Unit, Integration & E2E Tests, Explained

What the three levels of testing actually are, what each one catches and costs, and how to get the mix right so your suite stays fast and your failures point somewhere useful.

1. The Testing Pyramid The mental model behind the three levels: many small fast tests at the bottom, fewer big slow tests at the top - and why that shape gives you speed plus failures that point at the bug.
2. The Three Levels Unit, integration, and end-to-end tests one at a time: what each one catches, what each costs in speed and flakiness, and a concrete worked example of each.
3. Getting the Mix Right The practical strategy: lots of unit, some integration, a few E2E; how to decide which level a given risk belongs at; and why the 'ice-cream cone' anti-pattern of all-E2E quietly wrecks teams.