How to spy on static generator functions?

Question

I have an utility function that exposes a generator:

export class Utility {
    // provides a generator that streams 2^n binary combinations for n variables
    public static *binaryCombinationGenerator(numVars: number): IterableIterator<boolean[]> {
        for (let i = 0; i < Math.pow(2, numVars); i++) {
            const c = [];
           //fill up c
            yield c;
        }
    }
}

Now, I am using this generator in my code as follows:

myFuncion(input){
    const n = numberOfVariables(input);
    const binaryCombinations = Utility.binaryCombinationGenerator(n);
    let combination: boolean[] = binaryCombinations.next().value;
    while (till termination condition is met) {
      // do something and check whether termination condition is met         
      combination = binaryCombinations.next().value;
    }
}

In my unit tests (using Jasmine) I want to verify how many times the generator function is invoked (ie how many combinations are generated) before the termination. Below is what I have tried:

it("My spec", () => {
    //arrange
    const generatorSpy = spyOn(Utility, "binaryCombinationGenerator").and.callThrough();
    //act
    //assert
    expect(generatorSpy).toHaveBeenCalledTimes(16); // fails with: Expected spy binaryCombinationGenerator to have been called 16 times. It was called 1 times.
});

However, this assertion fails as binaryCombinationGenerator is indeed called once. What I actually want to spy on is the next method of IterableIterator .

However, I am not sure how to do that. Please suggest.

Answer 1

You could return a jasmine spy object from the Utility.binaryCombinationGenerator method

let binaryCombinationsSpy = jasmine.createSpyObject('binaryCombinations', ['next']);
binaryCombinationsSpy.next.and.returnValues(value1, value2);
spyOn(Utility, "binaryCombinationGenerator").and.returnValue(binaryCombinationsSpy);

expect(binaryCombinationsSpy.next).toHaveBeenCalledTimes(2);

Answer 2

I am posting this as an answer as this is what I have done to mock the generator function. This is based on @0mpurdy's answer .

To mock the generator function, we actually need to call a fake function, which will provide different (and limited, if applicable) values for each call of next() of the generator function.

This can be simply achieved by the following:

//arrange
const streamSpy= jasmine.createSpyObj("myGenerator", ["next", "counter"]);
streamSpy.counter = 0;
const values = [{ value: here }, { value: goes }, { value: your }, { value: limited }, 
                { value: values }]; // devise something else if it is an infinite stream

// call fake function each time for next
streamSpy.next.and.callFake(() => {
    if (streamSpy.counter < values.length) {
        streamSpy.counter++;
        return values[streamSpy.counter - 1];
    }
    return { value: undefined }; // EOS
});
spyOn(Utility, "myGenerator").and.returnValue(streamSpy);

...
//assert
expect(streamSpy.next).toHaveBeenCalledTimes(2);