Javascript數組的「字符串」索引 & for…in 和 for…of的區別

一句話結論：js中的數組是沒有「字符串」索引的，形如array['b'] = someValue只是在array對象上添加了屬性。

原本有幾個例子，然而搜到了MDN的文檔，因此摘一點：

下面摘自MDN
Difference between for...of and for...in

The for...in loop will iterate over all enumerable properties of an object.

for in 循環會遍歷一個對象上面的全部enumerable屬性。

The for...of syntax is specific to collections, rather than all objects. It will iterate in this manner over the elements of any collection that has a [Symbol.iterator] property.

for of 語法是針對集合的，而不是全部的對象。它會遍歷定義了[Symbol.iterator]屬性的集合的全部元素。

The following example shows the difference between a for...of loop and a for...in loop.

MDN的例子以下：

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
}