Extend a String class in ES6

Question

I can write the following in ES5:

String.prototype.something=function(){
  return this.split(' ').join('');
};

How do I do the same thing in ES6 using the new features?

I know that this is also a valid ES6. I want to know whether there's any other way of implementing such functions in ES6 which is shorter?
The above function is just an example.

Solution 1

In ES6 you can also do it with Object.assign() like this:

Object.assign(String.prototype, {
    something() {
        return this.split(' ').join();
    }
});

You can find more info to the method here.

Or you could use defineProperty (I think that would be better here):

Object.defineProperty(String.prototype, 'something', {
    value() {
        return this.split(' ').join();
    }
});

See the docs here.

See my comment to see when to use defineProperty vs Object.assign().

Solution 2

Your proposal works fine in ES6, is there something wrong with it?

If you want to actually extend String, instead of just adding a method to String itself, and get that warm ES6 feeling, you could try:

class MyString extends String {
    something() { return this.split(' ').join(''); }
}

However, you are going to quickly run into limitations on extending built-in classes. Chances are you will see the dreaded

TypeError: String.prototype.toString is not generic

error message (this is from babel-node).

Comments

[Qantas 94 Heavy]

Is the example code your actual use case or are you talking about extending String.prototype in general?

[Felix Kling]

All ES5 code is also valid ES6 code. So you can just use the same code.

[ritz078]

@Qantas94Heavy I was talking for a general case.

[KyleMit]

Might be worth noting that you can't update to arrow functions as the value for this is provided by their lexical scope.

[broofa]

It's worth noting that extending native objects like this, while not strictly prohibited, is pretty much never a good idea. See stackoverflow.com/questions/14034180/… for more. (tl;dr: Changes the behavior of the object in subtle and not-so-subtle ways that may break other code.)

[jehon]

Note that strings are immutable

[Admin]

What is the difference? What is the advantage? Why would he want to do this anyway instead of using defineProperty?

[AWolf]

Yes, you're right defineProperty would be better here. See this blog post. Especially the conclusion summarizes it pretty well. I'll add it to my answer in a minute.

[Qantas 94 Heavy]

I think this is an issue with babel (which can't be fixed easily AFAIK), in "proper" implementations with real subclassing support this should work properly.

[Qantas 94 Heavy]

Isn't defineProperty ES5 code? I really don't see how ES6 provides a better method of extending String than that (apart from subclassing).

[ritz078]

So that means that es5 is still the best method to implement such functions.

[ritz078]

It works fine in es6 but just wanted to know if any shorter way is available in es6.