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)
        })