你的第一个组件 | Your First Component

组件：UI 构成要素 | 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>: 在 Web 当中，HTML 允许我们使用其内置的标签集（如 <h1> 和 <li>）创建丰富的结构化文档：

<article>
  <h1>我的第一个组件</h1>
  <ol>
    <li>组件：UI 构成要素</li>
    <li>定义组件</li>
    <li>使用组件</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. <article> 表示这篇文章，<h1> 表示文章的标题，<ol> 以有序列表的形式表示文章的（缩写的）目录。每一个侧边栏、头像、模态框、下拉框的背后是都是像这样的（结合了用于样式的 CSS 和用于交互的 JavaScript 的）标签——你在 Web 上看到的每一个 UI 模块。

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. React 允许你将标签、CSS 和 JavaScript 组合成自定义“组件”，即 应用程序中可复用的 UI 元素。上文中表示目录的代码可以改写成一个能够在每个页面中渲染的 <TableOfContents /> 组件。实际上，使用的依然是 <article>、<h1> 等相同的 HTML 标签。

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: 就像使用 HTML 标签一样，你可以组合、排序和嵌套组件来绘制整个页面。例如，你正在阅读的文档页面就是由 React 组件构成的：

<PageLayout>
  <NavigationHeader>
    <SearchBar />
    <Link to="/docs">文档</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. 随着项目的发展，你会发现很多布局可以通过复用已经完成的组件来实现，从而加快开发进程。上文中提到的目录可以通过 <TableOfContents /> 组件添加到任意的画面中！你也可以使用 React 开源社区分享的大量组件（例如 Chakra UI 和 Material UI）来快速启动项目。

定义组件 | 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): 一直以来，创建网页时，Web 开发人员会用标签描述内容，然后通过 JavaScript 来增加交互。这种在 Web 上添加交互的方式能产生出色的效果。现在许多网站和全部应用都需要交互。React 最为重视交互性且使用了相同的处理方式：React 组件是一段可以 使用标签进行扩展 的 JavaScript 函数。如下所示（你可以编辑下面的示例）：

And here’s how to build a component: 以下是构建组件的方法：

第一步：导出组件 | 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!) export default 前缀是一种 JavaScript 标准语法（非 React 的特性）。它允许你导出一个文件中的主要函数以便你以后可以从其他文件引入它。欲了解更多关于导入的内容，请参阅 组件的导入与导出 章节！

第二步：定义函数 | Step 2: Define the function

With function Profile() { } you define a JavaScript function with the name Profile. 使用 function Profile() { } 定义名为 Profile 的 JavaScript 函数。

第三步：添加标签 | 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. 这个组件返回一个带有 src 和 alt 属性的 <img /> 标签。<img /> 写得像 HTML，但实际上是 JavaScript！这种语法被称为 JSX，它允许你在 JavaScript 中嵌入标签。

Return statements can be written all on one line, as in this component: 返回语句可以全写在一行上，如下面组件中所示：

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: 但是，如果你的标签和 return 关键字不在同一行，则必须把它包裹在一对括号中，如下所示：

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

使用组件 | 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: 现在你已经定义了 Profile 组件，你可以在其他组件中使用它。例如，你可以导出一个内部使用了多个 Profile 组件的 Gallery 组件：

浏览器所看到的 | What the browser sees

Notice the difference in casing: 注意下面两者的区别：

• <section> is lowercase, so React knows we refer to an HTML tag.
• <section> 是小写的，所以 React 知道我们指的是 HTML 标签。
• <Profile /> starts with a capital P, so React knows that we want to use our component called Profile.
• <Profile /> 以大写 P 开头，所以 React 知道我们想要使用名为 Profile 的组件。

And Profile contains even more HTML: <img />. In the end, this is what the browser sees: 然而 Profile 包含更多的 HTML：<img />。这是浏览器最后所看到的：

<section>
  <h1>了不起的科学家</h1>
  <img src="https://i.imgur.com/MK3eW3As.jpg" alt="Katherine Johnson" />
  <img src="https://i.imgur.com/MK3eW3As.jpg" alt="Katherine Johnson" />
  <img src="https://i.imgur.com/MK3eW3As.jpg" alt="Katherine Johnson" />
</section>

Nesting and organizing components | 嵌套和组织组件

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. 组件是常规的 JavaScript 函数，所以你可以将多个组件保存在同一份文件中。当组件相对较小或彼此紧密相关时，这是一种省事的处理方式。如果这个文件变得臃肿，你也可以随时将 Profile 移动到单独的文件中。你可以立即在 关于引入的页面 中学习如何做到这些。

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. 因为 Profile 组件在 Gallery 组件中渲染——甚至好几次！——我们可以认为 Gallery 是一个 父组件，将每个 Profile 渲染为一个“孩子”。这是 React 的神奇之处：你可以只定义组件一次，然后按需多处和多次使用。

export default function Gallery() {
  // 🔴 Never define a component inside another component!
  // 🔴 永远不要在组件中定义组件
  function Profile() {
    // ...
  }
  // ...
}

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

// ✅ Declare components at the top level
// ✅ 在顶层声明组件
function Profile() {
  // ...
}