Sinon unable to stub overloaded methods

Question

Given the following class:

class Foo {
  public bar(): Bar;
  public bar(string): Bar;
}

I want to stub only the first bar() method, how should I do that or is there any workarounds?

Answer 1

Not exactly

You can't do this. Typescript's overloaded methods don't exist at run time - they compile down to a single method.

From the typescript documentation:

TypeScript can only resolve a function call to a single overload

See this excellent answer to a different question for some discussion.

There's also this blog post which has the following example:

    public getHero(name: string): Hero {
        return /* some code here */
    }

    public getHero(name: string, skill: string): Hero {
        return /* some other code here */
    }

A breakpoint in the second function is hit when the first function is called, because Typescript has compiled away the two functions into one.

---

A workaround

However if you need exactly this behaviour - you can make a complex stub that will allow you to call the underlying implementation in some cases using wrappedMethod and callsFake (documentation link here):

        sinon
            .stub(foo, "bar");

         foo.bar.callsFake(function mockBar(maybeString?: string) {
             if (string === undefined) { /* stub behaviour */}
             else { return foo.bar.wrappedMethod(maybeString) }                
         });

---

An aside

As an aside, I would advise against complex stubbing, because now you are in a situation where it would be appropriate to write tests for your tests - in my view, it would be better to refactor the code so that this kind of stubbing is not necessary.