ts-mockito: thenResolve() with a mocked object not working

This was a “fun” one (if three days of pulling my hair out is fun, but I can’t get a haircut so I guess that’s OK 😃. I have an interface that has a method that returns a promise of another interface which is also mocked. When using thenResolve() (or thenReturn() with Promise.resolve()) any await on the promise fails to resolve. It works if any non-mocked object or primitive type is used.

So given (greatly simplfied):

interface Main {
  getChild(): Promise<Child>;
}

interface Child {
  name: string;
}

and then using this test (Mocha and Chai):

describe("mocking", function() {
  it("Should work", async function() {
        const mockedChild = mock<Child>();
        when(mockedChild.name).thenReturn("Sylvia");
        const child = instance(mockedChild);

        const mockedMain = mock<Main>();
        when(mockedMain.getPromise()).thenResolve(child);
        const main = instance(mockedMain);

        const result = await main.getPromise();  // <-- Here be dragons, this times outs
        console.log(`@@@@ val = '${result}'`);
        expect(result.name).to.equal("Sylvia");
  });
});

The test times out at the indicated line. I set the timeout to 10 seconds and it still times out.

Normally at this point I’d paste in more code, like the package.json etc. Or I’d create a separate, simplified project (which I did) and link it here (I can if you really want it) but I was so aggravated that I obviously didn’t understand what I was doing that I decided to figure out what I was doing wrong. It turns out, I wasn’t doing anything wrong, it was a slight by-product of the design of ts-mockito. I won’t call it a bug as everything is working exactly as designed. It’s more of an oversight of the subtleties of JavaScript and Promises.

BTW, one of the reasons I decided to figure it out on my own was because of another “bug” I’m going to report after this one pertaining to toString(). Another “fun” issue that made this one much harder for me.

So, what went “wrong.” If you read the Promise.resolve() description (I’m using the Mozilla one here because it’s easier to understand but I did read the JavaScript Spec and it says the same thing just using waaaaay more words) it says in part:

The Promise.resolve() method returns a Promise object that is resolved with a given value. If the value is a promise, that promise is returned; if the value is a thenable (i.e. has a “then” method), the returned promise will “follow” that thenable, adopting its eventual state; otherwise the returned promise will be fulfilled with the value. This function flattens nested layers of promise-like objects (e.g. a promise that resolves to a promise that resolves to something) into a single layer.

Promise.resolve() documentation

The bold is the interesting part. It says that it takes the resolve value, sees if it has a .then() method and if it does then uses the .then() method to resolve the value of the promise. Of course, because this is a mock of everything all of the objects that ts-mockito creates has a .then() method. Ruh-roh!

OK, now that is the problem. For bonus points, I have a solution. I think it’s right, and it does pass all the tests. It is in the attached PR. I have hand-patched my copy of ts-mockito and it works for me.

The basic solution is to have a list of what I’m calling defaultedPropertyNames which are properties that, if they are not explicitly set to return a value, will return undefined in the Proxy.get method. This is similar to the excludedPropertyNames in the .get() method that contains the hasOwnProperty value so we don’t override the one value we must have from JavaScript. The only difference is that my check comes after to check for a defined value for the property so if a when() is defined, that will be used.

BTW, in the PR there is another name in the defaultedPropertyNames list, Symbol(Symbol.toPrimitive) and that is to solve the next bug I’m opening, the fix was that same so I made them at the same time.