Specifying the type of this for functions

Following up on specifying the type of this in a class or an interface, functions and methods can now declare the type of this they expect.

By default the type of this inside a function is any.Starting with TypeScript 2.0, you can provide an explicit this parameter.this parameters are fake parameters that come first in the parameter list of a function:

function f(this: void) {
    // make sure `this` is unusable in this standalone function
}

this parameters in callbacks

Libraries can also use this parameters to declare how callbacks will be invoked.

Example

interface UIElement {
    addClickListener(onclick: (this: void, e: Event) => void): void;
}

this: void means that addClickListener expects onclick to be a function that does not require a this type.

Now if you annotate calling code with this:

class Handler {
    info: string;
    onClickBad(this: Handler, e: Event) {
        // oops, used this here. using this callback would crash at runtime
        this.info = e.message;
    };
}
let h = new Handler();
uiElement.addClickListener(h.onClickBad); // error!

—noImplicitThis

A new flag is also added in TypeScript 2.0 to flag all uses of this in functions without an explicit type annotation.