摘要：對(duì)象設(shè)置初始狀態(tài)默認(rèn)為等待狀態(tài)用數(shù)組來保存成功函數(shù)設(shè)置為成功狀態(tài)設(shè)置為失敗狀態(tài)調(diào)用回調(diào)函數(shù)增加方法根據(jù)狀態(tài)執(zhí)行成功失敗方法執(zhí)行成功方法執(zhí)行失敗方法將成功失敗方法保存在數(shù)組里

promise的意思是承諾。承諾理解為某個(gè)時(shí)候一些條件滿足后，會(huì)兌現(xiàn)一件事情。

//為了方便理解我編一個(gè)小故事
//先假裝我有一個(gè)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(huì)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(diǎn)休息");
        }
    },3000);

});
 
 //第一個(gè)函數(shù)接收sucess函數(shù)傳來的成功信息
 //第二個(gè)函數(shù)接收fail函數(shù)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(huì)傳入一個(gè)回調(diào)函數(shù)，會(huì)伴著對(duì)象創(chuàng)建被立即調(diào)用。
這個(gè)function是承諾的主體內(nèi)容。一般我們會(huì)做一些判斷或者異步請(qǐng)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(zhí)行了success或fail后會(huì)執(zhí)行對(duì)應(yīng)的成功失敗方法。 （這里理解為吧想好告訴她）

根據(jù)promis規(guī)范promise Promise/A

Promise設(shè)置了三個(gè)狀態(tài)，"pending"、"resolved"、"rejected"。

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

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

    callback(resolve,reject);
    //調(diào)用Promise回調(diào)函數(shù)
}

module.exports = Promise;

增加Promise.then方法

//根據(jù)Promise狀態(tài)執(zhí)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(zhí)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(zhí)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(shù)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);
      }
    });
  }
}