ReactJS has revolutionized the way we build user interfaces by introducing components, self-contained and reusable pieces of code that form the foundation of every React application. This blog serves as a comprehensive guide to understanding React components, exploring their types, the role of props and state, and how they integrate seamlessly to create scalable, maintainable, and interactive UIs.
Dive into the fundamentals of React components, the modular, reusable building blocks that make ReactJS a game-changer for modern web development. Learn about their types, usage, lifecycle, and best practices to create dynamic and interactive web applications effortlessly.
React components are like Lego blocks, versatile and reusable, enabling you to assemble your application piece by piece. They encapsulate structure, style, and behavior, ensuring each part of your app is modular and easy to manage.
Types of Components in ReactJS
1. Function-Based Components : These simple JavaScript functions return JSX and are perfect for lightweight, presentational tasks.
Example:
function Greeting() {
return <h1>Hello, World!</h1>;
}
export default Greeting;
When to Use:
For static content or components without state or lifecycle methods.
2. Class-Based Components : Ideal for handling local state and lifecycle methods, these components extend React.Component.
Example:
import React, { Component } from 'react';
class Welcome extends Component {
render() {
return <h1>Welcome to React!</h1>;
}
}
export default Welcome;
When to Use:
When more advanced features like lifecycle methods are required (though Hooks have largely replaced this need).
Props: The Bridge Between Components
Props let you pass dynamic data between components, enabling reusability and interaction.
Example:
function Greeting(props) {
return <h1>Hello, {props.name}!</h1>;
}
Usage:
<Greeting name="John" />
Stateful vs. Stateless Components
Stateless Components:
Depend only on props.
Lightweight and ideal for UI rendering.
Stateful Components:
Manage their own internal state, making them dynamic and interactive.
Achieved through either class-based components or Hooks like useState in function-based components.
Lifecycle of React Components
Class-based components go through three main phases:
Mounting: The component is created and added to the DOM.
Methods: constructor, componentDidMount.
Updating: Props or state changes trigger updates.
Method: componentDidUpdate.
Unmounting: The component is removed from the DOM.
Method: componentWillUnmount.
Function-based components replicate this with the useEffect Hook.
Best Practices
Break Down UI Into Small Components:
Keep components small and focused on a single task for better readability and reusability.
Leverage Props for Data Sharing:
Pass data efficiently between components.
Favor Functional Components:
Use functional components with Hooks to streamline development.
Minimize Stateful Logic:
Where possible, rely on props and stateless designs to reduce complexity.
Conclusion
Components are the essence of ReactJS, offering unparalleled flexibility and scalability in building modern UIs. By mastering function-based and class-based components, props, state, and lifecycle methods, you'll unlock the full potential of React.
What’s Next?
Interview Questions and Answers
Basic Questions
Q.1 What are the main advantages of using React components?
Answer:
Reusability of code.
Modularity for easier debugging and maintenance.
Scalability for building complex applications.
Q.2 What are the differences between props and state?
Answer:
Props are immutable and used for passing data between components.
State is mutable and used for managing data within a component.
Q.3 How do you define a functional component in React?
Answer:
A functional component is a JavaScript function that returns JSX, defining the UI structure. Example:
function MyComponent() {
return <h1>Hello!</h1>;
}
Q.4 What is JSX? Why is it used in React?
Answer:
JSX stands for JavaScript XML, allowing developers to write HTML-like syntax directly in JavaScript. It improves readability and makes UI code declarative.
Q.5 What are key props in React and why are they important?
Answer:
Key props are unique identifiers used in lists to help React optimize rendering. They improve performance by tracking changes in elements.
Q.6 How do you handle events in React?
Answer:
Events in React are handled using camelCase syntax and functions. Example:
<button onClick={handleClick}>Click Me</button>
Q.7 Can you pass functions as props in React? How?
Answer:
Yes, functions can be passed as props and called inside the child component. Example:
function Parent() {
const greet = () => alert("Hello!");
return <Child greet={greet} />;
}
Q.8 What are default props in React?
Answer:
Default props allow setting default values for a component's props. Example:
MyComponent.defaultProps = { name: "Guest" };
Q.9 What is the role of the React.Fragment?
