36.一个promise

　　var Promise = function(){
　　　　this.doneList = [];
　　　　this.failList = [];
　　　　this.state = 'pending';
　　};

Promise.prototype = {
        constructor:'Promise',
        resolve:function(){
            this.state = 'resolve';
            var list = this.doneList; 
             for(var i = 0, len = list.length; i < len; i++) {  
                list[0].call(this);  
                list.shift();  
            }
        },  
        reject: function() {  
            this.state = 'rejected';  
            var list = this.failList;  
            for(var i = 0, len = list.length; i < len; i++){  
                list[0].call(this);  
                list.shift();  
            }  
        },  
        done: function(func) {  
            if(typeof func === 'function') {  
                this.doneList.push(func);  
            }  
            return this;  
        },  
        fail: function(func) {  
            if(typeof func === 'function') {  
                this.failList.push(func);  
            }  
            return this;  
        },  
        then: function(doneFn, failFn) {  
            this.done(doneFn).fail(failFn);  
            return this;  
        },  
        always: function(fn) {  
            this.done(fn).fail(fn);  
            return this;  
        }
    };