摘要：對象設置初始狀態默認為等待狀態用數組來保存成功函數設置為成功狀態設置為失敗狀態調用回調函數增加方法根據狀態執行成功失敗方法執行成功方法執行失敗方法將成功失敗方法保存在數組里

promise的意思是承諾。承諾理解為某個時候一些條件滿足后，會兌現一件事情。

//為了方便理解我編一個小故事
//先假裝我有一個女朋友
//她承諾如果她爸媽不回來就給我就可以去幫她修電腦 否則...
let promise = new Promise(function (success,fail) {
    //她說三秒后會告訴我今晚爸媽回不回來
    let herParentBack = Math.random() > 0.5 ? true : false;
    setTimeout(function(){
        if(herParentBack){
            success("我馬上過來來幫你修你電腦");
        }else{
            fail("你要早點休息");
        }
    },3000);

});
 
 //第一個函數接收sucess函數傳來的成功信息
 //第二個函數接收fail函數傳來的失敗信息

promise.then(function (sucessMsg) {
    console.log(sucessMsg);
},function (fail) {
    console.log(failMsg);
})
//我馬上過來幫你修電腦

new Promise會傳入一個回調函數，會伴著對象創建被立即調用。
這個function是承諾的主體內容。一般我們會做一些判斷或者異步請求。（在這里要等女友回消息，我想好怎么回答她。）

promise.then方法在promise執行了success或fail后會執行對應的成功失敗方法。 （這里理解為吧想好告訴她）

根據promis規范promise Promise/A

Promise設置了三個狀態，"pending"、"resolved"、"rejected"。

//Promise對象設置初始狀態
function Promise(callback) {
    var self = this;
    //默認為等待狀態
    self.status = "pending";
    self.value = undefined;
    self.reason = undefined;
    //用數組來保存成功函數
    self.onResolvedCallBacks = [];
    self.onRejectedCallbacks = [];

    function resolve(value){
        if(self.status === "pending"){
            //設置為成功狀態
            self.status = "resolved";
            self.value = value;
            self.onResolvedCallBacks.forEach(item=>item(self.value));
        }
    }
    function reject(reason) {
        if(self.status === "pending"){
            //設置為失敗狀態
            self.status = "rejected";
            self.reason = reason;
            self.onRejectedCallbacks.forEach(item=>item(self.reason));
        }
    }

    callback(resolve,reject);
    //調用Promise回調函數
}

module.exports = Promise;

增加Promise.then方法

//根據Promise狀態執行成功失敗方法
Promise.prototype.then = function (onFulfilled,onRejected) {
    let self = this;
    onFulfilled = typeof onFulfilled == "function"?onFulfilled:function(value){return value};
  onReject = typeof onReject=="function"?onReject:function(reason){throw reason;}
    if(self.status === "resolved"){
        return new Promise(function (resolve,reject) {
            try {
                let x = onFullFilled(self.value);
                if(x instanceof Promise){
                    x.then(resolve,reject);
                }else{
                    resolve(x);
                }
            }
            catch(e) {
                reject(e);
            }
        })
        //執行成功方法
    }else if(self.status == "rejected"){
        return new Promise(function (resolve,reject) {
            try {
                let x = onRejected(self.reason);
                if(x instanceof Promise){
                    x.then(resolve,reject);
                }else{
                    resolve(x);
                }
            }
            catch(e) {
                reject(e)
            }
        })
        //執行失敗方法
    }
    if(self.status === "pending"){
        return new Promise(function (reslove,reject) {
            self.onResolvedCallBacks.push(function () {
                let x = onFullFilled(self.value);
                if(x instanceof Promise){
                    x.then(resolve,reject);
                }else{
                    resolve(x);
                }
            })
            self.onRejectedCallbacks.push(function () {
                let x = onRejected(self.reason);
                if(x instanceof Promise){
                    x.then(resolve,reject);
                }else{
                    resolve(x);
                }
            })
        })
        //將成功失敗方法保存在數組里
    }
}

Promise.all

Promise.all = all;
function all(iterable) {
  var self = this;
  if (!isArray(iterable)) {
    return this.reject(new TypeError("must be an array"));
  }

  var len = iterable.length;
  var called = false;
  if (!len) {
    return this.resolve([]);
  }

  var values = new Array(len);
  var resolved = 0;
  var i = -1;
  var promise = new this(INTERNAL);

  while (++i < len) {
    allResolver(iterable[i], i);
  }
  return promise;
  function allResolver(value, i) {
    self.resolve(value).then(resolveFromAll, function (error) {
      if (!called) {
        called = true;
        doReject(promise, error);
      }
    });
    function resolveFromAll(outValue) {
      values[i] = outValue;
      if (++resolved === len && !called) {
        called = true;
        doResolve(promise, values);
      }
    }
  }
}

Promise.race

Promise.race = function(iterable) {
  var self = this;
  if (!isArray(iterable)) {
    return this.reject(new TypeError("must be an array"));
  }

  var len = iterable.length;
  var called = false;
  if (!len) {
    return this.resolve([]);
  }

  var i = -1;
  var promise = new this(INTERNAL);

  while (++i < len) {
    resolver(iterable[i]);
  }
  return promise;
  function resolver(value) {
    self.resolve(value).then(function (response) {
      if (!called) {
        called = true;
        doResolve(promise, response);
      }
    }, function (error) {
      if (!called) {
        called = true;
        doReject(promise, error);
      }
    });
  }
}