What is JSX ?
官网定义：JSX 是 React.createElement(components, props, ...children) 函数的语法糖
<Mybutton color="blue" shadowSize={2}>Click Me</Mybutton>上方函数会被编译为下方代码：
React.createElement( Mybutton, {color: 'blue', shadowSize: 2}, 'Click Me')关上 React 的源码，找到 createElement 函数:
function createElement(type, config, children) { var propName; // Reserved names are extracted var props = {}; var key = null; var ref = null; var self = null; var source = null; if (config != null) { if (hasValidRef(config)) { ref = config.ref; { warnIfStringRefCannotBeAutoConverted(config); } } if (hasValidKey(config)) { { 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. var childrenLength = arguments.length - 2; if (childrenLength === 1) { props.children = children; } else if (childrenLength > 1) { var childArray = Array(childrenLength); for (var i = 0; i < childrenLength; i++) { childArray[i] = arguments[i + 2]; } { if (Object.freeze) { Object.freeze(childArray); } } props.children = childArray; } // Resolve default props if (type && type.defaultProps) { var defaultProps = type.defaultProps; for (propName in defaultProps) { if (props[propName] === undefined) { props[propName] = defaultProps[propName]; } } } { if (key || ref) { var 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);}从源码中，咱们能够看到 createElement 函数接管三个参数: type(标签类型)、config(标签中的属性)、children(子标签)
然而，在编写 JSX 的时候，一个标签通常会蕴含多个子标签，那么该函数是如何接管这些子标签呢 ?
咱们关注源码的这一部分:
var childrenLength = arguments.length - 2; if (childrenLength === 1) { props.children = children; } else if (childrenLength > 1) { var childArray = Array(childrenLength); for (var i = 0; i < childrenLength; i++) { childArray[i] = arguments[i + 2]; } { if (Object.freeze) { Object.freeze(childArray); } } props.children = childArray; } // Resolve default props咱们晓得，arguments 对象中蕴含有所有参数，那么 childrenLength 就是排除前两个参数之外残余参数的个数
若 childrenLength 为 1，则只有一个子元素（能够是文字也能够是新的JSX） 若 childrenLength 大于 1，则创立一个长度为 childrenLength 的数组，利用 for 循环将arguments 中的对象增加到数组中
简化版 React.createElement
ReactElement 对象定义：
function ReactElement(type, key, props) { return { $$typeof: Symbol.for('react.element'), type, key, props }}createElement() 函数实现：
function createElement(type, config, children) { const props = {}; if (config) { // 将 config 中的键值对增加到 props 中 for (propName in config) { if (hasOwnProperty.call(config, propName)) { props[propName] = config[propName]; } } } const childrenLength = arguments.length - 2; // 多个children应用数组的模式 if (childrenLength === 1) { props.children = children; } else if (childrenLength > 1) { const childArray = Array(childrenLength); for (let i = 0; i < childrenLength; i++) { childArray[i] = arguments[2 + i]; } props.children = childArray; } return ReactElement(type, null, props);}当仅有一个 DOM 节点时，调用 createElement 函数：
let a = createElement( 'div', {width:'20px', height: '20px'},)打印出a，其后果为：
{ '$$typeof': Symbol(react.element), type: 'div', key: null, props: { width: '20px', height: '20px' }}因为并没有子元素，所以 props 中没有 children 属性
当父节点含有多个子节点时：
let a = createElement( 'div', {width:'20px', height: '20px'}, createElement( 'p' ), createElement( 'a' ),)打印出a，其后果为：
{ '$$typeof': Symbol(react.element), type: 'div', key: null, props: { width: '20px', height: '20px', children: [ [Object], [Object] ] }}能够看到，当含有多个子元素时，children 以数组的模式存储这些子元素。