flaky-test-detective

Flaky Test Detective

Diagnose and eliminate flaky tests systematically.

Common Flaky Test Patterns

1. Timing Issues

// ❌ Flaky: Race condition
test("should load user data", async () => {
  render(<UserProfile userId="123" />);

  // Race condition - might pass or fail
  expect(screen.getByText("John Doe")).toBeInTheDocument();
});

// ✅ Fixed: Wait for element
test("should load user data", async () => {
  render(<UserProfile userId="123" />);

  await waitFor(() => {
    expect(screen.getByText("John Doe")).toBeInTheDocument();
  });
});

// ❌ Flaky: Fixed timeout
test("should complete animation", async () => {
  render(<AnimatedComponent />);
  await new Promise((resolve) => setTimeout(resolve, 500)); // Brittle!
  expect(element).toHaveClass("animated");
});

// ✅ Fixed: Wait for condition
test("should complete animation", async () => {
  render(<AnimatedComponent />);
  await waitFor(
    () => {
      expect(element).toHaveClass("animated");
    },
    { timeout: 2000 }
  );
});

2. Shared State

// ❌ Flaky: Global state pollution
let userId = "123";

test("test A", () => {
  userId = "456"; // Modifies global
  // ...
});

test("test B", () => {
  expect(userId).toBe("123"); // Fails if test A runs first!
});

// ✅ Fixed: Isolated state
test("test A", () => {
  const userId = "456"; // Local variable
  // ...
});

test("test B", () => {
  const userId = "123";
  expect(userId).toBe("123");
});

// ❌ Flaky: Database not cleaned
test("should create user", async () => {
  await db.user.create({ email: "[email protected]" });
  // No cleanup!
});

test("should create another user", async () => {
  await db.user.create({ email: "[email protected]" }); // Fails! Duplicate
});

// ✅ Fixed: Proper cleanup
afterEach(async () => {
  await db.user.deleteMany();
});

3. Randomness

// ❌ Flaky: Random data
test("should sort users", () => {
  const users = generateRandomUsers(10); // Different each time!
  const sorted = sortUsers(users);
  expect(sorted[0].name).toBe("Alice"); // Might not be Alice
});

// ✅ Fixed: Deterministic data
test("should sort users", () => {
  const users = [
    { name: "Charlie", age: 30 },
    { name: "Alice", age: 25 },
    { name: "Bob", age: 35 },
  ];
  const sorted = sortUsers(users);
  expect(sorted[0].name).toBe("Alice");
});

// ✅ Fixed: Seeded randomness
import { faker } from "@faker-js/faker";

beforeEach(() => {
  faker.seed(12345); // Same data every time
});