Promise/A+规范

1. 三种状态 pending || fullfiled(resolved) || rejected
2. 当处于pending状态的时候，可以转移到fullfiled(resolved)或者rejected状态
3. 当处于fullfiled(resolved)状态或者rejected状态的时候，就不可变

• 必须有一个then异步执行方法，then接收2个参数并且必须返回一个promise：

1. onFullfiled 用来接收promise成功的值
2. onRejected 用语接收promise失败的原因
3. promise.prototype.then = function(onFullfiled,onRejected){}

面试够用版

       function myPromise(constructor) {            let self = this;            self.status = 'pending';            self.value = undefined;            self.reason = undefined;            function resolve(value) {                if (self.status === 'pending') {                    self.value = value;                    self.status = 'resolved'                }            }            function reject(reason) {                if (self.status === 'pending') {                    self.reason = reason;                    self.status = 'rejected'                }            }            try {                constructor(resolve, reject)            } catch (e) {                reject(e)            }        }        myPromise.prototype.then = function(onFullfiled, onRejected) {            let self = this;            switch (self.status) {                case 'resolved':                    onFullfiled(self.value)                    break;                case 'rejected':                    onRejected(self.reason)                    break;                default:            }        }        var p = new myPromise((resolve, reject) => {            resolve(1)        })        p.then((x) => {            console.log(x)        })