I have an authorizer#getToken method that returns a token. The first time it gets called, it authenticates by posting to an API; each call thereafter returns the token that was received from the API call. I need to test that the private method that hits the API does not get called (via #authenticate) when a token already exists. #sessionIsValid is the function that checks for an existing token. Here's my test:

        it('does not authenticate', async () => {
            authorizer['sessionIsValid'] = jest.fn(() => true);

            await authorizer.getToken();
            expect(authorizer['authenticate']).not.toHaveBeenCalled();
        });

Jest is complaining with

    Matcher error: received value must be a mock or spy function

I've tried jest.spyOn(authorizer, 'authenticate') but get Argument of type '"authenticate"' is not assignable to parameter of type '"getToken"'.

I can't quite connect the dots here. Any thoughts on how to accomplish this?