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Creating and nesting components
创建和嵌套组件

React apps are made out of components. A component is a piece of the UI (user interface) that has its own logic and appearance. A component can be as small as a button, or as large as an entire page.
React 应用是由组件组成的。一个组件是 UI（user interface）的一部分，它有自己的逻辑和外观。一个组件可以小到一个按钮，或者大到一个整个页面。

React components are JavaScript functions that return markup:
React 组件是 JavaScript 函数，它们返回标记：

function MyButton() {
  return (
    <button>I'm a button</button>
  );
}

Now that you’ve declared MyButton, you can nest it into another component:
现在你已经声明了 MyButton ，你可以用它嵌套到另一个组件中：

export default function MyApp() {
  return (
    <div>
      <h1>Welcome to my app</h1>
      <MyButton />
    </div>
  );
}

Notice that <MyButton /> starts with a capital letter. That’s how you know it’s a React component. React component names must always start with a capital letter, while HTML tags must be lowercase.
请注意， <MyButton /> 以大写字母开头。这就是你知道它是 React 组件的方式。React 组件名称必须始终以大写字母开头，而 HTML 标签必须是小写。

Have a look at the result:
查看结果：

The export default keywords specify the main component in the file. If you’re not familiar with some piece of JavaScript syntax, MDN and javascript.info have great references.
export default 关键词指定了文件中的主要组件。如果您不熟悉某些 JavaScript 语法，MDN 和 javascript.info 有很好的参考资料。

Writing markup with JSX
使用 JSX 编写标记

The markup syntax you’ve seen above is called JSX. It is optional, but most React projects use JSX for its convenience. All of the tools we recommend for local development support JSX out of the box.
您上面看到的标记语法称为 JSX。它是可选的，但大多数 React 项目使用 JSX 以其便利性。我们推荐的所有本地开发工具都开箱即用支持 JSX。

JSX is stricter than HTML. You have to close tags like <br />. Your component also can’t return multiple JSX tags. You have to wrap them into a shared parent, like a <div>...</div> or an empty <>...</> wrapper:
JSX 比 HTML 更严格。你必须像 <br /> 一样关闭标签。你的组件也不能返回多个 JSX 标签。你必须将它们包装到一个共享父级中，比如一个 <div>...</div> 或一个空的 <>...</> 包装器：

function AboutPage() {
  return (
    <>
      <h1>About</h1>
      <p>Hello there.<br />How do you do?</p>
    </>
  );
}

If you have a lot of HTML to port to JSX, you can use an online converter.
如果您有大量 HTML 要移植到 JSX，可以使用一个在线转换器。

Adding styles   添加样式

In React, you specify a CSS class with className. It works the same way as the HTML class attribute:
在 React 中，您通过 className 指定一个 CSS 类。它的工作方式与 HTML class 属性相同：

<img className="avatar" />

Then you write the CSS rules for it in a separate CSS file:
然后您在单独的 CSS 文件中为其编写 CSS 规则：

/* In your CSS */
.avatar {
  border-radius: 50%;
}

React does not prescribe how you add CSS files. In the simplest case, you’ll add a <link> tag to your HTML. If you use a build tool or a framework, consult its documentation to learn how to add a CSS file to your project.
React 并不规定您如何添加 CSS 文件。在最简单的情况下，您将向您的 HTML 添加一个 <link> 标签。如果您使用构建工具或框架，请查阅其文档以了解如何将 CSS 文件添加到您的项目中。

Displaying data

JSX lets you put markup into JavaScript. Curly braces let you “escape back” into JavaScript so that you can embed some variable from your code and display it to the user. For example, this will display user.name:

return (
  <h1>
    {user.name}
  </h1>
);

You can also “escape into JavaScript” from JSX attributes, but you have to use curly braces instead of quotes. For example, className="avatar" passes the "avatar" string as the CSS class, but src={user.imageUrl} reads the JavaScript user.imageUrl variable value, and then passes that value as the src attribute:

return (
  <img
    className="avatar"
    src={user.imageUrl}
  />
);

You can put more complex expressions inside the JSX curly braces too, for example, string concatenation:

In the above example, style={{}} is not a special syntax, but a regular {} object inside the style={ } JSX curly braces. You can use the style attribute when your styles depend on JavaScript variables.

Conditional rendering

In React, there is no special syntax for writing conditions. Instead, you’ll use the same techniques as you use when writing regular JavaScript code. For example, you can use an if statement to conditionally include JSX:

let content;
if (isLoggedIn) {
  content = <AdminPanel />;
} else {
  content = <LoginForm />;
}
return (
  <div>
    {content}
  </div>
);

If you prefer more compact code, you can use the conditional ? operator. Unlike if, it works inside JSX:

<div>
  {isLoggedIn ? (
    <AdminPanel />
  ) : (
    <LoginForm />
  )}
</div>

When you don’t need the else branch, you can also use a shorter logical && syntax:

<div>
  {isLoggedIn && <AdminPanel />}
</div>

All of these approaches also work for conditionally specifying attributes. If you’re unfamiliar with some of this JavaScript syntax, you can start by always using if...else.

