直接代码

// 声明函数
function MyPromise(fun) {this.state = 'Pending';    // 状态基this.value = '';   // 成功回调的默认值this.successFun = [];    // 成功回调函数数组this.errFun = [];   var _self = this;// 成功时候执行function resolve(value) {// 这里需要注意，判断返回值是不是一个新的promise,  如果是，需要把这个promise.then执行得到成功的值，//返回的值在继 续往下走if (value && value instance of MyPromise) {value.then(resolve)return}_self.state = 'Fulfilled'_self.value = valueif (_self.successFun.length > 0) {_self.successFun.forEach(cb => {cb(value)}) }};function reject(err) {_self.value = err_self.state = 'Rejected'_self.errFun.forEach(cb => {cb(err)}) };fun(resolve, reject);return _self;
}// 在原型对象上，声明then方法
MyPromise.prototype.then = function(success, err) {var _self = this;var promise2;// then 方法都是返回新promiseswitch(this.state) {case 'Fulfilled':promise2 = new MyPromise(function(resolve, reject) {resolve(success(_self.value))})breakcase 'Rejected':promise2 = new MyPromise(function(resolve, reject) {reject(err(_self.value))})breakcase 'Pending':promise2 = new MyPromise(function(resolve, reject) {_self.successFun.push(function() {resolve(success(_self.value))})_self.errFun.push(function() {reject(err(_self.value))})})break}return promise2
}// 调用
var p = new MyPromise(function(res, rej) {setTimeout(function(){res(2222)}, 1000)
})var a = p.then(function (result) {console.log(result)return new MyPromise(function(res, rej) {setTimeout(function() {res(3333)}, 2000)res(33333)})
})var b = a.then(function (result) {console.log(result)return 44444
})var c = b.then(function (result) {console.log(result)return 55555
})
复制代码

注意

• then的声明阶段是把新返回的promise的resolve方法的引用提交上一个promise成功回调里面等待执行
• 执行then的成功回调，得到上个reslove传来的值，处理得到的值传给下一个resovle(value)