Loose assertions on arguments passed to function with Jest

#JavaScript #Jest #Unit Tests

Problem

Consider the following function and passing test

function foo(bar, arg) {
  bar(Math.random(), arg);
}

test("bar is called", () => {
  const barSpy = jest.fn();
  foo(barSpy, "fake-argument");

  expect(barSpy).toHaveBeenCalled();
});

foo is a function that calls bar with a random first argument and arg which is just passed through.

The test passes as expected, but what if we wanted to check that the second argument is equal to arg.

expect(...).toHaveBeenCalledWith(...args) requires the test to provide the exact list of arguments passed to our jest.fn spy during the call.

### Solution

Use the asymetric matcher expect.anything() to ignore the first argument passed to foo.

test("Match any argument", () => {
  const barSpy = jest.fn();
  foo(barSpy, "fake-argument");

  expect(barSpy).toHaveBeenCalledWith(expect.anything(), "fake-argument");
});

### Better Solution

Use expect.any(constructor) to check that the first argument is a number

test("Match any number", () => {
  const barSpy = jest.fn();
  foo(barSpy, "fake-argument");

  expect(barSpy).toHaveBeenCalledWith(expect.any(Number), "fake-argument");
});