What is the purpose of the `isNaN()` function?

0-2 years experience

1. 1How would you check that a value entered in a form field is a number before you add it to a total?
2. 2What does isNaN('123') return and why?
3. 3If you call isNaN(undefined), what result do you get and what does it mean?

2-5 years experience

1. 1We have a function that sometimes throws NaN errors when processing user input. Walk me through how you'd use isNaN to debug the problem.
2. 2Why might using isNaN on a string like '0' cause unexpected validation behavior in a feature you’re building?
3. 3Explain the difference between isNaN and Number.isNaN and when you’d prefer one over the other in production code.

5-8 years experience

1. 1Our analytics pipeline processes millions of numeric strings. Discuss the correctness and performance implications of using the global isNaN versus Number.isNaN across the codebase.
2. 2Design a reusable validation utility that safely distinguishes numeric from non‑numeric values, handling edge cases such as empty strings, null, and objects.
3. 3How would you refactor a legacy codebase that heavily relies on isNaN to improve reliability without breaking existing behavior?

8+ years experience

1. 1At a platform level we need strict numeric validation across many services written in JavaScript/TypeScript. What architectural changes would you propose to replace isNaN usage, ensure type safety, and keep backward compatibility?
2. 2Discuss the trade‑offs of introducing a lint rule or compiler plugin that bans the global isNaN in favor of Number.isNaN, including impact on developer workflow and CI pipelines.
3. 3How would you plan a migration strategy for deprecating isNaN in a large monorepo, balancing risk, testing coverage, and documentation updates?