improving-weak-tests

Improving Weak Tests

Core principle

Every rewrite must answer: what plausible mutation does the new test catch that the old one did not? If you cannot name that mutation in one sentence, the rewrite is not done.

The catalogue below is written in TypeScript against jest/vitest matchers (toEqual, toHaveBeenCalledWith, expect.any), but the weak-assertion patterns are language-agnostic — the same rewrites apply to pytest in Python (see the pytest note under rule 1).

The catalogue

1. toBeTruthy() → exact shape

// WEAK
expect(await saveUser(input)).toBeTruthy();

// STRONG
const result = await saveUser(validInput);
expect(result).toEqual({
  id: expect.any(String),
  email: validInput.email,
  createdAt: expect.any(Date),
});

Catches: saveUser returning {} or echoing the input.

The same weakness in pytest is a bare truthy assert; rewrite it to pin the exact shape:

# WEAK
assert save_user(valid_input)

# STRONG
from datetime import datetime

result = save_user(valid_input)
assert result.email == valid_input.email
assert isinstance(result.id, str)
assert isinstance(result.created_at, datetime)

2. "mock was called" → state + args + output

// WEAK
await vm.saveUser(input);
expect(httpClient.post).toHaveBeenCalled();

// STRONG
await vm.saveUser(input);
expect(httpClient.post).toHaveBeenCalledWith(`${baseUrl}/users`, input);
expect(vm.users()).toContainEqual(expect.objectContaining({ email: input.email }));
expect(vm.isFormVisible()).toBe(false);
expect(vm.errorMessage()).toBeNull();

Catches: the handler not reloading, not closing the form, or silently failing.

3. Happy path only → add failure path + negative space

it('keeps form open and exposes error when create fails', async () => {
  vm.openCreateForm();
  const promise = vm.saveUser(validInput);
  httpTesting.expectOne('POST', `${baseUrl}/users`)
    .error(new ProgressEvent('error'));
  await promise;
  expect(vm.isFormVisible()).toBe(true);
  expect(vm.errorMessage()).toMatch(/create failed|error/i);
  expect(vm.isSaving()).toBe(false);
  expect(vm.users()).toEqual([]);
});

it('does not issue a write when input is invalid', async () => {
  const promise = vm.saveUser(invalidInput);
  await promise;
  httpTesting.expectNone('POST', `${baseUrl}/users`);
  expect(vm.errorMessage()).toMatch(/invalid/i);
});