重寫了JavaScript數組Array的幾個原生方法

使用原型，重寫了數組的push、pop、shift、unshift幾個簡單的方法。
我在這拋磚引玉，期待大神更多對象的其餘的方法擴展

Array.prototype.push = function(){

                //this: 1,2,3
                //arguments: 4,5,6

                for (var i = 0; i < arguments.length; i++) {
                    this[this.length] = arguments[i];
                }
                return this.length;

            }

Array.prototype.pop = function(num){

                if (num) {
                    this.length = this.length-num
                }else{
                    this.length = this.length-1;
                }
                
                return this.length;
                
            }

Array.prototype.shift = function(){

    
                for (var i = 1; i < this.length; i++) {
                    this[i-1] =this[i];
                }
                this.length = this.length -1;
                return this.length;
                
            }

Array.prototype.unshift = function(){


                for (var i = this.length-arguments.length-1; i >= 0; i--) {   
                    this[arguments.length+i] = this[i];
                }

                for (var j = 0; j < arguments.length; j++) {
                    this[j] =arguments[j];
                }
                return this.length;
                
            }

            var arr = new Array();
            arr = [1,2,3];
            arr.push(4,5,6);
            alert(arr);
            arr.pop();
            alert(arr);
            arr.shift();
            alert(arr);//2,3,4,5
            arr.unshift("a","b");
            alert(arr);//a,b,2,3,4,5