In this article, we will analyze the causes of flaky tests in Chai, explore debugging techniques, and provide best practices to ensure reliable assertions in asynchronous test scenarios.

Understanding Flaky Tests in Chai

Flaky tests occur when assertions behave inconsistently due to race conditions, timing issues, or improper async handling. Common causes include:

• Assertions running before an asynchronous operation completes.
• Not returning a promise inside a test, leading to premature completion.
• Using time-dependent checks without proper waiting mechanisms.
• Failing to handle rejected promises correctly.

Common Symptoms

• Tests pass sometimes and fail randomly without code changes.
• Assertions evaluating undefined due to premature execution.
• Test runner completing before async operations finish.
• Unhandled promise rejections causing false positives.

Diagnosing Flaky Chai Assertions

1. Ensuring Async Functions Return a Promise

Check if async tests properly return promises:

it("should resolve with correct data", function() {
    return fetchData().then(data => {
        expect(data).to.have.property("name", "Test");
    });
});

2. Debugging with Deliberate Delays

Introduce artificial delays to test timing dependencies:

it("should wait for async operation", async function() {
    await new Promise(resolve => setTimeout(resolve, 1000));
    expect(await fetchData()).to.have.property("status", "success");
});

3. Catching Unhandled Promise Rejections

Ensure promise rejections are properly handled:

it("should reject with error", function() {
    return expect(fetchData(true)).to.eventually.be.rejectedWith(Error);
});

4. Checking Test Execution Order

Log test execution steps to verify order:

beforeEach(() => console.log("Setup"));
it("should execute assertions correctly", () => console.log("Test"));

5. Verifying Time-Dependent Assertions

Use chai-as-promised to ensure reliable async checks:

it("should resolve correctly", function() {
    return expect(fetchData()).to.eventually.have.property("status", "success");
});

Fixing Flaky Chai Assertions

Solution 1: Using Async/Await Correctly

Ensure async tests wait for completion:

it("should work with async/await", async function() {
    const data = await fetchData();
    expect(data).to.have.property("status", "success");
});

Solution 2: Ensuring Proper Promise Chaining

Always return the promise in async tests:

it("should return promise correctly", function() {
    return fetchData().then(data => {
        expect(data).to.have.property("id");
    });
});

Solution 3: Using chai-as-promised for Async Assertions

Enable proper async error handling:

const chaiAsPromised = require("chai-as-promised");
chai.use(chaiAsPromised);
it("should be rejected with an error", function() {
    return expect(fetchData(true)).to.eventually.be.rejectedWith(Error);
});

Solution 4: Using Timeouts Carefully

Ensure enough time for async operations to complete:

it("should wait long enough", function(done) {
    setTimeout(() => {
        expect(true).to.be.true;
        done();
    }, 500);
});

Solution 5: Isolating State Between Tests

Reset states to avoid shared data conflicts:

beforeEach(() => resetDatabase());

Best Practices for Stable Chai Tests

• Always return a promise or use async/await in asynchronous tests.
• Use chai-as-promised for fluent async assertions.
• Avoid using setTimeout without proper control mechanisms.
• Log execution flow to ensure test order consistency.
• Isolate test cases by resetting shared state between runs.

Conclusion

Flaky tests in Chai can lead to unreliable test suites and difficult debugging. By correctly handling async assertions, using promise-based assertions, and ensuring proper test execution flow, developers can build more stable and consistent automated tests.

FAQ

1. Why do my Chai tests sometimes pass and sometimes fail?

Async operations may not have completed before assertions execute, leading to race conditions.

2. How do I ensure Chai properly waits for async operations?

Use async/await or return a promise inside the test.

3. What is chai-as-promised and how does it help?

It extends Chai to handle promises properly, avoiding unhandled rejections.

4. How do I prevent state leaks between tests?

Use beforeEach to reset the test environment before each test.

5. Can setTimeout fix async timing issues in Chai?

It can introduce artificial delays but should be avoided in favor of proper promise handling.