React, a popular JavaScript library for building user interfaces, is known for its component-based architecture. A React component is essentially an independent, reusable piece of code that returns HTML to be rendered on the page. These components are the building blocks of React applications, enabling developers to create complex UIs from small, isolated pieces of code.

Here is a functional “Hello World!” React component:

import React from 'react';

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

export default HelloWorld;

React components are always uppercase like <React> while React requires HTML to be formatted in lowercase like <html>. Using lower case in HTML is a common convention but is not required by HTML, the lowercase requirement comes from React.

This simple component encapsulates all the necessary markup for a greeting. In React, components like HelloWorld are always capitalized. This naming convention is more than just a stylistic choice; it’s crucial for distinguishing React components from HTML tags, which are written in lowercase. This distinction is rooted in React’s JSX syntax, a JavaScript extension that allows mixing HTML with JavaScript. While lowercase in HTML tags is a common convention but not a strict requirement of HTML, React enforces this rule to differentiate between its components and standard HTML elements.

One of the most powerful features of React components is their composability. Components can be nested within each other, allowing for complex UIs to be built out of simpler, reusable pieces. This nesting capability not only promotes code reusability but also enhances the maintainability and scalability of the application. Consider the following example where the HelloWorld component is nested inside another component:

export default function HelloPage() {
  return (
    <div>
      <h1>I have something to say.</h1>
      <HelloWorld />
    </div>
  );
}

In this example, HelloPage is a parent component that encapsulates the HelloWorld child component. This hierarchical structure is a key aspect of React’s design philosophy. It allows developers to break down complex interfaces into smaller, manageable components, each responsible for a specific part of the UI.