一个promise实例:

var getJSON = function(url) {var promise = new Promise(function(resolve, reject) {// XHR对象发现ajax请求var client = new XMLHttpReqeust();client.open("GET", url);client.onreadystatechange = handler;client.responseType = "json";client.setRequestHeader("Accept", "application/json");client.send();function hanlder(){if (this.readyState !== 4) {return;}if (this.status === 200) {resolve(this.response);} else {reject(new Error(this.statusText));}};});return promise;
}
getJSON("/posts.json").then(function(json) {console.log("Content: " + json);
}, function(error) {console.error("出错了!', error);
});

上述在调用new Promise时,传递了一个函数function(resolve, reject),此函数在规范中被称为exector执行器.
所以,首先:需传入一个exector执行器:

function Promise(exector) {//...
}

确定Promise内部exector的作用:
可以看出原生的exector中传入了2个参数,resolve和reject.第一个代表成功,第二个代表失败.

function Promise(exector) {let self = this;this.value = undefined;this.reason = reason;// 执行成功function resolve(value) {self.value = value;}// 执行失败function reject(reason) {self.reason = reason;}exector(resolve, reject);
}

添加状态:
promise的执行过程是不可逆,因此需要一个status来记录其状态,初始时为padding,成功了为resolve,失败了为reject

function Promise(exector) {let self = this;this.status = "padding";this.value = value;this.reason = reason;// 成功function resolve(value) {if(self.status === "padding") {self.value = value;self.status = "resolved";}}// 失败function  reject(reason) {if(self.status === "padding") {self.reason = reason;self.status = "reject";}}// 对异常的处理try {exector(resolve, reject);} catch(e) {reject(e)}
}

原型上添加then方法:
注意到Promise实例的使用是p.then(onFulfilled, onRejected),可以在前面定义的Promise的原型上添加then方法.

Promise.prototype.then = function(onFulfilled, onRejected) {let self = this;if(this.status === "resolved") {onFulfilled(self.value);}if(this.status === "rejected") {onRejected(self.value);}
}

新增2个数组完成异步操作:
上面编写的Promise的调用是同步的,但一般都是异步使用Promise,故需在对Promise和其原型进行一定的修改.
如果异步,则处于padding状态,将回调函数fn保存在数组中!

function Promise(exector) {let self = this;this.status = "padding";this.value = undefined;this.reason = undefined;// 存储then中成功的回调函数this.onResolvedCallbacks = [];// 存储then中失败的回调函数this.onRejectedCallbacks = [];// 成功执行function resolve(value) {if(self.status === "padding") {self.value = value;self.status = "resolved";// 成功后遍历then中成功的所有回调函数self.onResolvedCallbacks.forEach(fn => fn());}}// 失败执行function reject(reason) {if(self.status === "padding") {self.reason = reason;self.status = "rejected";// 失败后遍历then中失败的所有回调函数self.onRejectedCallbacks.forEach(fn => fn());}}// 对异常进行处理try {exector(resolve, reject);} catch(e) {reject(e)}
}// Promise.prototype.then
Promise.prototype.then = function(onFulfilled, onRejected) {let self = this;// 成功if (this.status === "resolved") {onFulfilled(self.value);}// 失败if (this.status === "rejected") {onRejected(self.reason);}// paddingif (this.status === "padding") {// 推进onResolvedCallbacks数组this.onResolvedCallbacks.push( () => {onFulfilled(self.value);})this.onRejectedCallbacks.push( () => {onRejected(self.reason);})}
}

参考《ES6标准入门》(第3版) P276
参考 https://www.jianshu.com/p/4b126518c26d