ReactNote03：JSX本质及源码解析

JSX 的本质

实际上，jsx 仅仅只是 React.createElement(component, props, …children) 函数的语法糖。所有的 jsx 语法都会被转换为 React.createElement 语法的调用。

先来看一下几行 jsx 代码：

<div>
  <div>nihao</div>
  <h2>you are the best</h2>
  <ul>
    <li>1</li>
    <li>2</li>
  </ul>
</div>

那么它们转换成 js 代码会是什么样的呢？

React.createElement("div", null, React.createElement("div", null, "nihao"),
                        React.createElement("h2", null, "you are the best"),
                        React.createElement("ul", null,
                            React.createElement("li", null, "1"),
                            React.createElement("li", null, "2")))

好家伙，直接一个嵌套地狱，要是代码再多一点的话能把人看哭。还是 jsx 香

查看 React.createElement 的源码

源码位置：packages/react/src/ReactElement.js

/**
 * 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)) {
      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,
  );
}

createElement的三个参数：

• 1. type
  • 是当前 React 的类型，如果是标签元素，则用字符串表示，例如 “div”。如果是组件元素，则直接用组件名称
• 2. config
  • 所有 jsx 中的属性都在 config 中以对象的键值对存储
• 3. 存放标签中的内容，以数组的方式存储
  • 为什么是以数组的形式存储呢？
  • 因为 jsx 里面可以会传入很多的同级标签，createElement 并不是只能传入三个参数。为什么可以传入多个参数呢？这就要回到源码里面一探究竟了，毕竟源码之下，了无秘密。

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;
  }

这里就是上面 createElement 源码的一部分，也就是说 children 是一个数组，里面存储着第三个参数及以后的所有参数。