Promise原始碼分析

Promise原始碼分析
程式碼如下：

            /*
            我們要滿足狀態只能三種狀態：PENDING,FULFILLED,REJECTED三種狀態，且狀態只能由PENDING=>FULFILLED,或者PENDING=>REJECTED
            */
            var PENDING = 0;
            var FULFILLED = 1;
            var REJECTED = 2;
            /*
            value狀態為執行成功事件的入參，deferreds儲存著狀態改變之後的需要處理的函式以及promise子節點，建構函式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,
                //觸發改變promise狀態到FULFILLED
                resolve: function (result) {
                    this.status = FULFILLED;
                    this.value = result;
                    this.done();
                },
                //觸發改變promise狀態到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promise defferd裡面需要執行的函式
                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函式里面的事件
                返回promise物件
                defferd函式當前promise物件裡面
                */
                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;
                }
            };