Unit test a callback with QUnit async()

When working with callbacks you're usually doing async operations, so to unit test a callback you will need to use QUnit's async() method.

async(): Instruct QUnit to wait for an asynchronous operation.

So try this:

QUnit.test('should invoke callback', (assert) => {
  let done = assert.async();
  let callback = () => {
    assert.ok( true, 'test ok callback invoked' );
    done();
  };
  myFunction(callback);
});

And that's it! If myFunction does its job of invoking the callback then your test will pass.