ReactElement

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React 同时被 2 个专栏收录

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本文是React系列的第二部分，详细解析React.createElement的过程。它首先处理内置的props，然后处理剩余的配置属性，处理子节点，并根据需要赋予默认值。ReactElement的$$typeof标记其为React元素，type字段则区分不同类型的组件，包括原生DOM标签、类组件、函数组件以及React内置组件如Fragment和Suspense。

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React系列(2) ReactElement

react/src/ReactElement.js

React.createElement做了什么

先判断config中内置props（key,ref,self,source）存在且有效，则赋值给新声明的变量
再将config中非内置的props 给新声明的props（key和ref不会跟其他config中的变量一起被处理，而是单独作为变量出现在ReactElement上。）
判断如果孩子节点为一个，则赋值给props.children。如果孩子节点有多个，则将其以数组的形式赋值给props.children
如果存在默认值，且存在默认值的属性在props数组中为undefined，则将该属性赋值为默认值。（为null不赋值为默认值）
返回一个ReactElement对象

export function createElement(type, config, children) {
  let propName;

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

  let key = null;
  let ref = null;
  let self = null; // 用于dev
  let source = null; // 用于dev

  if (config != null) { // 处理config中的属性
    if (hasValidRef(config)) {
      // config里存在合理的ref就将其放入之前声明的ref变量
      ref = config.ref;
    }
    if (hasValidKey(config)) {
      // config里存在合理的key就将其放入之前声明的key变量
      key = '' + config.key;
    }

    self = config.__self === undefined ? null : config.__self;
    // config.__self存在就放入self 用于dev
    source = config.__source === undefined ? null : config.__source;
    // config.__source存在就放入source 用于dev

    // Remaining properties are added to a new props object
    for (propName in config) {
      if (
        hasOwnProperty.call(config, propName) &&
        !RESERVED_PROPS.hasOwnProperty(propName)
      ) {
        // 如果config里有props并且RESERVED_PROPS里没有，就将其放到props对象里
        /*
          const RESERVED_PROPS = {
            key: true,
            ref: true,
            __self: true,
            __source: true,
          };
        */
        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;
  // 除去type 和 config 剩下的是children
  if (childrenLength === 1) {
    // children的长度为1时props的children就是传入的最后一个参数children
    props.children = children;
  } else if (childrenLength > 1) {
    // children的长度大于1时，说明有多个子节点，就生成一个数组childArray，将所有children放入childArray里
    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 赋值为 childArray
    props.children = childArray;
  }

  // Resolve default props
  // 如果是继承于React.Component的class Comp时，可以设置defaultProps默认值。
  if (type && type.defaultProps) { // 如果组件存在并存在默认值
    const defaultProps = type.defaultProps; // 定义defaultProps为默认值
    for (propName in defaultProps) {
      if (props[propName] === undefined) { // props[propName]为null时 不使用默认值
        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);
      }
    }
  }
  // 返回一个ReactElement
  return ReactElement(
    type,
    key,
    ref,
    self,
    source,
    ReactCurrentOwner.current,
    props,
  );
}

ReactElement的内容

const ReactElement = function(type, key, ref, self, source, owner, props) {
  const element = {
    // This tag allows us to uniquely identify this as a React Element
    $$typeof: REACT_ELEMENT_TYPE,
    // $$typeof表示element的类型,通过creatElement创建的都是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;
};