ReactJS is one of the most powerful JavaScript libraries for building dynamic, high-performance user interfaces. Created by Facebook, it revolutionized frontend development by introducing a component-based architecture and virtual DOM.

This ReactJS Cheatsheet is designed to give developers a quick, structured, and easy-to-understand reference for all key features, including JSX, Components, Props, State, Hooks, and Routing.

What is ReactJS?

ReactJS is an open-source JavaScript library used to build interactive UIs for single-page and complex web applications. It allows developers to manage views efficiently by breaking the UI into reusable components.

✅ Key Features of ReactJS:

• Component-based architecture
• Virtual DOM for performance optimization
• Unidirectional data flow
• Support for JSX (JavaScript XML)
• Powerful hooks for managing state and side effects

ReactJS Basic Syntax

Creating Your First React Component

import React from 'react';

function Welcome() {
  return <h1>Hello, World!</h1>;
}

export default Welcome;

Render in index.js:

import React from 'react';
import ReactDOM from 'react-dom';
import Welcome from './Welcome';

ReactDOM.render(<Welcome />, document.getElementById('root'));

ReactJS JSX Syntax Explained

JSX allows you to write HTML-like syntax directly in JavaScript.

const element = <h2>React is Amazing!</h2>;

You can embed JavaScript expressions using {}:

const user = "Alice";
return <h3>Welcome, {user}!</h3>;

React Components — Functional and Class

Functional Component:

function Greeting() {
  return <h2>Hello React Functional Component!</h2>;
}

Class Component:

import React, { Component } from 'react';

class Greeting extends Component {
  render() {
    return <h2>Hello React Class Component!</h2>;
  }
}

React Props (Passing Data Between Components)

Props are used to pass data from a parent component to a child component.

function Welcome(props) {
  return <h3>Welcome, {props.name}</h3>;
}

function App() {
  return <Welcome name="John Doe" />;
}

React State (useState Hook)

State is used to manage dynamic data that can change during the app lifecycle.

import React, { useState } from 'react';

function Counter() {
  const [count, setCount] = useState(0);
  return (
    <div>
      <p>Clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>Click Me</button>
    </div>
  );
}

React useEffect Hook (Side Effects)

The useEffect() hook lets you handle side effects, such as fetching data or updating the DOM.

import React, { useState, useEffect } from 'react';

function DataFetcher() {
  const [data, setData] = useState([]);

  useEffect(() => {
    fetch("https://jsonplaceholder.typicode.com/posts")
      .then(res => res.json())
      .then(data => setData(data));
  }, []);

  return <div>{data.length} posts fetched</div>;
}

React Event Handling

function ClickButton() {
  const handleClick = () => alert("Button Clicked!");
  return <button onClick={handleClick}>Click Me</button>;
}

React Conditional Rendering

function Greeting({ isLoggedIn }) {
  return isLoggedIn ? <h2>Welcome back!</h2> : <h2>Please log in.</h2>;
}

React Lists and Keys

function NameList() {
  const names = ["Alice", "Bob", "Charlie"];
  return (
    <ul>
      {names.map((name, index) => <li key={index}>{name}</li>)}
    </ul>
  );
}

React Forms and Controlled Components

function FormExample() {
  const [value, setValue] = useState("");

  const handleSubmit = (e) => {
    e.preventDefault();
    alert(`Submitted: ${value}`);
  };

  return (
    <form onSubmit={handleSubmit}>
      <input value={value} onChange={(e) => setValue(e.target.value)} />
      <button type="submit">Submit</button>
    </form>
  );
}

React Router Cheatsheet

Install:

npm install react-router-dom

Example:

import { BrowserRouter as Router, Route, Routes, Link } from "react-router-dom";

function App() {
  return (
    <Router>
      <nav>
        <Link to="/">Home</Link> | <Link to="/about">About</Link>
      </nav>

      <Routes>
        <Route path="/" element={<Home />} />
        <Route path="/about" element={<About />} />
      </Routes>
    </Router>
  );
}

React Lifecycle Methods (Class Components)

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React Context API

Used for state management without prop drilling.

const UserContext = React.createContext();

function App() {
  return (
    <UserContext.Provider value="John">
      <Profile />
    </UserContext.Provider>
  );
}

function Profile() {
  const user = React.useContext(UserContext);
  return <h2>User: {user}</h2>;
}

React Performance Optimization Tips

• Use React.memo() to prevent unnecessary re-renders.
• Apply key props properly in lists.
• Lazy load components using React.lazy and Suspense.
• Keep components small and focused.
• Use PureComponent for class components.

Popular React Developer Tools

FAQ — ReactJS Cheatsheet

Q1: What is ReactJS used for?
React is used to build fast, reusable, and dynamic user interfaces for web and mobile apps.

Q2: Is ReactJS a library or framework?
ReactJS is a library, not a full-fledged framework like Angular.

Q3: What are hooks in React?
Hooks like useState and useEffect let you manage state and side effects in functional components.

Q4: How does React improve performance?
React uses a Virtual DOM to minimize direct DOM manipulation and re-rendering.

Q5: Can React work with backend technologies like PHP or Node.js?
Yes, React can easily integrate with APIs built in PHP, Node.js, or Python.