for/for of/for in

for

for循环可以将代码执行指定的次数。

var arr = [1, 2, 3, 4];Array.prototype.name = "angela";arr.sex = "girl";for (var i=0;i<arr.length;i++){document.write(arr[i] + ","); // 返回 1,2,3,4}

for in

for in返回的是key，遍历整个对象的原型链（包括后添加的）。

var arr = [1, 2, 3, 4];Array.prototype.name = "angela";arr.sex = "girl";for (let index in arr){document.write(index + ","); // 返回 0,1,2,3,name,sex}

for of

for of返回的是value，只遍历数组、字符串等有迭代对象（数组下标）的集合。

var arr = [1, 2, 3, 4];Array.prototype.name = "angela";arr.sex = "girl";arr[5] = 6;for (let value in arr){document.write(value + ","); // 返回 1,2,3,4,undefined,6}