Custom assertions are my favourite QUnit feature

This post is meant to share my favourite QUnit feature, custom assertions.

Compared to other library, QUnit has not a particularly rich assertions’ library; however that’s not a big deal, cause QUnit makes extremely simple to extend its core assertions collection.

QUnit.assert.typeof = function (subject, expected) {
  // The current value of the expression
  const actual = typeof subject;

  // It's a boolean;
  // It expresses wheter the test has passed, or not.
  const result = actual == expected;

  // That's the message that will be displayed in case the assertion fails
  const message = `Type mismatch. Expecting ${ expected }, got ${ actual }.`

  this.pushResult({
    result,
    actual,
    expected,
    message
  });
};

The important bit here is that we’re executing QUnit internal Assert#pushResult method.
The best part is that QUnit is 100% aware of this new assertion.

QUnit.test("An example", function (assert) {
  assert.typeof(() => 42, "function");
  assert.expect(1);
});

That is, we receive typeof as member of the assert parameter, and it still increments QUnit internal assertion count.

Custom assertions are great cause make extremely clear what a test is for, and avoid code duplication. They usually are also more concise that their counterpart.

assert.equal(typeof foo, "function", "foo must be a function");