Support specifying type of "this" in function implementations

[abergs]

Migrated issue from codeplex: https://typescript.codeplex.com/workitem/507

Currently typescript will type the "this" pointer in function callbacks as "any." Arrow syntax lets us capture "this" from the outer scope, and this can enable proper typing. It would be nice to be able to provide
an optional "ambient this" declaration in function signatures:

(declare this : MyType, first: number, second: string) : any;

The rules would be:

1. the declaration is obviously optional
2. if specified, should be first parameter
3. cannot be used with arrow syntax lambdas (should be compiler error)

I know you can cast as needed to get the intellisense but I'd rather not put this responsibility on the implementer of the function. Ideally it should be part of the definition.

Example with Sammy:

var app = $.sammy("#view", function() {
     // define default route
     this.get("#/", function() { ... } 
});

In the above case, typescript can only type this as any.

Thoughts?

[Bartvds]

There are many JS libraries out there that depend on using a specifically bound "this" in callbacks.

Note the codeplex issue had 68 votes.

[johnnyreilly]

👍

[abergs]

👍

[fdecampredon]

👍

[abergs]

@fdecampredon Have you moved the issue regarding exposing private types? That issue is still important for Facebook React compatibility, right?

[fdecampredon]

Not yet @abergs I'll do it

[basarat]

👍

[RyanCavanaugh]

+Needs Proposal (see https://github.com/Microsoft/TypeScript/wiki/Writing-Good-Design-Proposals)

The major design impediment we had here was that it was really unclear what the subtype relationships are for functions with a declared this parameter, and how the various DOM functions that might or might not care about this should act.

Some food for thought:

class MyClass {
    x = 5;
    fn1 = () => { console.log(this.x); }
    fn2() { console.log(this.x); }
}
declare function callme(f: (this: SomeType));

var m = new MyClass();
callme(m.fn1); // Allowed? Not allowed?
callme(m.fn2); // Allowed? Not allowed?
callme(window.focus); // Allowed? Not allowed?

[QueueHammer]

I like "this" for a couple of reasons. Currently there is no way to create practical private values in classes using TypeScript syntax. When the situation requires private members in a class, read "function constructed object", I end up using regular JavaScript syntax and use "this" to create my public members. However as an "any" there is no auto complete of values assigned to "this" in the class, nor can any sub value of an "any" have a type. Being able to define "this" as an interface would mitigate this. Or we could allow private members in classes... but those comments are for a different issue thread.