What is the difference between unit, integration, and end-to-end testing?

Question

Accepted Answer

**Unit** — single component. **Integration** — component interaction. **E2E** — complete flow.

**Unit Testing:**
- Test single function/component.
- Fast, isolated.
- Mock external dependencies.
- Most numerous.

```
Unit Test:
test("add", () => {
  expect(add(1, 2)).toBe(3);
});
(Single function)
```

**Integration Testing:**
- Test component interaction.
- Real dependencies.
- Slower than unit.
- Fewer tests.

```
Integration Test:
test("API + DB", () => {
  const user = createUser({name: "John"});
  expect(user.id).toBeDefined();
});
(Components together)
```

**End-to-End Testing:**
- Complete user flow.
- Real system.
- Slowest.
- Fewest tests.

```
E2E Test:
Login → Add to cart → Checkout
(Complete flow)
```

**Test Pyramid:** Many unit → fewer integration → few E2E.

Systems Analyst: What is the difference between unit, integration, and end-to-end testing?