Why is extending native objects a bad practice?

Every JS opinion leader says that extending the native objects is a bad practice. But why? Do we get a perfomance hit? Do they fear that somebody does it "the wrong way", and adds enumerable types to Object, practically destroying all loops on any object?

Take TJ Holowaychuk's should.js for example. He adds a simple getter to Object and everything works fine (source).

Object.defineProperty(Object.prototype, 'should', {
  set: function(){},
  get: function(){
    return new Assertion(Object(this).valueOf());
  },
  configurable: true
});

This really makes sense. For instance one could extend Array.

Array.defineProperty(Array.prototype, "remove", {
  set: function(){},
  get: function(){
    return removeArrayElement.bind(this);
  }
});
var arr = [0, 1, 2, 3, 4];
arr.remove(3);

Are there any arguments against extending native types?

When you extend an object, you change its behaviour.

Changing the behaviour of an object that will only be used by your own code is fine. But when you change the behaviour of something that is also used by other code there is a risk you will break that other code.

When it comes adding methods to the object and array classes in javascript, the risk of breaking something is very high, due to how javascript works. Long years of experience have taught me that this kind of stuff causes all kinds of terrible bugs in javascript.

If you need custom behaviour, it is far better to define your own class (perhaps a subclass) instead of changing a native one. That way you will not break anything at all.

The ability to change how a class works without subclassing it is an important feature of any good programming language, but it is one that must be used rarely and with caution.

There's no measurable drawback, like a performance hit. At least nobody mentioned any. So this is a question of personal preference and experiences.

The main pro argument: It looks better and is more intuitive: syntax sugar. It is a type/instance specific function, so it should be specifically bound to that type/instance.

The main contra argument: Code can interfere. If lib A adds a function, it could overwrite lib B's function. This can break code very easily.

Both have a point. When you rely on two libraries that directly change your types, you will most likely end up with broken code as the expected functionality is probably not the same. I totally agree on that. Macro-libraries must not manipulate the native types. Otherwise you as a developer won't ever know what is really going on behind the scenes.

And that is the reason I dislike libs like jQuery, underscore, etc. Don't get me wrong; they are absolutely well-programmed and they work like a charm, but they are big. You use only 10% of them, and understand about 1%.

That's why I prefer an atomistic approach, where you only require what you really need. This way, you always know what happens. The micro-libraries only do what you want them to do, so they won't interfere. In the context of having the end user knowing which features are added, extending native types can be considered safe.

TL;DR When in doubt, don't extend native types. Only extend a native type if you're 100% sure, that the end user will know about and want that behavior. In no case manipulate a native type's existing functions, as it would break the existing interface.

If you decide to extend the type, use Object.defineProperty(obj, prop, desc); if you can't, use the type's prototype.

I originally came up with this question because I wanted Errors to be sendable via JSON. So, I needed a way to stringify them. error.stringify() felt way better than errorlib.stringify(error); as the second construct suggests, I'm operating on errorlib and not on error itself.

In my opinion, it's a bad practice. The major reason is integration. Quoting should.js docs:

OMG IT EXTENDS OBJECT???!?!@ Yes, yes it does, with a single getter should, and no it won't break your code

Well, how can the author know? What if my mocking framework does the same? What if my promises lib does the same?

If you're doing it in your own project then it's fine. But for a library, then it's a bad design. Underscore.js is an example of the thing done the right way:

var arr = [];
_(arr).flatten()
// or: _.flatten(arr)
// NOT: arr.flatten()