English for Vitest Developers

Vitest has become the default unit testing framework for Vite-based JavaScript and TypeScript projects, and it comes with a vocabulary that non-native English speakers often find confusing. Words like “spy”, “stub”, and “mock” are all used in testing but mean different things. Knowing how to use these terms correctly helps you write clearer PR descriptions, contribute to technical discussions, and read Vitest documentation without guessing.

Key Vocabulary

Mock A complete replacement for a real module or function that you control entirely in tests. In Vitest, vi.mock('module-name') replaces the whole module with auto-generated stubs you can configure. Example: “I mocked the email service so the test doesn’t send real emails to customers.”

Spy A wrapper around a real function that tracks how it was called — arguments, call count, return values — without replacing its behaviour. Created with vi.spyOn(). Example: “I added a spy on console.error to assert the component logs the right message on failure.”

Stub A simplified stand-in for a function that returns a pre-defined value, used when you do not want the real implementation to run. In Vitest, vi.fn() creates a stub function you can configure with .mockReturnValue(). Example: “The stub returns an empty array so the component renders its empty-state UI during the test.”

Snapshot A saved serialisation of a value — often a component’s rendered output — that Vitest compares against on subsequent test runs. If the output changes, the test fails until you review and update the snapshot. Example: “The snapshot test caught an unexpected class name change in the button component.”

Coverage Threshold A minimum percentage of code that must be exercised by tests. If coverage drops below the threshold, the CI pipeline fails. Configured per metric: lines, functions, branches, statements. Example: “Our branch coverage threshold is 80% — the new utility function dropped us to 76%, so we need more tests.”

Describe Block A describe() call that groups related tests under a shared label. Nesting describe blocks creates a readable hierarchy that appears in the test output. Example: “I wrapped the validation tests in a describe block called ‘when the form is empty’ to make the output easier to read.”

beforeEach / afterEach Hook Functions that run before or after every test in a suite. Used to set up shared state or clean up side effects so tests remain independent. Example: “The beforeEach hook resets the store to its initial state so each test starts from a known baseline.”

vi.fn() Vitest’s function for creating a mock function from scratch. It tracks calls and can be configured to return specific values, throw errors, or run a custom implementation. Example: “I passed a vi.fn() as the onSubmit prop and then asserted it was called once with the correct payload.”

Common Phrases

In code reviews:

• “This test is using a real HTTP client — can we replace it with a mock so the test doesn’t hit the network?”
• “The spy here never gets reset between tests; add a vi.restoreAllMocks() call in afterEach to prevent state leaking.”
• “The snapshot is quite large — if it changes frequently it will create noise in PRs. Consider a more targeted assertion instead.”

In standups:

• “I’m writing unit tests for the auth utility — I’m mocking the token service and asserting the refresh logic in isolation.”
• “Coverage is at 73% — I need to add tests for the error branches before this PR is ready to merge.”
• “The beforeEach hooks were duplicated across three test files, so I extracted them into a shared setup helper.”

In documentation:

• “Use vi.spyOn() when you want to observe calls to a real function without replacing its implementation.”
• “Run vitest --coverage to generate a coverage report; the thresholds are configured in vite.config.ts under test.coverage.”
• “Snapshots are stored in __snapshots__/ next to the test file; commit them alongside the test that created them.”

Phrases to Avoid

Confusing “mock” and “spy” — Non-native speakers often use “mock” for everything. If the original function still runs but you are watching it, say “spy.” If you replaced the function entirely, say “mock.” Reviewers will notice the difference: “I mocked fetch” vs. “I spied on fetch” are not the same action.

Saying “the test is wrong” when you mean the snapshot needs updating — When a snapshot fails because of an intentional UI change, the correct phrase is “update the snapshot” or “the snapshot is outdated.” Saying the test is wrong implies a bug in the test itself.

Saying “empty test” — If a test has no assertions, the correct term is a “passing test with no assertions” — or more commonly, you would call it an “incomplete test” or note that it “lacks assertions.” A truly empty test file is called a “placeholder” or “stub test file.”

Quick Reference