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How does `Promise.finally()` work? | Promises | javascript Interview Questions | NextRound.in

Questions

3 of 17

1What is the purpose of `Promise.promisify()`?

2How can you create a custom Promise with timeout functionality?

3How does `Promise.finally()` work?

4How do you convert a callback-based function into a Promise-based function?

5What is the purpose of `Promise.all()`, `Promise.race()`, `Promise.any()`, `Promise.allSettled()`?

6How do you handle errors in a chain of Promises?

7What is the purpose of `Promise.resolve()` and `Promise.reject()`?

8How do you create a new Promise in JavaScript?

9Explain the three states of a Promise.

10What is the purpose of the `async` and `await` keyword in JavaScript??

11What is Promise chaining?

12How can you handle multiple Promises concurrently without using `Promise.all()`?

13What is a Promise in JavaScript?

14Explain the concept of 'callback hell' and how to mitigate it.

15How do you handle the result of a Promise?

16Do all `async` functions return Promises? Why or why not?

17How do you handle errors when using `async/await`?

javascript

Event Loop and Runtime

Concurrency and Threading

Bytecode and Parsing

Execution Context

String Operations

Array Operations

Call Apply Bind

Iterator and Generator

Garbage Collector

Functional Programming

Higher Order Functions

03 / 17

How does `Promise.finally()` work?

Difficulty: 5/10

error handling, resource cleanup, promise chaining

The `.finally()` method is used to attach a callback that will be executed regardless of whether the Promise is fulfilled or rejected.

javascript

Scenario Questions

0-2 years experience

1You need to fetch user data and then hide a loading spinner regardless of success or failure. How would you use Promise.finally() to ensure the spinner is hidden?
2If you have a promise that resolves, and you attach a finally handler that returns a value, what will the downstream then receive?

2-5 years experience

1We have a function that retries an API call up to three times on failure. After all attempts, we need to log the duration of the whole operation. How would you incorporate Promise.finally() into the retry logic, and what pitfalls might you watch out for?

Follow-up Questions

What happens if the finally callback returns a rejected promise?
Can you use finally with async/await syntax, and how does it differ from a try/finally block?
How does finally affect the propagation of the original error or value?

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2During a recent bug, a promise chain stopped executing after a finally block. Explain why that might happen and how you would debug it.

5-8 years experience

1Our microservice uses a pool of database connections that must be released after each async operation. Describe how you would design a utility that wraps any async function with Promise.finally() to guarantee connection release, and discuss any edge cases with rejected promises or async/await.
2We are considering replacing custom try/finally patterns with Promise.finally() across the codebase. What performance or compatibility concerns would you evaluate before a large‑scale rollout?

8+ years experience

1A legacy codebase targets ES5 and uses a polyfill for Promise.finally(). As we migrate to a new platform, what architectural considerations arise when deciding whether to keep the polyfill, replace it with native finally, or refactor to async/await with try/finally? Discuss impact on cross‑team contracts and testing.
2Our organization wants to enforce a policy that all network requests must have a cleanup step (e.g., abort controller) even when using third‑party libraries that may not expose promises directly. How would you design a wrapper or abstraction that leverages Promise.finally() to provide this guarantee at a system level?