React Interview Questions (Beginner level)
Tags: react, javascript, interview, beginners
Published: 2024-06-22 15:24:33

Here are some beginner-level React interview questions that you might encounter in your next interview. Good luck, and I hope this material proves helpful in your preparation.

## **What is React?**

React is a JavaScript library for building user interfaces, focusing on component-based architecture, virtual DOM for performance, JSX syntax for rendering, and one-way data flow. It's widely used for creating interactive web applications efficiently.

It is used for handling view layer for web and mobile apps based on components in a declarative approach.

## **What are the key characteristics of React?**

- Component-based architecture.
- Virtual DOM for efficient rendering.
- JSX for declarative UI syntax.
- Unidirectional data flow.
- Hooks for functional components.
- Rich ecosystem with libraries like Redux and React Router.

## **What is JSX?**

JSX stands for JavaScript XML. It's an extension to JavaScript syntax that allows developers to write HTML-like code within JavaScript. JSX makes it easier to create and manipulate the DOM structure in React applications.

- Combines JavaScript and HTML: JSX allows you to write HTML elements and JavaScript together in a single file.
- Syntax extension: It provides syntactic sugar for React.createElement(component, props, ...children) function calls.
- Compile-time transformation: JSX code is transformed into regular JavaScript objects during build time using tools like Babel.

> Code-snippet of JSX:

```
import React from 'react';

const element = <h1>Hello, JSX!</h1>;

function App() {
  return (
    <div>
      <h1>Welcome to my React App</h1>
      <p>This is a paragraph rendered using JSX.</p>
      {element}
    </div>
  );
}

export default App;
```
In the above example, `<h1>Hello, JSX!</h1>` is JSX syntax that gets transformed into React.createElement('h1', null, 'Hello, JSX!') behind the scenes. This simplifies the process of writing and visualizing UI components in React applications.

## **What is state in React?**

In React, state is a built-in object that allows components to keep track of their internal data. It represents the mutable data that influences the component's rendering and behavior. The important point is whenever the state object changes, the component re-renders.

## **What are props in React?**

In React, props (short for properties) are a way of passing data from one component to another. They are similar to function arguments in JavaScript or parameters in other programming languages. Props are read-only and help to make components more dynamic and reusable by allowing them to be configured with different values.

_OR_

Data passed from a parent component to a child component. They are immutable (read-only) and allow components to be customizable and reusable.

> Example
Consider a parent component passing a name prop to a child component:

```
// ParentComponent.jsx
import React from 'react';
import ChildComponent from './ChildComponent';

function ParentComponent() {
  const name = "Alice";

  return (
    <div>
      <ChildComponent name={name} />
    </div>
  );
}

export default ParentComponent;
```

```
// ChildComponent.jsx
import React from 'react';

function ChildComponent(props) {
  return <p>Hello, {props.name}!</p>;
}

export default ChildComponent;

```
- ParentComponent: Renders ChildComponent and passes a name prop with the value "Alice".
- ChildComponent: Receives name as a prop (props.name) and displays a greeting using that prop (Hello, {props.name}!).

> Above Example

- name is a prop passed from ParentComponent to ChildComponent.
- Props allow ChildComponent to dynamically display different names based on what ParentComponent passes.

## **What is the difference between state and props?**

> State

- Managed internally by the component itself.
- Can be updated using setState() method in class components or using Hooks in functional components.
- Changes trigger re-rendering of the component and its children.
- Mutable (can be modified within the component).
- Used for managing internal state and data that may change over time.
- Enhances component reusability when used effectively with local state management.

> Props

- Passed from parent to child component.
- Read-only (immutable) within the receiving component.
- Used to configure a component and provide data from parent to child.
- Components become more reusable as they can be configured differently via props.
- Cannot be modified by the receiving component.
- Used for passing data and callbacks between components in a component hierarchy.

## **What are react fragments?**

React Fragments provide a way to group multiple children elements without adding extra nodes to the DOM. They allow you to return multiple elements from a component's render method without needing to wrap them in a container element like a `<div>` or `<span>`. Fragments were introduced in React 16.2 as a lightweight syntax for grouping elements.

