摘要：執(zhí)行的時候呢即可如何處理鏈式的且保證順序每個后面鏈一個對象該對象包含子三個屬性當父狀態(tài)改變完畢執(zhí)行完相應的的時候呢，拿到子在等待這個子狀態(tài)改變，在執(zhí)行相應的。

setTimeout(function(){
    var a=100;
    console.log(a);
    setTimeout(function () {
        var b=200;
        console.log(b)
        setTimeout(function () {
            var c=300;
            console.log(c)
        }, 1000);
    }, 1000);
},1000);   

 new Promise(function (resolve, reject) {
    setTimeout(function () {
        var a=100;
        resolve(a);
    }, 1000);
}).then(function (res) {
    console.log(res);
    return new Promise(function (resolve, reject) {
        setTimeout(function () {
            var b=200;
            resolve(b);
        }, 1000);
    })
}).then(function (res) {
    console.log(res);
    return new Promise(function (resolve, reject) {
        setTimeout(function () {
            var c=300
            resolve(c);
        }, 1000);
    })
}).then(function (res) {
        console.log(res);
    }
    )

讓a,b,c的值能在then里面的回調(diào)接收到

在連續(xù)調(diào)用異步，如何確保異步函數(shù)的執(zhí)行順序

將resolve/reject函數(shù)和onfulfiled/onrejected放入同一個對象(promise對象)里面,resolve/reject的時候?qū)alue設(shè)置this.value=xxx。onfulfiled/onrejected執(zhí)行的時候呢,onfulfiled(this.value)即可

每個promise后面鏈一個對象該對象包含onfulfiled,onfulfiled,子promise三個屬性.

當父promise 狀態(tài)改變完畢,執(zhí)行完相應的onfulfiled/onfulfiled的時候呢，拿到子promise,在等待這個子promise狀態(tài)改變，在執(zhí)行相應的onfulfiled/onfulfiled。依次循環(huán)直到當前promise沒有子promise

            /*
            我們要滿足狀態(tài)只能三種狀態(tài)：PENDING,FULFILLED,REJECTED三種狀態(tài)，且狀態(tài)只能由PENDING=>FULFILLED,或者PENDING=>REJECTED
            */
            var PENDING = 0;
            var FULFILLED = 1;
            var REJECTED = 2;
            /*
            value狀態(tài)為執(zhí)行成功事件的入?yún)ⅲ琩eferreds保存著狀態(tài)改變之后的需要處理的函數(shù)以及promise子節(jié)點，構(gòu)造函數(shù)里面應該包含這三個屬性的初始化
             */
            function Promise(callback) {
                this.status = PENDING;
                this.value = null;
                this.defferd = [];
                setTimeout(callback.bind(this, this.resolve.bind(this), this.reject.bind(this)), 0);
            }
            
            Promise.prototype = {
                constructor: Promise,
                //觸發(fā)改變promise狀態(tài)到FULFILLED
                resolve: function (result) {
                    this.status = FULFILLED;
                    this.value = result;
                    this.done();
                },
                //觸發(fā)改變promise狀態(tài)到REJECTED
                reject: function (error) {
                    this.status = REJECTED;
                    this.value = error;
                },
                //處理defferd
                handle: function (fn) {
                    if (!fn) {
                        return;
                    }
                    var value = this.value;
                    var t = this.status;
                    var p;
                    if (t == PENDING) {
                         this.defferd.push(fn);
                    } else {
                        if (t == FULFILLED && typeof fn.onfulfiled == "function") {
                            p = fn.onfulfiled(value);
                        }
                        if (t == REJECTED && typeof fn.onrejected == "function") {
                            p = fn.onrejected(value);
                        }
                    var promise = fn.promise;
                    if (promise) {
                        if (p && p.constructor == Promise) {
                            p.defferd = promise.defferd;
                        } else {
                            p = this;
                            p.defferd = promise.defferd;
                            this.done();
                        }
                    }
                    }
                },
                //觸發(fā)promise defferd里面需要執(zhí)行的函數(shù)
                done: function () {
                    var status = this.status;
                    if (status == PENDING) {
                        return;
                    }
                    var defferd = this.defferd;
                    for (var i = 0; i < defferd.length; i++) {
                        this.handle(defferd[i]);
                    }
                },
                /*儲存then函數(shù)里面的事件
                返回promise對象
                defferd函數(shù)當前promise對象里面
                */
                then: function (success, fail) {
                   var o = {
                        onfulfiled: success,
                        onrejected: fail
                    };
                    var status = this.status;
                    o.promise = new this.constructor(function () {
            
                    });
                    if (status == PENDING) {
                        this.defferd.push(o);
                    } else if (status == FULFILLED || status == REJECTED) {
                        this.handle(o);
                    }
                    return o.promise;
                }
            };

v8官方實現(xiàn)

github