for in 循环，对象与数组的不同之处

 

//数组

var arr=[1,2,9];

        function join(array,sign){

            var str = "";

            for(var item in array){

                console.log(typeof item);//string

                console.log(typeof array.length);//number

                console.log(item);//0 1 2  item 是String类型

                if(item==array.length-1){

                    sign = "";

                }

                str += array[item] + sign;

            }

            return str;

        }

        console.log(join(arr,"#"));

        join(arr);

//对象

        var obj={

        a:1,

        b:2,

        c:3,

        d:{

            a:1,b:2

        }

    }

    for(var prop in obj){

        // prop 就是obj下的所有属性名 key

        // 每循环一次就将属性名付给prop变量

        console.log(prop,obj[prop]);

    }