Jest Async Testing Essentials Quiz Quiz

Challenge your understanding of asynchronous testing in Jest with focused questions on async functions, promises, and async testing methods. This quiz evaluates key concepts that are vital for writing effective asynchronous tests in modern JavaScript projects.

1. Async Function Testing Techniques

   Which method is recommended to test an async function that returns a promise in Jest?

   1. Return the promise from the test function
   2. Place setTimeout inside a describe block
   3. Use expect.assertTrue instead of expect
   4. Omit any return statement in the test

   Explanation: Returning the promise from the test function ensures that Jest knows when the asynchronous operation is finished. Placing setTimeout in a describe block does not control test timing. Using expect.assertTrue is syntactically incorrect; expect should be used. Omitting the return statement may cause tests to pass before the promise is resolved or rejected.

2. Testing Rejected Promises

   When testing that an async function throws or rejects, which Jest matcher is most appropriate to use with async/await?

   1. await expect(promise).rejects.toThrow()
   2. expect(promise).toThrowSync()
   3. expect(promise).throwsAsync()
   4. await expect(promise).resolves.toThrow()

   Explanation: The correct way to assert that a promise rejects with an error is using await expect(promise).rejects.toThrow(). toThrowSync() is not a valid Jest matcher and applies to synchronous code. throwsAsync() is similarly invalid. resolves.toThrow() is incorrect because toThrow is for errors thrown, not resolved values.

3. Using done Callback in Async Tests

   In which scenario should you use the done callback in Jest tests for async code?

   1. When using callback-based asynchronous APIs
   2. When testing async functions that return promises
   3. When using synchronous code only
   4. When expecting Jest to automatically resolve timeouts

   Explanation: The done callback is used when testing asynchronous code that uses callbacks instead of promises, allowing you to signal when the test is complete. For promise-based or async/await code, you should not use done. Using done is unnecessary for synchronous code and will not help Jest handle timeouts automatically.

4. Combining Multiple Async Assertions

   If you need to test that multiple async operations (promises) resolve as expected in a single Jest test, what is the best approach?

   1. Use Promise.all and return or await the result
   2. Nest each operation inside separate it blocks
   3. Call expect for each without awaiting
   4. Place all operations inside a describe statement

   Explanation: Using Promise.all and returning or awaiting the combined result ensures that all async operations are completed before the test ends. Nesting operations in separate it blocks splits them into separate tests. Calling expect without awaiting risks the test finishing before the assertions run. describe is for grouping tests, not for handling async logic.

5. Handling Timers in Asynchronous Jest Tests

   Which method helps simulate or control setTimeout and setInterval in Jest async tests?

   1. Use fake timers with jest.useFakeTimers()
   2. Wrap each timer call in a promise
   3. Replace setTimeout with setImmediate
   4. Increase the test timeout with jest.timeout()

   Explanation: jest.useFakeTimers() allows you to simulate and manage timer functions, making it easier to test timing-dependent async code. Wrapping timers in promises does not control their execution. setImmediate is a separate method, not a replacement for setTimeout. There's no jest.timeout() method to change test timeout.