JavaScript —— 关于for in 与 for of 的区别

for in是ES5标准，遍历key，遍历的是数组的索引（即键名）；

for of是ES6标准，遍历value，遍历的是数组元素值；

Object.prototype.objCustom = function () {}; 
Array.prototype.arrCustom = function () {};

let iterable = [3, 5, 7];
iterable.foo = "hello";

for (let i in iterable) {
  console.log(i);
}
 //  0, 1, 2, "foo", "arrCustom", "objCustom"

for (let i of iterable) {
  console.log(i);
｝
  // 3, 5, 7