for in是ES5标准，遍历key. for of是ES6标准，遍历value.
for (
var key
in arr){
console.log(arr[key]);
}
for (
var value
of arr){
console.log(value);
}
一个比较神奇的例子：
Object.prototype.objCustom =
function () {};
Array.prototype.arrCustom =
function () {};
let iterable = [
3,
5,
7];
iterable.foo =
"hello";
for (
let i
in iterable) {
console.log(i);
}
for (
let i of iterable) {
console.log(i);
