javascript - custom object prototype method turns into index in array loop

Question

Here's something weird. I have defined a custom prototype method for object called getSize:

if (!Object.prototype.getSize) {
    Object.prototype.getSize = function() {
        var size = 0, key;
        for (key in this) {
            if (this.hasOwnProperty(key)) size++;
        }
        return size;
    };
}

It gives you the size of an object. Pretty simple so far, right? Here's where I get confused:

var l = ["foo", "bar"]
for (var i in l) {
    console.log(i);
}

results are:

0
1
getSize

Why is getSize there???

edit I know that arrays in javascript are objects too. My question is why is the method turned into an index, instead of remaining a method. It doesn't make any sense to me...

elclanrs · Accepted Answer · 2014-09-29 08:35:17Z

4

Because getSize is an enumerable property, you could define it to be non-enumerable:

Object.defineProperty(Object.prototype, 'getSize', {
  enumerable: false,
  value: function() {
    var size = 0, key;
    for (key in this) {
      if (this.hasOwnProperty(key)) size++;
    }
    return size;
  }
});

But you could also do:

Object.keys(object).length;

answered Sep 29, 2014 at 8:35

elclanrs

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4 Comments

yuvi Over a year ago

I didn't know methods have an enumerable definition. This is very cool, thanks for the answer! I'll use defineProperty from now on

yuvi Over a year ago

btw, Object.keys(l) returns ['0', '1'] even if I don't set enumerable: false - how come?

Dagg Nabbit Over a year ago

@yuvi because Object.keys doesn't use the prototype chain. All of this stuff is well documented on MDN.

yuvi Over a year ago

@DaggNabbit thanks. MDN is well documented, but that doesn't mean you always know where to look or what this and that means. Obviously, Object.keys doesn't go through the same process as for...in, because then you would get the same results. I'm wondering what that process is though (or does that happen on a very low level of code?)

antyrat · Accepted Answer · 2014-09-29 08:32:40Z

3

Because ["foo", "bar"] is an Object in JavaScript too. Try to run typeof ["foo", "bar"] in console. It will return "object" to you. So you have extended Arrays methods too. Once you have done this you will need to check for hasOwnProperty in all for..in iterations as well.

This happens to all global objects in JavaScript

answered Sep 29, 2014 at 8:32

antyrat

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2 Comments

yuvi Over a year ago

I know that arrays are objects too. But why doesn't it inherit the method, why is it turned into an index? That makes no sense

antyrat Over a year ago

elclanrs answer to this. You can read more about enumerables at MDN developer.mozilla.org/en-US/docs/Web/JavaScript/…

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