理解 React 中的 JSX

[myLightLin]

JSX 是什么

引用 React 官网对 JSX 的定义：

JSX 是 JavaScript 的一种语法扩展，它和模板语言很接近，但是它充分具备 JavaScript 的能力

在 React 里，JSX 大概长这样：

function App(props) {
  return (
    <div>
      <div className="title">我是标题</div>
      <div className="content">我是内容</div>
    </div>
  )
}

上面这段代码，会经过 Babel 编译，然后转换为 React.createElement() 函数调用。所以接下来我们要探究 createElement 这个函数。

React.createElement

React.createElement 函数的源码如下：

/**
 * Create and return a new ReactElement of the given type.
 * See https://reactjs.org/docs/react-api.html#createelement
 */
export function createElement(type, config, children) {
  let propName;

  // Reserved names are extracted
  const props = {};

  let key = null;
  let ref = null;
  let self = null;
  let source = null;

  if (config != null) {
    if (hasValidRef(config)) {
      ref = config.ref;

      if (__DEV__) {
        warnIfStringRefCannotBeAutoConverted(config);
      }
    }
    if (hasValidKey(config)) {
      if (__DEV__) {
        checkKeyStringCoercion(config.key);
      }
      key = '' + config.key;
    }

    self = config.__self === undefined ? null : config.__self;
    source = config.__source === undefined ? null : config.__source;
    // Remaining properties are added to a new props object
    for (propName in config) {
      if (
        hasOwnProperty.call(config, propName) &&
        !RESERVED_PROPS.hasOwnProperty(propName)
      ) {
        props[propName] = config[propName];
      }
    }
  }

  // Children can be more than one argument, and those are transferred onto
  // the newly allocated props object.
  const childrenLength = arguments.length - 2;
  if (childrenLength === 1) {
    props.children = children;
  } else if (childrenLength > 1) {
    const childArray = Array(childrenLength);
    for (let i = 0; i < childrenLength; i++) {
      childArray[i] = arguments[i + 2];
    }
    if (__DEV__) {
      if (Object.freeze) {
        Object.freeze(childArray);
      }
    }
    props.children = childArray;
  }

  // Resolve default props
  if (type && type.defaultProps) {
    const defaultProps = type.defaultProps;
    for (propName in defaultProps) {
      if (props[propName] === undefined) {
        props[propName] = defaultProps[propName];
      }
    }
  }
  if (__DEV__) {
    if (key || ref) {
      const displayName =
        typeof type === 'function'
          ? type.displayName || type.name || 'Unknown'
          : type;
      if (key) {
        defineKeyPropWarningGetter(props, displayName);
      }
      if (ref) {
        defineRefPropWarningGetter(props, displayName);
      }
    }
  }
  return ReactElement(
    type,
    key,
    ref,
    self,
    source,
    ReactCurrentOwner.current,
    props,
  );
}

这个方法接收三个入参：

export function createElement(type, config, children)

• type: 传入的节点类型，比如 div, p 等，也可以是 React 组件
• config: 配置参数，比如组件的一些 props 等信息
• children: 嵌套的子组件内容

这个 createElement 函数主要做了如下工作：

1. 处理 key，ref，source，self 内部参数
2. 从 config 中提取可以放进 props 的参数
3. 通过 arguments.length - 2 判断是传入了一个子数组项还是多个，计算得到 props.children
4. 如果 type 传了 defaultProps，从中提取属性放入到 props 中
5. 发起 ReactElement 函数调用

ReactElement

接下来要接着追一下 ReactElement 函数，源码如下：

/**
 * Factory method to create a new React element. This no longer adheres to
 * the class pattern, so do not use new to call it. Also, instanceof check
 * will not work. Instead test $$typeof field against Symbol.for('react.element') to check
 * if something is a React Element.
 *
 * @param {*} type
 * @param {*} props
 * @param {*} key
 * @param {string|object} ref
 * @param {*} owner
 * @param {*} self A *temporary* helper to detect places where `this` is
 * different from the `owner` when React.createElement is called, so that we
 * can warn. We want to get rid of owner and replace string `ref`s with arrow
 * functions, and as long as `this` and owner are the same, there will be no
 * change in behavior.
 * @param {*} source An annotation object (added by a transpiler or otherwise)
 * indicating filename, line number, and/or other information.
 * @internal
 */
function ReactElement(type, key, ref, self, source, owner, props) {
  const element = {
    // This tag allows us to uniquely identify this as a React Element
    $$typeof: REACT_ELEMENT_TYPE,

    // Built-in properties that belong on the element
    type: type,
    key: key,
    ref: ref,
    props: props,

    // Record the component responsible for creating this element.
    _owner: owner,
  };

  if (__DEV__) {
    // The validation flag is currently mutative. We put it on
    // an external backing store so that we can freeze the whole object.
    // This can be replaced with a WeakMap once they are implemented in
    // commonly used development environments.
    element._store = {};

    // To make comparing ReactElements easier for testing purposes, we make
    // the validation flag non-enumerable (where possible, which should
    // include every environment we run tests in), so the test framework
    // ignores it.
    Object.defineProperty(element._store, 'validated', {
      configurable: false,
      enumerable: false,
      writable: true,
      value: false,
    });
    // self and source are DEV only properties.
    Object.defineProperty(element, '_self', {
      configurable: false,
      enumerable: false,
      writable: false,
      value: self,
    });
    // Two elements created in two different places should be considered
    // equal for testing purposes and therefore we hide it from enumeration.
    Object.defineProperty(element, '_source', {
      configurable: false,
      enumerable: false,
      writable: false,
      value: source,
    });
    if (Object.freeze) {
      Object.freeze(element.props);
      Object.freeze(element);
    }
  }

  return element;
}

这个 ReactElement 函数看起来非常简短，除去那些 DEV 环境的处理代码，它其实只做了一件事，就是返回下面这个 element 对象：

const element = {
    // This tag allows us to uniquely identify this as a React Element
    $$typeof: REACT_ELEMENT_TYPE,

    // Built-in properties that belong on the element
    type: type,
    key: key,
    ref: ref,
    props: props,

    // Record the component responsible for creating this element.
    _owner: owner,
  };

这就是一个 React 元素节点，到这里来，我们就可以知道 JSX 的真面目了：Babel 把我们写的 JSX 编译为 React.createElement 调用，然后这个函数调用后又返回了一个 ReactElement 元素实例，也就是虚拟 DOM 节点，那这样的虚拟节点是怎么转变为真实 DOM 的呢？这就是 ReactDOM.render 干的活了。

总结

• JSX 是 JavaScript 的扩展语法，它允许我们使用类 HTML 的语法来编写 UI 模板
• JSX 会经由 babel 编译为 React.createElement 调用，这个函数二次处理参数，然后返回一个 ReactElement 对象实例
• ReactElement 对象实例也就是虚拟 DOM 节点，它还要经过 ReactDOM.render 才能变成页面上的 DOM