高阶函数的应用：手写Promise源码（一）

按照promise/A+规范
1、promise有三种状态，pending，fulfilled，rejected， 且状态只能从pending变为fulfilled或从pending变为rejected，状态一旦发生变化就不能改变
2、Fulfilled：promise必须有一个不可变的终值value
3、Rejected：promise必须拥有一个不可变的据因reason
4、promise是具有then方法的对象或函数，该方法接受两个函数参数，分别为onFulfilled，onRejected，可以访问到终值和据因
5、promise是立即执行的

const PENDING = 'Pending'
const FULFILLED = 'Fulfilled'
const REJECTED = 'Rejected'
class Promise {
  // 高阶函数，函数的参数是函数，高阶函数可以实现参数的预置
  // 函数的参数中的函数为函数的声明
  // executor是立即执行的函数，参数被预置在了constructor中
  constructor(executor) {
    this.state = PENDING
    this.value = undefined         // 成功的结果
    this.reason = undefined        // 失败的理由
    
    const resolve = (value) => {
      if(this.state === PENDING) {
        this.state = FULFILLED
        this.value = value
      }
    }

    const reject = (reason) => {
      if(this.state === PENDING) {
        this.state = REJECTED
        this.reason = reason
      }
    }
   
    // 此处为executor函数的调用。参数传入resolve和reject两个函数
    try {
      executor(resolve, reject)
    } catch(err) {
      reject(err)
    }
  }
  then(onFulfilledCallback, onRejectedCallback) {
    onFulfilledCallback = typeof onFulfilledCallback === 'function' ? onFulfilledCallback : value => value
    onRejectedCallback = typeof onRejectedCallback === 'function' ? onRejectedCallback : reason => reason

    if(this.state === FULFILLED) {
   		onFulfilledCallback(this.value)
    }
    
    if(this.state === REJECTED) {
    	onRejectedCallback(this.reason)
    }
  }
}

new Promise((resolve, reject) => {
	resolve(1)
}).then((res) => {
	console.log(res)
})

执行结果：1