Promise error handling in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of .catch() in a JavaScript Promise?

.catch() is used to handle errors or rejections that happen in a Promise chain. It catches any error thrown or rejection from the Promise and lets you respond to it.

beginner

How does try...catch work with async/await for error handling?

When using async/await, you can wrap the await call inside a try block. If the Promise rejects, the error is caught in the catch block, allowing you to handle it cleanly.

beginner

What happens if you don't handle a rejected Promise?

If a Promise rejects and you don't handle it with .catch() or try...catch, it causes an unhandled promise rejection. This can crash your program or cause unexpected behavior.

intermediate

Can you chain multiple .catch() handlers on a Promise?

No, only the first .catch() in the chain will catch the error. After an error is caught, the chain continues normally unless you throw another error.

intermediate

How can you handle errors in multiple Promises running in parallel?

You can use Promise.allSettled() to wait for all Promises to finish, then check which ones failed. Or use Promise.all() with a .catch() to handle if any reject.

Which method is used to catch errors in a Promise chain?

A .catch()

B .then()

C .finally()

D .resolve()

.catch() is specifically designed to handle errors or rejections in Promises.

What happens if a Promise rejects but no .catch() is provided?

A The error is ignored silently

B An unhandled promise rejection occurs

C The Promise automatically retries

D The Promise resolves with undefined

Without a .catch(), rejected Promises cause unhandled promise rejections, which can crash the program.

How do you handle errors when using async/await?

A Use <code>.catch()</code> after the await

B Use <code>finally</code> block only

C Errors are handled automatically

D Wrap the <code>await</code> call in a <code>try...catch</code> block

Errors from await can be caught using try...catch blocks.

Which Promise method lets you wait for all Promises to finish regardless of success or failure?

A Promise.allSettled()

B Promise.race()

C Promise.all()

D Promise.resolve()

Promise.allSettled() waits for all Promises to settle, whether fulfilled or rejected.

If you want to handle errors only once in a Promise chain, where should you place .catch()?

A At the start of the chain

B After every <code>.then()</code>

C At the end of the chain

D You don't need <code>.catch()</code>

Placing .catch() at the end catches any error from the entire chain.

Explain how to handle errors in a Promise chain using .catch().

Think about how you catch mistakes in a sequence of steps.

Describe how try...catch works with async/await for error handling.

Imagine waiting for a task and catching problems as if it was normal code.