Support final classes (non-subclassable)

[Zorgatone]

I was thinking it could be useful to have a way to specify that a class should not be subclassed, so that the compiler would warn the user on compilation if it sees another class extending the original one.

On Java a class marked with final cannot be extended, so with the same keyword on TypeScript it would look like this:

final class foo {
    constructor() {
    }
}

class bar extends foo { // Error: foo is final and cannot be extended
    constructor() {
        super();
    }
}

[mhegazy]

a class with private constructor is not extendable. consider using this instead.

[Zorgatone]

From what I recalled I was sure the compiler didn't like the private keyword on the constructor. Maybe I'm not using the paste version though

[mhegazy]

This is a new feature, will be released in TS 2.0, but you can try it using typescript@next. see #6885 for more details.

[Zorgatone]

Ok thank you

[duanyao]

Doesn't private constructor also make a class not instantiatable out of the class? It's not a right answer to final class.

[mhegazy]

Java and/or C# uses the final class to optimize your class at runtime, knowing that it is not going to be specialized. this i would argue is the main value for final support. In TypeScript there is nothing we can offer to make your code run any better than it did without final.
Consider using comments to inform your users of the correct use of the class, and/or not exposing the classes you intend to be final, and expose their interfaces instead.

[0815fox]

I do not agree with that, instead I agree with duanyao. Private does not solve that issue, because I also want classes which are final to be instanciateable using a constructor. Also not exposing them to the user would force me to write additional factories for them. For me the main value of final support is, that it prevents users from making mistakes.
Arguing like that: What does TypeScript offer to make my code run faster, when I use types in function signatures? Isn't it also only for preventing users from making mistakes? I could write comments describing which types of values a user should pass in as a parameter. It's a pitty, that such extensions like a final keyword are just pushed away, because on my opinion it collides with the original intension of typescript: make JavaScript safer by adding a compilation level, which performs as many checks as possible to avoid as many mistakes upfront as possible. Or did I misunderstand the intention of TypeScript?