Components

Components are independent, reusable pieces of UI. Each component returns JSX describing a part of the interface.

Function Components

The modern way to write React components:

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

Use PascalCase for component names. The name must start with a capital letter.

Rendering a Component

Use the component as a JSX tag:

function App() {
  return (
    <div>
      <Greeting />
      <Greeting />
    </div>
  );
}

Each <Greeting /> renders independently.

Composing Components

Components can nest and compose freely:

function Header() {
  return <h1>My App</h1>;
}

function Content() {
  return <p>Welcome to React.</p>;
}

function App() {
  return (
    <div>
      <Header />
      <Content />
    </div>
  );
}

Build complex UIs from small, focused pieces.

Component Tree

React renders components as a tree. App is the root, with children branching down:

App
├── Header
├── Content
└── Footer

Each component manages its own piece of the tree.

One Component Per File

Keep components in separate files:

src/
  components/
    Header.jsx
    Footer.jsx
  App.jsx

This keeps code organized and easy to find.

Best Practices

1. Name components PascalCase — UserProfile, not userProfile
2. Keep components small — One job per component
3. Extract when复用 occurs — If you copy-paste, make a component
4. One component per file — Easier to navigate and maintain

Common Mistakes

1. Lowercase component names — <greeting /> is treated as an HTML tag
2. Making components too large — Split into smaller pieces
3. Putting everything in App.jsx — Extract to separate files
4. Forgetting to export — Component won’t be importable