Can you extend a function in TypeScript?

Question

I help maintain a JavaScript library that produces spy functions which allow you to inspect how a passed-in function was called (mainly for use in unit testing).

The library creates a function that has additional properties on it that allow you to inspect the calls.

Is it possible to create a TypeScript definition that will allow the function to be passed in to methods that require a function AND have extra properties?

This is invalid, but something like:

class Spy extends function {
    wasCalled: () => boolean;
    ...
}

Which would allow me to pass a spy into a function with this signature:

function subjectUnderTest(callback:() => void) {
    ...
}

Answer 1

Yes, the TypeScript handbook calls this a "hybrid type", because it's a combination of a function type and a regular interface.

interface Spy {
    (foo: string, bar: number) : boolean; // Just an example
    wasCalled() : boolean;
}

var spy : Spy = createASpySomehow();
var result = spy("foo", 123);
if (spy.wasCalled()) {
    // ...
}