react jsx

const rootElement =
  React.createElement('div', {},
    React.createElement('h1', {style: {color: 'red'}}, 'The world is yours'),
    React.createElement('p', {}, 'Say hello to my little friend')
  )

ReactDOM.render(rootElement, document.getElementById('app'))

const RootElement = (
  <div>
    <h1 style={{color:'red'}}>The world is yours</h1>
    <p>Say hello to my little friend</p>
  </div>
);

ReactDOM.render(RootElement, document.getElementById('app'));

// type可以是react组件，也可以是span之类的标签字符串
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 (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
    // 把config传进去的对象的键值对添加到props中，注意剔除key和ref
    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.
  // children 可能是1个或者多个元素
  // 如果是一个则直接把子元素赋值给children属性
  // 如果是多个元素，则先把元素转换为数组，然后赋值给children属性
  // 所以我们在使用this.props.children属性的时候，可能是一个对象，也可能一个数组，也可能是undefined
  // 绝不能当成一个数组直接使用
  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];
    }
    props.children = childArray;
  }

  // Resolve default props
  // 把defaultProps添加到props中，注意检查时候已经包含对应的props了
  if (type && type.defaultProps) {
    const defaultProps = type.defaultProps;
    for (propName in defaultProps) {
      if (props[propName] === undefined) {
        props[propName] = defaultProps[propName];
      }
    }
  }
  return ReactElement(
    type,
    key,
    ref,
    self,
    source,
    ReactCurrentOwner.current,
    props,
  );
}

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,

    // 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,
  };
  return element;
};

<div>
  <span>foo</span>
  bar
  <span>bar</span>
</div>

<div>
  <span>foo</span>
  {' '}
  bar
  {' '}
  <span>bar</span>
</div>

const foo = {id : 'bar'};
return <div {...foo} />