Declarations for function-scoped class expressions

[awerlang]

TLDR: How to generated .d.ts from function-scoped class expressions?

Consider the following snippet:

class MyService {
    constructor(private dependency) { }
    myMethod() {
        return this.dependency.someMethod();
    }
}

function myFactory(dependency) {
    return new MyService(dependency);
}

As you can see, I have to make a field for a dependency. This is so C# (I like C#) but I'd prefer the JavaScript way in JavaScript.

Then I make a closure:

function myFactory2(dependency) {
    return new class MyService {
        myMethod() {
            return dependency.someMethod();
        }
    };
}

I'm aware that I get a new constructor function each time and there's no prototype sharing, but I don't care. I see a high potential in this pattern for true information hiding. It is even better for inheritance. A base class could evolve without breaking derived classes.

To make it fit on TypeScript, however, automatic generation of declarations or somehow compiler being able to interpret this pattern is needed I think. An identifier for this factory type is needed.

One should be able to do the following then, based on the second snippet:

function MyController(myService: myFactory2) {
}

[ahejlsberg]

This is slated for the 1.6 release: https://github.com/Microsoft/TypeScript/wiki/Roadmap.

[ahejlsberg]

Also see #497.