What is the difference between unit, integration, and end-to-end testing? When would you write each?

The Testing Pyramid

/  E2E  \        ← Few, slow, expensive
       / Integration \   ← Some, moderate speed
      /   Unit Tests  \  ← Many, fast, cheap

Types Compared

AspectUnitIntegrationE2E

ScopeSingle function/componentMultiple units togetherFull user flow
Speed<10ms100ms-1s5-30s
DependenciesMockedReal (DB, API)Real browser + server
FlakinessNoneLowHigh
CostLowMediumHigh
ConfidenceLow (isolated)MediumHigh (real scenario)

When to Write Each

Unit Tests — Business logic

// Test pure functions, calculations, formatters
describe("formatSalary", () => {
  it("formats yearly salary", () => {
    expect(formatSalary(120000, "yearly")).toBe("$120,000/year");
  });
  it("formats monthly salary", () => {
    expect(formatSalary(5000, "monthly")).toBe("$5,000/month");
  });
});
// 100 tests run in <1 second

Integration Tests — API + Database

// Test API routes with real database
describe("POST /api/applications", () => {
  it("creates an application", async () => {
    const res = await fetch("/api/applications", {
      method: "POST",
      body: JSON.stringify({ company: "Google", position: "SWE" }),
      headers: { Authorization: Bearer ${testToken} },
    });
    expect(res.status).toBe(201);
    const data = await res.json();
    expect(data.company).toBe("Google");
  });
});
// Uses real database (test instance)

E2E Tests — Critical User Flows

// Playwright test
test("user can create and view application", async ({ page }) => {
  await page.goto("/login");
  await page.fill('[name="email"]', "test@example.com");
  await page.fill('[name="password"]', "password123");
  await page.click('button[type="submit"]');

  await page.goto("/dashboard/applications");
  await page.click("text=Add Application");
  await page.fill('[name="company"]', "Google");
  await page.fill('[name="position"]', "SWE");
  await page.click("text=Save");

  await expect(page.locator("text=Google")).toBeVisible();
});

Recommended Ratio

70% Unit — Fast feedback, catches logic bugs

20% Integration — API contract, DB queries

10% E2E — Critical paths only (login, checkout, core flow)