Javascript for-in for-of

for-of是ES6引進的新語法，for-in循環是遍歷對象的屬性，for-of循環是遍歷實現iterator接口的成員

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
}

通過上面的例子可以看出，for-of會忽略不可迭代的屬性