手写一个promise

promise是什么？

promise是一个解决回调地狱的函数，是一个强大的异步回调方案。es6将其弄可以直接使用！

我们知道promise中共有三种状态

pending 过渡态
fulfilled 完成态
rejected 失败态

function Promise(exector) {
            let self = this;
            //status表示一种状态
            let status = "pending";
            let value = undefined;
            let reason = undefined;
            //成功执行
            function resolve(value) {
                if (status == 'pending') {
                    self.value = value;
                    self.status = "resolve";
                }
            }
            //执行失败
            function reject(reason) {
                if (status == 'pending') {
                    self.reason = reason;
                    self.status = "reject"
                }
            }
            //对异常操作
            try {
                exector(resolve, reject)
            } catch (e) {
                reject(e)
            }
            //设置promise的then方法
            Promise.prototype.then = function(reject, resolve) {
                let self = this;
                if (this.status == 'resolve') {
                    reject(self.value)
                }
                if (this.status == 'reject') {
                    resolve(self.reason)
                }
            }
        }
        //new 一个promise  进行测试  
        let promise = new Promise((reject, resolve) => {
            resolve("return resolve");
        });
        promise.then(data => {
            console.log(`success${data}`);

        }, err => {
            console.log(`err${err}`);

        })

简单 的 promise就实现了