What is React? A Beginner's Guide to Understanding the Basics
Posted on March 26, 2025 • 6 min read • 1,228 wordsLearn the basics of React, a powerful JavaScript library for building dynamic UIs with reusable components, state management, and hooks.
React is one of the most widely used JavaScript libraries for building user interfaces, particularly single-page applications. It was developed by Facebook and is maintained by both Facebook and a large community of developers. React has become a go-to solution for creating fast and interactive web applications, enabling developers to efficiently build complex user interfaces with less code.
React is a JavaScript library primarily used for building user interfaces (UIs). It allows developers to build components, which are reusable, self-contained units of code that represent parts of the user interface. These components can be composed together to form complex UIs, which are easier to manage and scale.
Before diving deeper into React, it’s important to understand what a JavaScript library is. A JavaScript library is a collection of pre-written JavaScript code that helps developers perform common tasks more easily. React is a library specifically designed for building UIs, handling state management, and making applications interactive.
It’s essential to distinguish between a JavaScript library and a framework. While both provide tools to help developers build applications, a framework dictates the structure of your application, providing a more opinionated approach to development. React, however, is a library, meaning it offers flexibility and leaves much of the architectural decision-making to the developer.
In order to get the most out of React, it’s essential to understand some of its core concepts. These concepts form the foundation of React development.
React applications are built using components, which are the building blocks of a user interface. Components are reusable and can be thought of as functions that return HTML (or JSX, which we’ll discuss later). There are two main types of components in React:
Functional components are simple JavaScript functions that return JSX (a syntax extension that allows you to write HTML-like code within JavaScript). They are ideal for rendering UI elements and are the simplest way to define a component.
function Welcome() {
return <h1>Hello, React!</h1>;
}Class components are more powerful but also more complex. They provide additional features such as local component state and lifecycle methods, which allow you to control the behavior of the component over time.
class Welcome extends React.Component {
render() {
return <h1>Hello, React!</h1>;
}
}JSX is a syntax extension for JavaScript that allows you to write HTML-like code inside JavaScript. It makes the code easier to read and write, particularly when building UI components.
const element = <h1>Hello, world!</h1>;JSX may look like HTML, but it’s actually JavaScript. This means you can embed expressions and call functions within JSX, which makes it much more powerful than traditional HTML.
One of React’s most powerful features is the Virtual DOM (Document Object Model). The Virtual DOM is an in-memory representation of the actual DOM. React uses it to optimize updates to the real DOM by comparing the current state of the UI with a new state and then updating only the parts of the DOM that have changed. This process is known as “reconciliation.”
Updating the real DOM is slow, especially when dealing with large applications. The Virtual DOM helps mitigate performance issues by reducing the number of updates to the real DOM. React makes these updates efficiently, leading to faster rendering and a smoother user experience.
State and props are fundamental concepts in React, and they determine how data flows within a React application.
State is a JavaScript object used to store data that can change over time. State is local to the component in which it is defined and is used to represent dynamic data (e.g., user inputs, API responses, etc.).
class Counter extends React.Component {
constructor(props) {
super(props);
this.state = { count: 0 };
}
render() {
return <h1>{this.state.count}</h1>;
}
}Props (short for properties) are read-only values that are passed from parent components to child components. They allow you to pass data down the component tree.
function Greeting(props) {
return <h1>Hello, {props.name}!</h1>;
}React components have lifecycle methods that allow developers to run code at different stages of a component’s life, from creation to destruction. These methods are particularly useful for fetching data, setting up subscriptions, or cleaning up when the component is no longer needed.
componentDidMount(): Called once the component is rendered.componentDidUpdate(): Called after the component updates.componentWillUnmount(): Called right before the component is removed from the DOM.With the release of React 16.8, a new feature called hooks was introduced. Hooks allow you to use state and other React features in functional components, which were previously only available in class components. Hooks have quickly become an essential part of React development.
The useState hook allows functional components to manage local state. Here’s a basic example:
import React, { useState } from 'react';
function Counter() {
const [count, setCount] = useState(0);
return (
<div>
<p>You clicked {count} times</p>
<button onClick={() => setCount(count + 1)}>Click me</button>
</div>
);
}The useEffect hook is used to perform side effects in functional components, such as fetching data or subscribing to events. It replaces lifecycle methods like componentDidMount and componentDidUpdate.
import React, { useEffect, useState } from 'react';
function DataFetcher() {
const [data, setData] = useState(null);
useEffect(() => {
fetch('https://api.example.com/data')
.then((response) => response.json())
.then((data) => setData(data));
}, []); // Empty array means this runs only once, like componentDidMount
return <div>{data ? <pre>{JSON.stringify(data)}</pre> : 'Loading...'}</div>;
}React has several advantages that contribute to its popularity among developers:
Since React is component-based, you can build reusable pieces of UI. This helps to reduce redundancy, makes your code more maintainable, and speeds up development.
Thanks to the Virtual DOM, React updates the UI efficiently and reduces the number of costly DOM manipulations. This results in high performance, especially for large applications.
React has a vast ecosystem of libraries, tools, and resources, which makes it easier to solve problems and find support. Additionally, React’s large community contributes to its growth and the development of new features.
React’s one-way data binding ensures that data flows in a single direction. This makes it easier to understand the flow of data through the application and helps to debug issues more effectively.
React offers robust developer tools, including the React Developer Tools extension for Chrome and Firefox. These tools help developers inspect the component tree, track state changes, and debug their applications.
React is a powerful and flexible JavaScript library that allows developers to build dynamic, responsive, and efficient user interfaces. By understanding its core concepts, including components, JSX, state and props, and hooks, you’ll be well-equipped to start developing your own React applications.
React is a valuable tool to have, With its component-based architecture, high performance, and large community, React is set to remain a dominant force in web development for years to come.