Are Jest timer mocks broken in Stencil 1.0.2? - unit-testing

Question

I'm using StencilJS v1.0.2 and it runs unit tests in Jest. I have the following code but it doesn't seem to behave as expected. Am I using the API correctly?
function ok( delay: number ) {
setTimeout(() => {
return 3;
}, delay);
}
test( 'timer', () => {
jest.useFakeTimers();
const result = ok( 1000 );
jest.advanceTimersByTime( 999 );
expect( result ).toBeUndefined();
jest.runAllTimers();
expect( result ).toBe( 3 );
} );
the test should pass but instead it fails on the last expect statement

Answer 1

This line:
const result = ok( 1000 );
...sets result to the return value of ok...
...but ok doesn't return anything so result is always undefined.
So the first expect passes:
expect( result ).toBeUndefined(); // Success!
...but the second expect fails since result will always be undefined:
expect( result ).toBe( 3 ); // <= FAILS since result is always undefined
The issue is that setTimeout just calls the function passed to it.
If the function returns a value then it just gets lost so this line:
return 3;
...doesn't actually do anything.
A simple way to fix the test is to have ok return an object...
...and have the function passed to setTimeout set a property on the object like this:
function ok(delay: number) {
const result: { val?: number } = {};
setTimeout(() => {
result.val = 3; // <= set a property on result
}, delay);
return result;
}
test('timer', () => {
jest.useFakeTimers();
const result = ok(1000);
jest.advanceTimersByTime(999);
expect(result.val).toBeUndefined(); // Success!
jest.runAllTimers();
expect(result.val).toBe(3); // Success!
});