What is JSX ?
官方定义：JSX 是 react.createElement(components, props, …children) 函数的语法糖
Click Me
上方函数会被编译为下方代码：
React.createElement(
Mybutton,
{color: ‘blue’, shadowSize: 2},
‘Click Me’
)
打开 React 的源码，蓝狮官网找到 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，我们可以看到 createElement 函数接收三个参数: type(标签类型)、config(标签中的属性)、children(子标签)
但是，在编写 JSX 的时候，一个标签通常会包含多个子标签，那么该函数是如何接收这些子标签呢 ?
我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
我们知道，arguments 对象中包含有所有参数，那么 childrenLength 就是排除前两个参数之外剩余参数的个数
若 childrenLength 为 1，则只有一个子元素（可以是文字也可以是新的JSX） 若 childrenLength 大于 1，则创建一个长度为 childrenLength 的数组，利用 for 循环将arguments 中的对象添加到数组中
简化版 React.createElement
ReactElement 对象定义：
function ReactElement(type, key, props) {
return {
$$typeof: Symbol.for(‘react.element’),
type,
key,
props
}
}
createElement() 函数实现：
function createElement(type, config, children) {
const props = {};
if (config) {
// 将 config 中的键值对添加到 props 中
for (propName in config) {
if (hasOwnProperty.call(config, propName)) {
props[propName] = config[propName];
}
}
}
const childrenLength = arguments.length – 2;
// 多个children使用数组的形式
if (childrenLength === 1) {
props.children = children;
} else if (childrenLength > 1) {
const childArray = Array(childrenLength);
for (let i = 0; i < childrenLength; i++) {
childArray[i] = arguments[2 + i];
}
props.children = childArray;
}
return ReactElement(type, null, props);
}
当仅有一个 dom 节点时，蓝狮注册调用 createElement 函数：
let a = createElement(
‘div’,
{width:’20px’, height: ’20px’},
)
打印出a，其结果为：
{
‘$$typeof’: Symbol(react.element),
type: ‘div’,
key: null,
props: { width: ’20px’, height: ’20px’ }
}
由于并没有子元素，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，其结果为：
{
‘$$typeof’: Symbol(react.element),
type: ‘div’,
key: null,
props: { width: ’20px’, height: ’20px’, children: [ [Object], [Object] ] }
}
可以看到，当含有多个子元素时，children 以数组的形式存储这些子元素。
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