实现一个简易版的Promise

// 三个常量⽤于表示状态
const PENDING = 'pending'
const RESOLVED = 'resolved'
const REJECTED = 'rejected'
function MyPromise(fn) {
	const that = this
	this.state = PENDING
 	// value 变量⽤于保存 resolve 或者 reject 中传⼊的值
 	this.value = null
 	// ⽤于保存 then 中的回调，因为当执⾏完 Promise 时状态可能还是等待中，这时候应该把
 	that.resolvedCallbacks = []
 	that.rejectedCallbacks = []
 	function resolve(value) {
 		// ⾸先两个函数都得判断当前状态是否为等待中
 		if(that.state === PENDING) {
 			that.state = RESOLVED
 			that.value = value
			// 遍历回调数组并执⾏
			that.resolvedCallbacks.map(cb=>cb(that.value))
		 }
	 }
 	function reject(value) {
		if(that.state === PENDING) {
 			that.state = REJECTED
 			that.value = value
 			that.rejectedCallbacks.map(cb=>cb(that.value))
 		 }
	 }
 	// 完成以上两个函数以后，我们就该实现如何执⾏ Promise 中传⼊的函数了
 	try {
 		fn(resolve,reject)
 	}cach(e){
		reject(e)
 	} 
 }
// 最后我们来实现较为复杂的 then 函数
MyPromise.prototype.then = function(onFulfilled,onRejected){
 	const that = this
 	// 判断两个参数是否为函数类型，因为这两个参数是可选参数
 	onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : v=>v
 	onRejected = typeof onRejected === 'function' ? onRejected : e=>throw e
 	// 当状态不是等待态时，就去执⾏相对应的函数。如果状态是等待态的话，就往回调函数中 push 
 	if(this.state === PENDING) {
 		this.resolvedCallbacks.push(onFulfilled)
 		this.rejectedCallbacks.push(onRejected)
 	}
 	if(this.state === RESOLVED) {
 		onFulfilled(that.value)
	}
 	if(this.state === REJECTED) {
		onRejected(that.value)
 	}
 }