Introduction

Chai provides a flexible way to write assertions using `expect`, `should`, and `assert`, making it a popular choice for unit and integration testing. However, when dealing with asynchronous code, incorrect assertion strategies can lead to race conditions, unhandled promise rejections, or misleading test results. These issues often go unnoticed in small test suites but can cause significant instability in large-scale applications. This article explores common pitfalls with async assertions in Chai, debugging techniques, and best practices for writing stable asynchronous tests.

Common Causes of Async Assertion Failures in Chai

1. Missing `return` in Promise-Based Tests

When testing asynchronous functions, forgetting to return a promise can cause tests to pass prematurely before assertions are executed.

Problematic Scenario

// Test passes even if the assertion fails because the promise is not returned
it("should fetch user data", () => {
  fetchUser().then(user => {
    expect(user).to.have.property("name");
  });
});

Solution: Return the Promise or Use `async/await`

// Correct approach with return
it("should fetch user data", () => {
  return fetchUser().then(user => {
    expect(user).to.have.property("name");
  });
});

// Alternative approach using async/await
it("should fetch user data", async () => {
  const user = await fetchUser();
  expect(user).to.have.property("name");
});

Returning the promise ensures that Mocha (or another test runner) waits for the test to complete before marking it as passed.

2. Uncaught Promise Rejections

Failing to properly handle rejected promises can cause tests to hang or fail unexpectedly.

Problematic Scenario

// Test does not handle rejections properly
it("should handle API failure", async () => {
  const user = await fetchUser(); // If this rejects, test will crash
  expect(user).to.be.null;
});

Solution: Use `.catch()` or `expect().eventually`

// Properly handle rejections using try/catch
it("should handle API failure", async () => {
  try {
    await fetchUser();
  } catch (error) {
    expect(error).to.be.an("error");
  }
});

Using `try/catch` ensures that errors are properly handled and don’t cause the test to crash.

3. Assertion Execution Before Async Completion

Assertions executed before an async operation completes can result in false negatives.

Problematic Scenario

// Assertion executes before the promise resolves
it("should check data exists", () => {
  let data;
  fetchData().then(response => {
    data = response;
  });
  expect(data).to.exist; // Test fails because `data` is still undefined
});

Solution: Use `async/await` to Ensure Proper Timing

it("should check data exists", async () => {
  const data = await fetchData();
  expect(data).to.exist;
});

Using `await` ensures that the assertion executes only after the data has been fetched.

4. Using `.eventually` Without `chai-as-promised`

Chai’s `.eventually` assertion requires the `chai-as-promised` plugin, and using it incorrectly can result in test failures.

Problematic Scenario

// Using `.eventually` without `chai-as-promised`
it("should resolve with correct value", () => {
  expect(fetchValue()).to.eventually.equal("expected value");
});

Solution: Use `chai-as-promised` and Return the Assertion

const chaiAsPromised = require("chai-as-promised");
chai.use(chaiAsPromised);

it("should resolve with correct value", () => {
  return expect(fetchValue()).to.eventually.equal("expected value");
});

Returning the `expect` statement ensures that Mocha correctly waits for the promise to resolve.

5. Mixing `done` Callback with Promises

Using the `done` callback with promises can result in duplicate test completions or hanging tests.

Problematic Scenario

// Calling `done()` inside a promise chain can cause issues
it("should fetch user info", done => {
  fetchUser().then(user => {
    expect(user).to.have.property("name");
    done();
  });
});

Solution: Use `async/await` or Return the Promise Instead

// Correct approach without `done`
it("should fetch user info", async () => {
  const user = await fetchUser();
  expect(user).to.have.property("name");
});

Avoid using `done` with promises; use `async/await` instead for cleaner and more reliable async tests.

Best Practices for Writing Stable Async Tests in Chai

1. Always Return Promises in Async Tests

Ensure the test runner properly handles asynchronous execution.

Example:

it("should return user", () => {
  return expect(fetchUser()).to.eventually.have.property("name");
});

2. Use `async/await` for Readability

Using `async/await` simplifies test logic and avoids nested `.then()` chains.

Example:

it("should return correct user", async () => {
  const user = await fetchUser();
  expect(user.name).to.equal("John");
});

3. Handle Promise Rejections Explicitly

Ensure failed promises don’t cause unhandled exceptions.

Example:

it("should reject with an error", async () => {
  await expect(fetchUser()).to.be.rejectedWith(Error);
});

4. Use `chai-as-promised` for Clean Assertions

Enabling `chai-as-promised` simplifies async assertions.

Example:

chai.use(require("chai-as-promised"));

5. Avoid Using `done` with Promises

Use `async/await` instead to prevent duplicate test completions.

Conclusion

Flaky tests and inconsistent assertions in Chai are often caused by improper handling of asynchronous operations. By returning promises, using `async/await`, properly handling rejections, and leveraging `chai-as-promised`, developers can ensure stable and reliable test behavior. Adopting best practices for async assertions eliminates race conditions, reduces test flakiness, and improves the overall quality of test suites.