Promises and Async/Await in Mocha: Testing Best Practices Quiz

Explore core concepts and best practices for handling asynchronous code using Promises and Async/Await in Mocha test suites. Enhance your understanding of error handling, test structure, and common pitfalls when testing async JavaScript functions.

1. Properly Returning a Promise in Mocha

   Which of the following is the correct way to ensure a Mocha test waits for an asynchronous function returning a Promise to resolve before finishing?

   1. Return the Promise from the test function
   2. Call the 'done' callback and return the Promise
   3. Use setTimeout to delay test completion
   4. Wrap the test code in a try-catch without returning anything

   Explanation: Returning a Promise from the test function allows Mocha to wait for its resolution before ending the test, ensuring accurate async behavior. Calling 'done' and returning the promise may lead to unpredictable results, as combining the two is discouraged. Using setTimeout introduces arbitrary delays without guaranteeing correctness. Wrapping code in a try-catch but not returning the promise leads Mocha to finish the test before the asynchronous code completes.

2. Async/Await Syntax in Mocha Tests

   When writing a Mocha test using async/await, which function declaration format enables proper error handling and test completion?

   1. Declare the test function with the 'async' keyword
   2. Use 'await' inside a plain function without 'async'
   3. Wrap the test code with Promise.resolve() only
   4. Omit both 'async' and 'await' from the test function

   Explanation: Declaring the test function with 'async' allows the use of 'await' within the body and ensures that any thrown errors are handled by Mocha. Using 'await' in a non-async function results in a syntax error. Wrapping code with Promise.resolve() doesn't enable await functionality or error handling. Omitting both 'async' and 'await' treats all code as synchronous, so asynchronous operations may be missed.

3. Error Handling in Asynchronous Mocha Tests

   If an asynchronous function being tested with async/await throws an error, what should you do to ensure Mocha correctly recognizes the test as failed?

   1. Allow the error to be thrown within the async function
   2. Catch the error and call 'done()' regardless
   3. Suppress the error silently
   4. Log the error to the console only

   Explanation: Letting the error propagate within the async function lets Mocha detect test failure next time the promise rejects. Catching the error and calling 'done()' does not inform Mocha of a failure unless 'done' is called with the error, which is not shown here. Silently suppressing or only logging the error prevents Mocha from recognizing any issue, allowing potentially failing code to pass unnoticed.

4. Using 'done' with Promises in Mocha

   Which is the main drawback of combining the 'done' callback with Promises in a Mocha test function?

   1. It can cause unpredictable test results or multiple callbacks
   2. It always speeds up test execution
   3. It increases test reliability significantly
   4. It prevents any asynchronous code from running in tests

   Explanation: Mixing 'done' with Promises can confuse Mocha, causing the test to finish early, late, or even be called twice, leading to flaky tests. This practice does not speed up execution, make tests more reliable, or block asynchronous code, but rather does the opposite by introducing potential errors. Best practice is to use either Promises or 'done', but not both together.

5. Testing Functions Rejecting Promises

   Which is a recommended approach to assert that an asynchronous function rejects a Promise in a Mocha test using async/await?

   1. Use try-catch with 'await' and assert inside the catch block
   2. Call the function outside the async test
   3. Place the assertion after the awaited function without any catch
   4. Use synchronous assertions only

   Explanation: By awaiting the function inside a try-catch, you can catch the rejection and place assertions in the catch block to confirm the error was thrown as expected. Calling the function outside the test or without 'catch' will not allow you to handle the error or make assertions. Synchronous assertions do not capture asynchronous errors, thus failing to check the correct behavior.