Javascript for-in and built-in properties

Question

Why are the built-in properties on javascript objects not iterated through when using the for-in control block, while user-defined properties are?

For example:

var y = 'car';
for (var j in y)
{
    console.log(j);
}

Will print:

0
1
2

Even though String.prototype has properties for length, replace, substring, etc.

If extending the prototype, however, any new properties are iterated over:

String.prototype.foo = 7;
var y = 'car';
for (var j in y)
{
    console.log(j);
}

Will print:

0
1
2
foo

Answer 1

Basically, the built-in properties are marked as "non-enumerable" internally. This means, naturally, they aren't enumerated.

Edit: as Andy kindly pointed out, you can set enumerable : false in the current version of JavaScript using defineProperty. However, this seems to not be supported Opera at all; IE 8 only supports it on DOM objects and Safari only supports it on non-DOM objects (defineProperty on MDN (look towards bottom of file for browser support)).

All this cross-browser fun means that you probably shouldn't rely on this behavior if you need consistent browser support.

Here is how you could define a non-enumerable property:

Object.defineProperty(String.prototype, "foo", {value : 7, enumerable : false});

You don't actually need to include enumerable : false—it is false by default when calling defineProperty.

Answer 2

There is propertyIsEnumerable method, which returns a boolean indicating if the internal ECMAScript DontEnum attribute is set (also note the terrible disagreements amongst implementations). As said above, you cannot set DontEnum, but only query via propertyIsEnumerable.