## **What is the difference between Component and Element?**

> Component

- Definition: A component is a JavaScript function or class that optionally accepts inputs (props) and returns a React element.
- Purpose: It defines reusable UI pieces.
- Types: Functional components (using functions) and class components (using ES6 classes).
- Usage: Components manage their own state (with Hooks or setState for class components) and lifecycle.

> Example

```
function Welcome(props) {
  return <h1>Hello, {props.name}</h1>;
}

```

> Element

- Definition: An element is a plain object representation of a React component.
- Purpose: It describes what you want to see on the screen.
- Created with: JSX syntax or React.createElement() function.
- Immutable: Elements are immutable and represent the UI at a certain point in time.

> Example

```
const element = <h1>Hello, world!</h1>;
```




## **What is key in React?**

In React.js, the key prop is a special attribute used to give a unique identity to each element or component in an array or iterable list of children. It's primarily used by React internally to efficiently manage and update the component's UI when items are added, removed, or rearranged in a list.

Purpose of key:

**_Identifying Elements_**: When rendering multiple elements from an array or iterating over components, React uses keys to differentiate between items. This helps React determine which items have changed, are added, or are removed.

**_Optimizing Reconciliation_**: React uses keys during its reconciliation process (diffing algorithm) to minimize DOM updates. By having a stable identity for each item, React can efficiently update the UI without re-rendering unchanged components or losing component state.
 

## **What is prop drilling in React?**

Prop drilling in React refers to the process where props are passed from a higher-level component to a lower-level component through intermediary components that do not actually use the props themselves. This happens when data needs to be passed down multiple levels of nested components, even though some intermediate components don't need the data themselves.

Prop drilling is a common pattern in React where props are passed through multiple levels of nested components to reach a deeply nested child component that needs the data. While it's straightforward to implement, it can lead to code complexity and inefficiencies. Using React's Context API or state management libraries can provide cleaner solutions for managing and passing data across components in complex applications.

## **What are error boundaries?**

Error boundaries are React components that catch JavaScript errors anywhere in their child component tree, log those errors, and display a fallback UI instead of crashing the entire React application. They are used to manage and gracefully handle errors that occur during rendering, in lifecycle methods, and in constructors of React components.

## **What is Virtual DOM?**

The Virtual DOM (Document Object Model) is a concept in React and other virtual DOM-based libraries that represents a lightweight copy of the real DOM tree. It's a programming concept and an abstraction layer used for efficiently updating the UI.

- Definition: The Virtual DOM is a JavaScript representation of the actual DOM elements and their properties (attributes, styles, etc.) created by React.

- Purpose: It serves as an intermediary representation of the UI. When changes are made to the state or props of React components, React first updates the Virtual DOM rather than the real DOM directly.

- Working Principle: React compares the current Virtual DOM with a previous version (reconciliation process) to identify what has changed. This comparison is efficient because manipulating the Virtual DOM is faster than directly interacting with the actual browser DOM.

- Efficiency: Once React identifies the differences (diffing algorithm), it only updates the parts of the real DOM that have changed. This minimizes costly DOM manipulation operations and helps in achieving better performance.

- Example: Suppose you have a React component that updates its state. React will update the Virtual DOM first, compare it with the previous Virtual DOM state, and then apply the necessary changes to the real DOM.

## **What are the differences between controlled and uncontrolled components?**

Controlled and uncontrolled components are two different approaches to managing form input elements in React. The main differences lie in how they handle and manage state, especially with regards to form data.

> Controlled Components:

- State Handling: In controlled components, form data is handled by React state (typically within the parent component).
- Data Flow: The value of the form input elements (like `<input>`, `<textarea>`, `<select>`) is controlled by the state and is passed to the components as props.
- Event Handling: Changes to the form elements are handled using onChange event handlers, where the state is updated with each change.

> Uncontrolled Components:

- State Handling: Uncontrolled components rely on the DOM itself to manage form data.
- Ref Usage: References (ref) are typically used to access the DOM elements directly to get their values.
- Event Handling: Events like onSubmit, onClick, or directly accessing DOM events (element.value) are used to retrieve form data.

I hope these interview questions have been insightful and will help you prepare effectively, and also Feel free to bookmark 🔖 even if you don't need this for now. Best of luck in your upcoming interviews!

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