Build a function object with properties in TypeScript

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

This is easily achievable now (typescript 2.x) with Object.assign(target, source)

example:

The magic here is that Object.assign<T, U>(t: T, u: U) is typed to return the intersection T & U.

Enforcing that this resolves to a known interface is also straight-forward. For example:

interface Foo {
  (a: number, b: string): string[];
  foo: string;
}

let method: Foo = Object.assign(
  (a: number, b: string) => { return a * a; },
  { foo: 10 }
); 

which errors due to incompatible typing:

Error: foo:number not assignable to foo:string
Error: number not assignable to string[] (return type)

caveat: you may need to polyfill Object.assign if targeting older browsers.