for与forEach各自的用途

            var arr1 = new Array();
            var arr2 = new Array();
            arr1[0] = ‘第一个‘;
            arr1[1] = ‘第二个‘;
            arr2[‘name‘] = ‘姓名‘;
            arr2[‘age‘] = ‘年龄‘;
            
            for (var i = 0;i < arr1.length; i++) {
                console.log(i);   //0 , 1
                console.log(arr1[i]);   //第一个  , 第二个
            }
            
            for (var key in arr2) {
                console.log(arr2[key]);   //name , age
                console.log(arr2[key]);   //姓名  , 年龄
            }
            
            var obj = new Object();
            obj.name = "许文祥" ;
            obj.age = "22";
            for (var key in obj) {
                console.log(key); //name  ,   age
                console.log(obj[key])  //许文祥   ,  22
            }