Why is native datatype properties not shown on their corresponding prototypes?

Question

For example, an the Array datatype has a function called pop(), which I suppose is added using:

Array.prototype.pop = function(){ /* ... */ };

But as far as I know, the only way to make it non-enumerable is to do something like this:

Object.defineProperty(Array.prototype, "pop", { enumerable: false });

Which is not supported by all browsers.

Array.prototype.doSomething= function(){ };
var arr = [];

console.log(arr); // [doSomething: function]

So why does doSomething show up here, while pop() doesn't? Aren't they both added to the prototype?

Answer 1

MDN says:

A for...in loop does not iterate over non–enumerable properties. Objects created from built–in constructors like Array and Object have inherited non–enumerable properties from Object.prototype and String.prototype that are not enumerable, such as String's indexOf method or Object's toString method. The loop will iterate over all enumerable properties of the object itself and those the object inherits from its constructor's prototype (properties closer to the object in the prototype chain override prototypes' properties).

https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Statements/for...in

What is logged is supposedly coming from a for..in.. iteration, or similar.

The concept of non-enumerable properties pre-dates the feature of being to specify property enumerability right within Javascript.

MDN says:

ECMAScript 3 has an internal attribute called DontEnum. This attribute is attached to certain properties by default (§8.6.1).

The internal DontEnum attribute determines what is not to be enumerated by a for-in enumeration (§12.6.4). propertyIsEnumerable Test

https://developer.mozilla.org/en/docs/ECMAScript_DontEnum_attribute

EcmaScript 3 spec defines a lot of properties with a DontEnum attribute. http://bclary.com/2004/11/07/

This actually does not solve the whole puzzle, because for example Array.prototype.pop is not explicitly listed as having a DontEnum attribute, only Array.prototype itself is listed as so. It may be that the DontEnum attribute of native functions is implied, but I cannot find a reference for that. The first quote from MDN does for example describe String.prototype.indexOf as non-enumerable, while this isn't mentioned explicitly in the EcmaScript 3 spec either.