Unit tests
Fast tests of a single function or component in isolation.
Jest or Vitest with React Testing Library; many of them, run on every change.
Testing
What is the difference among unit tests, integration tests and end-to-end tests?
Unit tests check one piece in isolation, integration tests check pieces working together, and end-to-end tests drive the real app in a browser the way a user would.
- Technologies
- 3
- Difference questions
- 50
What each one is
Integration tests
Tests of several units together, such as a component with its data layer.
Catch wiring mistakes that unit tests miss; mock only external services.
End-to-end tests
Tests that drive the full application in a real browser.
Playwright or Cypress; slowest and fewest, covering critical user journeys.
At a glance
| Aspect | Unit tests | Integration tests | End-to-end tests |
|---|---|---|---|
| Scope | One unit | Several units | Whole app |
| Speed | Milliseconds | Seconds | Seconds to minutes |
| How many | Most | Some | Fewest |
| Tools | Jest, Vitest | Vitest + RTL, Supertest | Playwright, Cypress |
| Catches | Logic bugs | Wiring bugs | Broken user journeys |
The 10 most-asked difference questions
Subjective questions 10
1What is the difference among unit, integration and end-to-end tests?Asked almost every time
Unit tests check one function or component in isolation. Integration tests check several units working together, such as a component with its data layer or an API with its database. End-to-end tests drive the whole running application through a real browser as a user would.
2What is the testing pyramid?Asked almost every time
A guideline to have many fast unit tests, fewer integration tests and a small number of slow end-to-end tests, balancing confidence, speed and maintenance cost.
3What is the testing trophy and how does it differ?Asked almost every time
A model popularised for front-end apps that puts the biggest share in integration tests, plus static analysis at the base, because integration tests give high confidence at reasonable cost.
4Which tools fit each level in a JavaScript app?Asked almost every time
Unit: Vitest or Jest. Integration: Vitest or Jest with React Testing Library, Supertest for APIs, or a test database. End-to-end: Playwright or Cypress against the running app.
5Why not test everything end to end?Asked almost every time
End-to-end tests are slow, more fragile and harder to debug, so a large suite delays feedback. Most behaviour is checked faster and more precisely at lower levels.
6What should unit tests cover?Very often asked
Pure logic with clear inputs and outputs: calculations, validation rules, formatting, reducers and edge cases such as empty or extreme values.
7What should integration tests cover?Very often asked
Wiring between parts: a form submitting through a hook to a mocked API, a route handler writing to a test database, or several modules coordinating.
8What should end-to-end tests cover?Very often asked
Critical user journeys: sign-up and login, checkout or payment, core creation flows and anything where a failure would hurt users or revenue.
9How much should you mock at each level?Very often asked
Unit tests mock collaborators freely. Integration tests mock only external services such as payment providers. End-to-end tests mock as little as possible, sometimes only third parties.
10How fast is each kind?Very often asked
Unit tests run in milliseconds, integration tests in tens of milliseconds to seconds, and end-to-end tests in seconds to minutes each.
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