Testing async code in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main challenge when testing asynchronous code?

The main challenge is ensuring the test waits for the async operation to complete before checking results, so tests don't finish too early and give false results.

beginner

How do you tell a test framework like Jest to wait for an async function to finish?

You can return a Promise, use async/await syntax, or use a callback like 'done' to signal when the async work is complete.

beginner

What does the 'async' keyword do in a test function?

It allows you to use 'await' inside the test, making the test wait for asynchronous code to finish before continuing.

intermediate

Why should you avoid mixing callbacks and Promises in async tests?

Mixing them can cause confusion and errors, like tests finishing before async code runs, leading to unreliable test results.

intermediate

How can you test that an async function throws an error?

Use 'await expect(asyncFunc()).rejects.toThrow()' in Jest to check that the Promise rejects with an error.

Which keyword allows you to pause a test until an async operation completes?

A await

B async

C return

D done

The 'await' keyword pauses the test until the Promise resolves or rejects.

In Jest, how do you signal that an async test using callbacks is finished?

A Use 'async' keyword

B Return a Promise

C Call the 'done' callback

D Use 'await' keyword

Calling 'done' tells Jest the async test is complete when using callbacks.

What happens if you forget to wait for an async function in a test?

A Test may pass or fail incorrectly because it finishes too early

B Test will always fail

C Test will run synchronously

D Test will throw a syntax error

Without waiting, the test may finish before async code runs, causing wrong results.

Which Jest syntax checks that an async function throws an error?

A await expect(fn()).toThrow()

B expect(fn()).toThrow()

C expect(fn()).resolves.toThrow()

D await expect(fn()).rejects.toThrow()

'rejects.toThrow()' is used to test rejected Promises with errors.

What is a good practice when writing async tests?

A Always use callbacks only

B Use async/await or return Promises, but avoid mixing callbacks and Promises

C Never use async/await

D Ignore async behavior in tests

Using async/await or returning Promises keeps tests clear and reliable.

Explain how to write a test for an asynchronous function using async/await in Node.js.

Think about how to pause the test until the async work finishes.

Describe how to test that an async function throws an error using Jest.

Focus on how Jest handles rejected Promises in tests.