In this article, we will analyze the causes of flaky tests in Chai, explore debugging techniques, and provide best practices to ensure stable and deterministic test execution.

Understanding Flaky Tests in Chai

Flaky tests in Chai occur when assertions intermittently fail or produce inconsistent results due to race conditions, improper asynchronous handling, or shared state between tests. Common causes include:

• Using expect without properly handling promises.
• Asynchronous assertions executing before promises resolve.
• Test state leaking between runs due to global variables.
• Unreliable test order affecting dependent assertions.
• Timeout-related issues causing unpredictable failures.

Common Symptoms

• Tests pass locally but fail randomly in CI/CD pipelines.
• Assertions fail due to undefined or unexpected values.
• Errors such as AssertionError: expected undefined to equal value.
• Timeout errors in tests involving asynchronous operations.
• Intermittent behavior where re-running tests changes results.

Diagnosing Flaky Tests in Chai

1. Checking for Improper Promise Handling

Ensure assertions wait for promise resolution:

const fetchData = () => Promise.resolve("data");

it("should handle promises correctly", async () => {
    const result = await fetchData();
    expect(result).to.equal("data");
});

2. Ensuring Proper Use of done in Async Tests

Ensure tests signal completion properly:

it("should resolve asynchronously", function(done) {
    setTimeout(() => {
        expect(true).to.be.true;
        done();
    }, 100);
});

3. Detecting Test Order Dependencies

Ensure tests do not depend on execution order:

describe("Independent tests", () => {
    beforeEach(() => resetDatabase());
    it("should insert a record", () => {...});
    it("should delete a record", () => {...});
});

4. Avoiding Leaked Global State

Ensure tests do not modify shared state:

let counter = 0;

it("should not leak state", () => {
    counter++;
    expect(counter).to.equal(1); // May fail in repeated runs
});

5. Debugging Timeout Issues

Increase timeout for long-running async tests:

it("should complete within time", async function() {
    this.timeout(5000);
    await longRunningTask();
});

Fixing Flaky Tests in Chai

Solution 1: Using async/await for Promises

Ensure async assertions wait for promise resolution:

it("should return data correctly", async () => {
    const result = await fetchData();
    expect(result).to.equal("data");
});

Solution 2: Properly Using done in Callbacks

Ensure callback-based async tests call done() correctly:

it("should work with callbacks", function(done) {
    asyncFunction((err, result) => {
        expect(result).to.equal("success");
        done();
    });
});

Solution 3: Isolating Test State

Reset state before each test to avoid leaks:

beforeEach(() => {
    globalState = {};
});

Solution 4: Running Tests in a Fixed Order

Ensure deterministic execution order with mocha --sort:

mocha --sort

Solution 5: Adjusting Timeouts for Async Operations

Set appropriate timeouts for slow tests:

it("should handle long tasks", async function() {
    this.timeout(10000);
    await longRunningTask();
});

Best Practices for Stable Chai Testing

• Use async/await instead of callbacks for promise-based assertions.
• Ensure test state is reset before each run.
• Avoid depending on test execution order.
• Set explicit timeouts for long-running tests.
• Run tests multiple times in CI/CD to detect flakiness.

Conclusion

Flaky tests in Chai can lead to unreliable testing and CI/CD failures. By properly handling async assertions, isolating test state, and ensuring deterministic execution order, developers can build stable and predictable test suites.

FAQ

1. Why do my Chai tests fail intermittently?

Flaky tests often result from improper promise handling, shared state, or race conditions.

2. How do I fix asynchronous assertion failures in Chai?

Use async/await instead of relying on done() for promise-based tests.

3. What is the best way to debug flaky tests?

Run tests multiple times, check for state leaks, and use logging to track execution order.

4. Can timeouts cause tests to fail randomly?

Yes, improper timeout settings can lead to test failures under varying system loads.

5. How do I make Chai tests more stable?

Ensure proper async handling, reset test state, and avoid execution order dependencies.