Rendering lists

You will rely on JavaScript features like for loop and the array map() function to render lists of components.

For example, let’s say you have an array of products:

const products = [
  { title: 'Cabbage', id: 1 },
  { title: 'Garlic', id: 2 },
  { title: 'Apple', id: 3 },
];

Inside your component, use the map() function to transform an array of products into an array of <li> items:

const listItems = products.map(product =>
  <li key={product.id}>
    {product.title}
  </li>
);

return (
  <ul>{listItems}</ul>
);

Notice how <li> has a key attribute. For each item in a list, you should pass a string or a number that uniquely identifies that item among its siblings. Usually, a key should be coming from your data, such as a database ID. React uses your keys to know what happened if you later insert, delete, or reorder the items.

Responding to events

You can respond to events by declaring event handler functions inside your components:

function MyButton() {
  function handleClick() {
    alert('You clicked me!');
  }

  return (
    <button onClick={handleClick}>
      Click me
    </button>
  );
}

Notice how onClick={handleClick} has no parentheses at the end! Do not call the event handler function: you only need to pass it down. React will call your event handler when the user clicks the button.

Updating the screen

Often, you’ll want your component to “remember” some information and display it. For example, maybe you want to count the number of times a button is clicked. To do this, add state to your component.

First, import useState from React:

import { useState } from 'react';

Now you can declare a state variable inside your component:

function MyButton() {
  const [count, setCount] = useState(0);
  // ...

You’ll get two things from useState: the current state (count), and the function that lets you update it (setCount). You can give them any names, but the convention is to write [something, setSomething].

The first time the button is displayed, count will be 0 because you passed 0 to useState(). When you want to change state, call setCount() and pass the new value to it. Clicking this button will increment the counter:

function MyButton() {
  const [count, setCount] = useState(0);

  function handleClick() {
    setCount(count + 1);
  }

  return (
    <button onClick={handleClick}>
      Clicked {count} times
    </button>
  );
}

React will call your component function again. This time, count will be 1. Then it will be 2. And so on.

If you render the same component multiple times, each will get its own state. Click each button separately:

Notice how each button “remembers” its own count state and doesn’t affect other buttons.

Using Hooks

Functions starting with use are called Hooks. useState is a built-in Hook provided by React. You can find other built-in Hooks in the API reference. You can also write your own Hooks by combining the existing ones.

Hooks are more restrictive than other functions. You can only call Hooks at the top of your components (or other Hooks). If you want to use useState in a condition or a loop, extract a new component and put it there.

Sharing data between components

In the previous example, each MyButton had its own independent count, and when each button was clicked, only the count for the button clicked changed:

However, often you’ll need components to share data and always update together.

To make both MyButton components display the same count and update together, you need to move the state from the individual buttons “upwards” to the closest component containing all of them.

In this example, it is MyApp:

Now when you click either button, the count in MyApp will change, which will change both of the counts in MyButton. Here’s how you can express this in code.

First, move the state up from MyButton into MyApp:

export default function MyApp() {
  const [count, setCount] = useState(0);

  function handleClick() {
    setCount(count + 1);
  }

  return (
    <div>
      <h1>Counters that update separately</h1>
      <MyButton />
      <MyButton />
    </div>
  );
}

function MyButton() {
  // ... we're moving code from here ...
}

Then, pass the state down from MyApp to each MyButton, together with the shared click handler. You can pass information to MyButton using the JSX curly braces, just like you previously did with built-in tags like <img>:

export default function MyApp() {
  const [count, setCount] = useState(0);

  function handleClick() {
    setCount(count + 1);
  }

  return (
    <div>
      <h1>Counters that update together</h1>
      <MyButton count={count} onClick={handleClick} />
      <MyButton count={count} onClick={handleClick} />
    </div>
  );
}

The information you pass down like this is called props. Now the MyApp component contains the count state and the handleClick event handler, and passes both of them down as props to each of the buttons.

Finally, change MyButton to read the props you have passed from its parent component:

function MyButton({ count, onClick }) {
  return (
    <button onClick={onClick}>
      Clicked {count} times
    </button>
  );
}

When you click the button, the onClick handler fires. Each button’s onClick prop was set to the handleClick function inside MyApp, so the code inside of it runs. That code calls setCount(count + 1), incrementing the count state variable. The new count value is passed as a prop to each button, so they all show the new value. This is called “lifting state up”. By moving state up, you’ve shared it between components.

Next Steps

By now, you know the basics of how to write React code!

Check out the Tutorial to put them into practice and build your first mini-app with React.