Looping through javascript object converts keys to string [duplicate]

Question

When I loop through an object in Javascript to extract its keys, why do the keys convert to string when they were intended to be integers ?

obj = {1:'a', 2:'b'};
arr = [];
for(var key in obj){
  if (obj.hasOwnProperty(key)){
    arr.push(key);
  }
}

Now arr is [ "1", "2" ] instead of [1, 2]

Answer 1

It's not the loop that is converting the keys; it's the fact that keys can only be strings. You cannot have any other type of key. If your key isn't a string, JavaScript will convert it to a string when you use it as a property name.

Consider:

key = {
  toString: function () { return "Blah" }
};

myObject = {}

myObject[key] = "value";

// writes "Blah"
document.write(Object.keys(myObject));

Note that if you didn't provide a toString, the keys would have been the string "[object Object]".