frontend-study-book

Your First Component

Components are one of the core concepts of React. They are the foundation upon which you build user interfaces (UI), which makes them the perfect place to start your React journey!
foundation [/faʊnˈdeɪʃ(ə)n] 基础

You will learn

Components: UI building blocks

On the Web, HTML lets us create rich structured documents with its built-in set of tags like <h1> and <li>:

<article>
  <h1>My First Component</h1>
  <ol>
    <li>Components: UI Building Blocks</li>
    <li>Defining a Component</li>
    <li>Using a Component</li>
  </ol>
</article>

This markup represents this article <article>, its heading <h1>, and an (abbreviated) table of contents as an ordered list <ol>. Markup like this, combined with CSS for style, and JavaScript for interactivity, lies behind every sidebar, avatar, modal, dropdown—every piece of UI you see on the Web.
represent [ˌreprɪˈzent] 表示
abbreviated [əˈbrivieɪtɪd] 缩写的

React lets you combine your markup, CSS, and JavaScript into custom “components”, reusable UI elements for your app. The table of contents code you saw above could be turned into a <TableOfContents /> component you could render on every page. Under the hood, it still uses the same HTML tags like <article>, <h1>, etc.

Just like with HTML tags, you can compose, order and nest components to design whole pages. For example, the documentation page you’re reading is made out of React components:

<PageLayout>
  <NavigationHeader>
    <SearchBar />
    <Link to="/docs">Docs</Link>
  </NavigationHeader>
  <Sidebar />
  <PageContent>
    <TableOfContents />
    <DocumentationText />
  </PageContent>
</PageLayout>

As your project grows, you will notice that many of your designs can be composed by reusing components you already wrote, speeding up your development. Our table of contents above could be added to any screen with <TableOfContents />! You can even jumpstart your project with the thousands of components shared by the React open source community like Chakra UI and Material UI.

Defining a component

Traditionally when creating web pages, web developers marked up their content and then added interaction by sprinkling on some JavaScript. This worked great when interaction was a nice-to-have on the web. Now it is expected for many sites and all apps. React puts interactivity first while still using the same technology: a React component is a JavaScript function that you can sprinkle with markup. Here’s what that looks like (you can edit the example below):
traditionally [trəˈdɪʃənəli] 传统上\

export default function Profile() {
  return (
    <img
      src="https://i.imgur.com/MK3eW3Am.jpg"
      alt="Katherine Johnson"
    />
  )
}

And here’s how to build a component:

Step 1: Export the component

The export default prefix is a standard JavaScript syntax (not specific to React). It lets you mark the main function in a file so that you can later import it from other files. (More on importing in Importing and Exporting Components!)
standard [ˈstændərd] 标准

Step 2: Define the function

With function Profile() { } you define a JavaScript function with the name Profile.

Pitfall
React components are regular JavaScript functions, but their names must start with a capital letter or they won’t work!

Step 3: Add markup

The component returns an <img /> tag with src and alt attributes. <img /> is written like HTML, but it is actually JavaScript under the hood! This syntax is called JSX, and it lets you embed markup inside JavaScript.

Return statements can be written all on one line, as in this component:

return <img src="https://i.imgur.com/MK3eW3As.jpg" alt="Katherine Johnson" />;

But if your markup isn’t all on the same line as the return keyword, you must wrap it in a pair of parentheses:
parentheses [pəˈrenθəsiz] 括号

return (
  <div>
    <img src="https://i.imgur.com/MK3eW3As.jpg" alt="Katherine Johnson" />
  </div>
);

Pitfall
Without parentheses, any code on the lines after return will be ignored!

Using a component

Now that you’ve defined your Profile component, you can nest it inside other components. For example, you can export a Gallery component that uses multiple Profile components:

function Profile() {
  return (
    <img
      src="https://i.imgur.com/MK3eW3As.jpg"
      alt="Katherine Johnson"
    />
  );
}

export default function Gallery() {
  return (
    <section>
      <h1>Amazing scientists</h1>
      <Profile />
      <Profile />
      <Profile />
    </section>
  );
}

What the browser sees

Notice the difference in casing:

Amazing scientists

Katherine Johnson Katherine Johnson Katherine Johnson 

### Nesting and organizing components
Components are regular JavaScript functions, so you can keep multiple components in the same file. This is convenient when components are relatively small or tightly related to each other. If this file gets crowded, you can always move `Profile` to a separate file. You will learn how to do this shortly on the [page about imports](https://react.dev/learn/importing-and-exporting-components).

Because the `Profile` components are rendered inside `Gallery`—even several times!—we can say that `Gallery` is a **parent component**, rendering each `Profile` as a “child”. This is part of the magic of React: you can define a component once, and then use it in as many places and as many times as you like.

**Pitfall**\
Components can render other components, but you must never nest their definitions:
```jsx
export default function Gallery() {
  // 🔴 Never define a component inside another component!
  function Profile() {
    // ...
  }
  // ...
}

The snippet above is very slow and causes bugs. Instead, define every component at the top level:

export default function Gallery() {
  // ...
}

// ✅ Declare components at the top level
function Profile() {
  // ...
}

When a child component needs some data from a parent, pass it by props instead of nesting definitions.

Recap

You’ve just gotten your first taste of React! Let’s recap some key points.
recap [ˈriːkæp] 总结

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