Answer:
It allows grouping multiple elements without adding extra nodes to the DOM. Example:
<React.Fragment>
<h1>Title</h1>
<p>Description</p>
</React.Fragment>
Q.10 What is the virtual DOM in React?
Answer:
The virtual DOM is a lightweight copy of the actual DOM that React uses to optimize updates by only changing what’s necessary.
Advanced Questions
Q.11 What is the difference between useState and useReducer?
Answer:
useState is simpler and used for basic state management.
useReducer is more powerful, handling complex state logic similar to Redux.
Q.12 Explain the role of the useEffect Hook.
Answer:
useEffect is used to handle side effects such as data fetching, DOM manipulation, or setting subscriptions in function-based components.
Q.13 How can you optimize a React application’s performance?
Answer:
Use memoization (React.memo, useMemo).
Optimize state updates and re-renders.
Use lazy loading and code-splitting.
Avoid unnecessary API calls.
Q.14 What are controlled and uncontrolled components?
Answer:
Controlled Components: Manage form data through state.
Uncontrolled Components: Use ref for direct DOM access.
Q.15 How does React handle conditional rendering?
Answer:
Conditional rendering can be handled using:
Ternary operators: {condition ? <ComponentA /> : <ComponentB />}.
Short-circuit evaluation: {condition && <ComponentA />}.
Q.16 What are higher-order components (HOCs) in React?
Answer:
HOCs are functions that take a component and return a new enhanced component. Example: withAuth(MyComponent).
Q.17 What is the useContext Hook?
Answer:
It provides access to context values without using a consumer wrapper. Example:
const value = useContext(MyContext);
Q.18 Explain lazy loading in React.
Answer:
Lazy loading delays the loading of components until they are needed, improving performance. Example:
const LazyComponent = React.lazy(() => import('./LazyComponent'));
Q.19 What are React portals?
Answer:
Portals enable rendering child components into a DOM node outside the parent’s hierarchy. Example:
ReactDOM.createPortal(child, domNode);
Q.20 What is the significance of React’s reconciliation process?
Answer:
The reconciliation process compares the virtual DOM with the real DOM and updates only the changed parts, optimizing performance.
In our upcoming blog, we’ll explore props, events, and states in detail to create highly interactive applications.
Multiple Choice Questions (MCQs)
Basic MCQs
1.Which is not a type of React component?
A) Class-based Component
B) Function-based Component
C) Directive-based Component
D) Pure Component
Answer: C) Directive-based Component
2.What does JSX stand for?
A) JavaScript Syntax Extension
B) JavaScript XML
C) Java Syntax Extension
D) None of the above
Answer: B) JavaScript XML
3.What is the purpose of React.Fragment?
A) To manage state
B) To group multiple elements without adding extra DOM nodes
C) To handle events
D) None of the above
Answer: B) To group multiple elements without adding extra DOM nodes
4.Which of the following is true about props in React?
A) Props are mutable.
B) Props are passed in a unidirectional flow.
C) Props can only pass numbers.
D) Props are used to define state.
Answer: B) Props are passed in a unidirectional flow
5.Which method is not part of the React component lifecycle?
A) componentWillMount
B) componentDidUpdate
C) componentWillUnmount
D) componentDidStart
Answer: D) componentDidStart
Advanced MCQs
6.What does useEffect replace in function-based components?
A) componentDidMount
B) componentDidUpdate
C) componentWillUnmount
D) All of the above
Answer: D) All of the above
7.What is the output of the following code?
function App() {
const [count, setCount] = React.useState(0);
return <button onClick={() => setCount(count + 1)}>{count}</button>;
}
A) Button displays 0 initially, then increments by 1 on each click.
B) Button displays 0 and does not update.
C) Button throws an error.
D) None of the above.
Answer: A) Button displays 0 initially, then increments by 1 on each click.
8.Which Hook is used to manage global state across components?
A) useState
B) useEffect
C) useContext
D) useMemo
Answer: C) useContext
9.What is the purpose of React.lazy()?
A) To lazy load CSS files.
B) To lazy load JavaScript variables.
C) To lazy load components.
D) None of the above.
Answer: C) To lazy load components.
10.Which of the following enhances components with extra functionality?
A) Pure Components
B) Higher-Order Components (HOCs)
C) React Portals
D) Context API
Answer: B) Higher-Order Components (HOCs)