Typing `this` in TypeScript

Question

Is there a way to set the type of this in TypeScript?

I'm sure there's plenty of uses for it, but in my particular case it's for typing a JS library, and writing a plugin for it.

For example:

// Some library
declare var SomeLib: sl.ISomeLibStatic;
declare module sl {
    interface ISomeLib extends ISomeLibApi {
        baseFn(): void;
    }

    interface ISomeLibStatic {
        new (): ISomeLib;
        API: ISomeLibApi;
    }

    interface ISomeLibApi {
    }
}

// Plugin file
declare module sl {
    // Extend the API signature declaration
    interface ISomeLibApi {
        extraFn(): void;
    }
}

module sl {
    // Implement the function
    SomeLib.API.extraFn = function () {
        // Get typing on this here
        this.baseFn();
    };
}

Anybody knows a way to do this without a variable like: var typedThis: ISomeLib = this; ?

Currently the only way I found was having cast it on every usage <ISomeLib>this which is cumbersome, and not defined in the type of the function.

Answer 1

There is no way to hint to the language service what context to apply to the function.

The only mechanism that will cleanly supply type checking and auto-completion without resulting in runtime artefacts is:

(<ISomeLib>this).baseFn();

Which compiles down to the desired:

this.baseFn();

Answer 2

It can be done as follows:

function foo(this: MyType) { 
}

You can also use this: this to enforce the function to be called on the declaring context, or this: void to prevent the usage of this.

However, the target version for this feature is TypeScript 2.0 (although it is already implemented on the latest develop).

See here for details.