The Testing Pyramid

Three altitudes of testing

Everything so far was unit testing: one function, dependencies replaced by doubles. Two more levels exist above it.

• an integration test wires several real pieces together and checks the seam between them: the parser feeding the calculator, your code talking to a real (test) database
• an end-to-end (E2E) test drives the whole running system the way a user would: open the browser, click checkout, see the receipt

Going up the levels buys realism and costs speed and stability. A unit test runs in microseconds and fails for one reason. An E2E test takes seconds or minutes, needs the whole app running, and can fail for a dozen unrelated reasons (a slow server, a renamed button).

What to test where

The pyramid is a budget, and each level has a specialty:

• unit: all your business logic and edge cases. The lesson 3-1 checklist lives here, where probes cost microseconds
• integration: the seams. Anywhere two components exchange data, or your code meets a real database, file system, or API contract
• E2E: two or three critical user journeys, like sign up, pay, see the dashboard. These are smoke tests: quick checks whose only question is whether the assembled system basically works at all — the name comes from hardware engineers powering on a new circuit board and watching for literal smoke. Smoke coverage, not thoroughness

The classic failure mode inverts the pyramid: hundreds of slow E2E tests, few unit tests, and a suite nobody runs because it takes an hour and fails randomly. Keep the base wide and the top narrow.