Type annotation for `this` keyword in Typescript

Question

I have a standalone function that's meant to use the context provided by Function.prototype.call.

For example:

function foo () {
    return this.bar;
}

> foo.call({bar: "baz"})
baz

Is there a way to provide a Typescript type annotation for the this keyword in this scenario?

Answer 1

First of all, you can use the special this parameter syntax to identify the type of object you expect this to be:

function foo (this: {bar: string}) {
    return this.bar; // no more error
}

which helps if you call it directly:

foo(); // error, this is undefined, not {bar: string}

var barHaver = { bar: "hello", doFoo: foo };
barHaver.doFoo(); // acceptable, since barHaver.bar is a string

var carHaver = { car: "hello", doFoo: foo };
carHaver.doFoo(); // unacceptable, carHaver.bar is undefined

---

UPDATE for TS3.2+

TypeScript 3.2 introduced the --strictBindCallApply compiler option which strongly types the .call() method of functions. If you use this (or the --strict suite of compiler features which includes this), then the this parameter will also enforce that foo.call() behave as desired:

foo.call({ bar: "baz" }); // okay
foo.call({ baz: "quux" }); // error!

Playground link to code

---

pre TS3.2 answer follows:

But you want to use foo.call(). Unfortunately the Function.prototype.call() typing in TypeScript won't really enforce this restriction for you:

foo.call({ bar: "baz" }); // okay, but
foo.call({ baz: "quux" }); // no error, too bad!

Merging something better into TypeScript's Function declaration caused me problems, (First point of ugliness; you will need to cast foo to something) so you can try something like this:

interface ThisFunction<T extends {} = {}, R extends any = any, A extends any = any> {
  (this: T, ...args: A[]): R;
  call(thisArg: T, ...args: A[]): R;
}

A ThisFunction<T,R,A> is a function with a this of type T, a return value of type R, and a rest argument of type A[]. (Second point of ugliness: you can't easily specify multiple arguments of different types in a way that will be enforced by the type system.)

You can then cast foo to ThisFunction<{ bar: string }, string>, (Third point of ugliness: the type system just will not infer this types) and then finally use call():

(<ThisFunction<{ bar: string }, string>>foo).call({ bar: "baz" }); // okay, and
(<ThisFunction<{ bar: string }, string>>foo).call({ baz: "quux" }); // error, hooray!