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Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Forgetting to return or await a Promise inside a Mocha `it()` block, causing the test to pass falsely

// Wrong: test passes even if getUser() rejects or the assertion fails later
it('fetches a user', () => {
  getUser(1).then(user => expect(user.name).to.equal('Bob'));
});

// Correct: Mocha waits for the returned/awaited promise
it('fetches a user', async () => {
  const user = await getUser(1);
  expect(user.name).to.equal('Bob');
});

The Solution //

If an async assertion is not awaited (or the returned Promise isn't handed back to Mocha), the test function resolves immediately while the assertion is still pending, and Mocha marks it as passed before the real check ever runs. Always `await` async calls inside `it()`, or return the Promise chain directly.

The Error //

Mixing Chai's `expect` chainable words with Jest matcher syntax after migrating a suite

// Wrong: Chai syntax run under Jest
expect(result).to.equal(5); // TypeError: expect(...).to is undefined

// Correct: Jest's own matcher syntax
expect(result).toBe(5);

The Solution //

Chai uses English-like chains like `expect(x).to.equal(5)` while Jest matchers are direct calls like `expect(x).toBe(5)` — copy-pasting tests between the two frameworks without converting the assertion syntax produces a 'is not a function' error at runtime because Jest's expect() doesn't have a `.to` property, and vice versa.