Javascript for-in for-of

for-of是ES6引进的新语法，for-in循环是遍历对象的属性，for-of循环是遍历实现iterator接口的成员

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

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

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

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

通过上面的例子可以看出，for-of会忽略不可迭代的属性