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
});

4. Network Dependencies

// ❌ Flaky: Real API call
test("should fetch users", async () => {
  const users = await fetchUsers(); // External API!
  expect(users).toHaveLength(10); // Might fail if API down
});

// ✅ Fixed: Mocked API
test("should fetch users", async () => {
  server.use(
    http.get("/​api/​users", () => {
      return HttpResponse.json([
        { id: "1", name: "User 1" },
        { id: "2", name: "User 2" },
      ]);
    })
  );

  const users = await fetchUsers();
  expect(users).toHaveLength(2);
});

Flaky Test Detection Script

// scripts/​detect-flaky-tests.ts
import { execSync } from "child_process";

async function detectFlakyTests(iterations: number = 10) {
  const results = new Map<string, { passed: number; failed: number }>();

  for (let i = 0; i < iterations; i++) {
    console.log(`\nRun ${i + 1}/​${iterations}`);

    try {
      const output = execSync("npm test -- --reporter=json", {
        encoding: "utf-8",
      });

      const testResults = JSON.parse(output);

      testResults.testResults.forEach((file: any) => {
        file.assertionResults.forEach((test: any) => {
          const key = `${file.name}::${test.fullName}`;
          const stats = results.get(key) || { passed: 0, failed: 0 };

          if (test.status === "passed") {
            stats.passed++;
          } else {
            stats.failed++;
          }

          results.set(key, stats);
        });
      });
    } catch (error) {
      console.error("Test run failed:", error);
    }
  }

  // Analyze results
  console.log("\n🔍 Flaky Test Report\n");

  const flakyTests: string[] = [];

  results.forEach((stats, testName) => {
    if (stats.failed > 0 && stats.passed > 0) {
      const failureRate = (stats.failed / iterations) * 100;
      console.log(`❌ FLAKY: ${testName}`);
      console.log(`   Passed: ${stats.passed}/​${iterations}`);
      console.log(`   Failed: ${stats.failed}/​${iterations}`);
      console.log(`   Failure rate: ${failureRate.toFixed(1)}%\n`);
      flakyTests.push(testName);
    }
  });

  if (flakyTests.length === 0) {
    console.log("✅ No flaky tests detected!");
  } else {
    console.log(`\n🚨 Found ${flakyTests.length} flaky tests`);
    process.exit(1);
  }
}

detectFlakyTests(20); // Run tests 20 times

Root Cause Analysis

// Framework for analyzing flaky tests
interface FlakyTestAnalysis {
  testName: string;
  failureRate: number;
  symptoms: string[];
  rootCause: "timing" | "state" | "randomness" | "network" | "unknown";
  recommendation: string;
}

function analyzeTest(
  testName: string,
  errorMessages: string[]
): FlakyTestAnalysis {
  const analysis: FlakyTestAnalysis = {
    testName,
    failureRate: 0,
    symptoms: [],
    rootCause: "unknown",
    recommendation: "",
  };

  // Detect timing issues
  if (
    errorMessages.some(
      (msg) => msg.includes("timeout") || msg.includes("not found")
    )
  ) {
    analysis.symptoms.push("Timeout or element not found");
    analysis.rootCause = "timing";
    analysis.recommendation =
      "Add explicit waits using waitFor() or findBy* queries";
  }

  // Detect shared state
  if (
    errorMessages.some(
      (msg) =>
        msg.includes("already exists") || msg.includes("unique constraint")
    )
  ) {
    analysis.symptoms.push("Duplicate or existing data");
    analysis.rootCause = "state";
    analysis.recommendation =
      "Add beforeEach/​afterEach cleanup or use unique test data";
  }

  // Detect randomness
  if (
    errorMessages.some(
      (msg) => msg.includes("expected") && msg.includes("received")
    )
  ) {
    analysis.symptoms.push("Inconsistent values");
    analysis.rootCause = "randomness";
    analysis.recommendation =
      "Use deterministic test data or seed random generators";
  }

  // Detect network issues
  if (
    errorMessages.some(
      (msg) => msg.includes("network") || msg.includes("ECONNREFUSED")
    )
  ) {
    analysis.symptoms.push("Network or connection errors");
    analysis.rootCause = "network";
    analysis.recommendation = "Mock all network requests using MSW or similar";
  }

  return analysis;
}

Stabilization Guidelines

// Test stability checklist
const stabilityChecklist = {
  timing: [
    "Use waitFor() instead of fixed timeouts",
    "Use findBy* queries (built-in waiting)",
    "Set appropriate timeout values",
    "Wait for loading states to disappear",
  ],
  state: [
    "Clear database before each test",
    "Reset mocks after each test",
    "Use test-specific data (unique IDs)",
    "Avoid global variables",
  ],
  randomness: [
    "Use fixed seed for random generators",
    "Use deterministic test data",
    "Avoid Date.now() - mock time instead",
    "Generate IDs deterministically",
  ],
  network: [
    "Mock all API calls",
    "Use MSW for HTTP mocking",
    "Avoid real external services",
    "Test network errors explicitly",
  ],
  parallelism: [
    "Use isolated databases per test worker",
    "Avoid port conflicts (random ports)",
    "Dont share file system state",
    "Use test.concurrent cautiously",
  ],
};

Auto-Fix Patterns

// Automated fixes for common issues

// Fix 1: Add waitFor to assertions
function addWaitFor(code: string): string {
  // Replace: expect(screen.getByText('...')).toBeInTheDocument()
  // With: await waitFor(() => expect(screen.getByText('...')).toBeInTheDocument())

  return code
    .replace(
      /​expect\(screen\.getBy/​g,
      "await waitFor(() => expect(screen.getBy"
    )
    .replace(/\)\.toBeInTheDocument\(\)/​g, ").toBeInTheDocument())");
}

// Fix 2: Replace getBy with findBy
function replaceGetByWithFindBy(code: string): string {
  return code.replace(/​screen\.getBy/​g, "await screen.findBy");
}

// Fix 3: Add cleanup
function addCleanup(code: string): string {
  if (!code.includes("afterEach")) {
    const insertPoint = code.indexOf("test(");
    return (
      code.slice(0, insertPoint) +
      "afterEach(async () => {\n  await cleanup();\n});\n\n" +
      code.slice(insertPoint)
    );
  }
  return code;
}

Monitoring Flaky Tests in CI

# .github/​workflows/​test-stability.yml
name: Test Stability

on:
  schedule:
    - cron: "0 2 * * *" # Run nightly

jobs:
  stability-check:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/​checkout@v4

      - uses: actions/​setup-node@v4
        with:
          node-version: "20"

      - run: npm ci

      - name: Run tests 20 times
        run: |
          for i in {1..20}; do
            echo "Run $i/​20"
            npm test || echo "FAILED: Run $i"
          done

      - name: Analyze results
        run: npm run detect-flaky-tests

Best Practices

1. Explicit waits: Never use sleep/​timeout
2. Clean state: Reset between tests
3. Deterministic data: No randomness
4. Mock external deps: APIs, time, randomness
5. Run tests multiple times: Catch intermittent failures
6. Isolate tests: No shared state
7. Monitor CI: Track flaky test trends

Output Checklist

• Common patterns identified
• Root cause analysis performed
• Timing issues fixed (waitFor)
• Shared state eliminated (cleanup)
• Randomness removed (fixed seeds)
• Network mocked (MSW)
• Detection script implemented
• Stabilization guidelines documented
• CI monitoring configured