  • Controlled components: In a controlled component, form data is managed by the component's state. The component handles user input by updating its state and re-rendering the form with the updated values. The input elements' values are controlled by the component's state.
  • Uncontrolled components: In an uncontrolled component, form data is managed by the DOM itself. The input elements' values are controlled by the DOM, and not by the React component. To access form data, you use refs to access the input elements directly.

Controlled components are generally recommended as they provide better control and flexibility over form data in React applications.

5. What is the purpose of keys in React lists, and how should they be used?

Answer: Keys are identifiers used by React to uniquely associate items in lists with their corresponding DOM elements. They help React optimize the rendering process when updating, reordering, or deleting items in a list.

Keys should be unique, stable, and not change between renders. Ideally, use unique IDs from your data as keys. Avoid using array indices as keys, as they may cause rendering issues when items are added, removed, or reordered.

const dataList = =>
  <ListItem key={} item={item} />

By being well-prepared for these challenging React JS interview questions, you increase your chances of acing your interview and landing that coveted React developer job. Good luck!

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