es6 --- > 手写一个promise

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 === "rejected") {
        onFulfilled(self.value);
    }
    // 失败
    if (this.status === "resolved") {
        onRejected(self.reason);
    }
    // padding
    if (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