Explain the concept of a 'callback function' and provide an example of its usage.

0-2 years experience

1. 1Write a function fetchData(url, callback) that uses setTimeout to simulate an async request and then calls the callback with mock data. How would you verify it works?
2. 2If the callback you receive throws an exception, what happens to fetchData's execution? How could you protect against it?

2-5 years experience

1. 1We need a debounce utility that takes a function and a wait time and returns a debounced version using callbacks. Walk me through your implementation and any trade‑offs.
2. 2During a review you notice a function that accepts a callback but sometimes calls it twice, causing duplicate UI updates. How would you debug and fix the issue?
3. 3Why might a callback‑based API break when you start using Promise‑based code elsewhere? Explain the difference in flow.

5-8 years experience

1. 1Our event system stores callbacks in an array and we’re seeing memory growth under load. How would you redesign the callback handling to prevent leaks and improve performance?
2. 2Design an async task runner that executes an array of callbacks in series, handling errors and optional timeouts. What edge cases do you need to guard against?
3. 3When integrating a third‑party library that uses callbacks, how would you ensure proper cancellation and avoid dangling references in a large SPA?

8+ years experience

1. 1We’re migrating a legacy codebase full of nested callbacks to a Promise/async‑await architecture. What migration strategy would you propose to minimize risk and keep backward compatibility?
2. 2Design a cross‑team event bus that lets modules register callbacks with type safety and versioning. What architectural decisions keep it scalable and maintainable?
3. 3How would you assess the impact of replacing callback‑based streaming APIs with reactive streams on latency and resource usage across our microservices?