https://www.cnblogs.com/yangshifu/p/7377102.html#for-of
https://www.cnblogs.com/lvmh/p/6104397.html
第二條連結裡有分析不同遍歷方式的效率

 var arr = ["name","age","sex"];
        var obj ={
            name: "gong",
            sex: "male",
            age: 18,
            hobby:function(){
                console.log("play footbale")
            }
        }
``
        // for-in遍歷陣列和和物件
        for(key in obj){
            console.log("key:"+key);//name sex age hobby
            console.log("value:" + obj[key]);//gong male 18 footable
        }

        //forEach遍歷陣列，三個引數依次是陣列元素、索引、陣列本身
        arr.forEach(function(value,index,array){
            console.log("arr:"+value+" "+index + array);
        })

        //Object.keys()返回一個數組
        let arr2 = Object.keys(arr);
        console.log("arr2:"+arr2)//(0,1,2)陣列的索引組成的陣列物件；
        let arr3 = Object.keys(obj);
        console.log("arr3:"+arr3);//(name,sex,age,hobby)//物件的屬性名
        組成的陣列物件；