摘要：執(zhí)行的時(shí)候呢即可如何處理鏈?zhǔn)降那冶ＷC順序每個(gè)后面鏈一個(gè)對(duì)象該對(duì)象包含子三個(gè)屬性當(dāng)父狀態(tài)改變完畢執(zhí)行完相應(yīng)的的時(shí)候呢，拿到子在等待這個(gè)子狀態(tài)改變，在執(zhí)行相應(yīng)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(gè)對(duì)象(promise對(duì)象)里面,resolve/reject的時(shí)候?qū)alue設(shè)置this.value=xxx。onfulfiled/onrejected執(zhí)行的時(shí)候呢,onfulfiled(this.value)即可

每個(gè)promise后面鏈一個(gè)對(duì)象該對(duì)象包含onfulfiled,onfulfiled,子promise三個(gè)屬性.

當(dāng)父promise 狀態(tài)改變完畢,執(zhí)行完相應(yīng)的onfulfiled/onfulfiled的時(shí)候呢，拿到子promise,在等待這個(gè)子promise狀態(tài)改變，在執(zhí)行相應(yīng)的onfulfiled/onfulfiled。依次循環(huán)直到當(dāng)前promise沒(méi)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(diǎn)，構(gòu)造函數(shù)里面應(yīng)該包含這三個(gè)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(chǔ)存then函數(shù)里面的事件
                返回promise對(duì)象
                defferd函數(shù)當(dāng)前promise對(duì)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(shí)現(xiàn)

github