Testing a functional parameter two ways

test-double-function.md

Two ways of creating and asserting with a test double for a functional parameter

We are testing a function that takes a function as an argument. We don't need Jest to do its magic poke-in-replacements-for-imports magic. But we have the option of using jest.fn() to create a test double.

Example is in plain JS so that types don't clutter things.

Code under test (CUT):

const addOneAndApply = (a, fn) => {
  const oneAdded = a + 1;
  return fn(oneAdded);
};

Test double using an ordinary function

const fn = (n) => {
   expect(n).toBe(4);                   // A: Is the test double called with expected parameter?
   return 12345;                        // B: What does the test double return?
};

const actual = addOneAndApply(3, fn);   // C: Run the CUT

expect(actual).toBe(12345);             // D: Does the code under test return the expected value

• requires only standard JS knowledge
• you could give it real functionality return n * 2 (but see also Jest .mockImplementation())
• test terminates as soon as this test double fails an expect
• test double's response and expectations are both defined before the CUT is called

Test double using jest.fn()

const fn = jest.fn();
fn.mockReturnValue(12345);              // B: What does the test double return?

const actual = addOneAndApply(3, fn);   // C: Run the CUT

expect(fn).toHaveBeenCalledWith(4);     // A: Is the test double called with expected parameter?
expect(actual).toBe(12345);             // D: Does the code under test return the expected value

• requires knowledge of the Jest API
• test continues when unexpected parameters are supplied to fn() -- only fails when .toHaveBeenCalledWith() verifies later
• test double's response is defined before CUT is run, but expectations are defined after.