Build a function object with properties in TypeScript

Question

I want to create a function object, which also has some properties held on it. For example in JavaScript I would do:

var f = function() { }
f.someValue = 3;

Now in TypeScript I can describe the type of this as:

var f: { (): any; someValue: number; };

However I can't actually build it, without requiring a cast. Such as:

var f: { (): any; someValue: number; } =
    <{ (): any; someValue: number; }>(
        function() { }
    );
f.someValue = 3;

How would you build this without a cast?

Answer 1

Update: This answer was the best solution in earlier versions of TypeScript, but there are better options available in newer versions (see other answers).

The accepted answer works and might be required in some situations, but have the downside of providing no type safety for building up the object. This technique will at least throw a type error if you attempt to add an undefined property.

interface F { (): any; someValue: number; }

var f = <F>function () { }
f.someValue = 3

// type error
f.notDeclard = 3