es6 promise源码实现

摘要：执行的时候呢即可如何处理链式的且保证顺序每个后面链一个对象该对象包含子三个属性当父状态改变完毕执行完相应的的时候呢，拿到子在等待这个子状态改变，在执行相应的。

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);
    }
    )

让a,b,c的值能在then里面的回调接收到

在连续调用异步，如何确保异步函数的执行顺序

将resolve/reject函数和onfulfiled/onrejected放入同一个对象(promise对象)里面,resolve/reject的时候将value设置this.value=xxx。onfulfiled/onrejected执行的时候呢,onfulfiled(this.value)即可

每个promise后面链一个对象该对象包含onfulfiled,onfulfiled,子promise三个属性.

当父promise 状态改变完毕,执行完相应的onfulfiled/onfulfiled的时候呢，拿到子promise,在等待这个子promise状态改变，在执行相应的onfulfiled/onfulfiled。依次循环直到当前promise没有子promise

            /*
            我们要满足状态只能三种状态：PENDING,FULFILLED,REJECTED三种状态，且状态只能由PENDING=>FULFILLED,或者PENDING=>REJECTED
            */
            var PENDING = 0;
            var FULFILLED = 1;
            var REJECTED = 2;
            /*
            value状态为执行成功事件的入参，deferreds保存着状态改变之后的需要处理的函数以及promise子节点，构造函数里面应该包含这三个属性的初始化
             */
            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,
                //触发改变promise状态到FULFILLED
                resolve: function (result) {
                    this.status = FULFILLED;
                    this.value = result;
                    this.done();
                },
                //触发改变promise状态到REJECTED
                reject: function (error) {
                    this.status = REJECTED;
                    this.value = error;
                },
                //处理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();
                        }
                    }
                    }
                },
                //触发promise defferd里面需要执行的函数
                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]);
                    }
                },
                /*储存then函数里面的事件
                返回promise对象
                defferd函数当前promise对象里面
                */
                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官方实现

github