What is an array in JavaScript?

0-2 years experience

1. 1You receive an API response with a list of user IDs and need to display the third ID; how would you store and access it using a JavaScript array, and what happens if fewer than three IDs are returned?
2. 2Given `let nums = [1, 2, 3];`, what does `nums[5]` evaluate to and why?
3. 3How would you add a new element to the end of an array without mutating the original array?

2-5 years experience

1. 1A function doubles each number in an input array using a for‑loop but sometimes throws `undefined is not a function`. Walk me through how you'd debug the issue.
2. 2When merging two sorted arrays you notice duplicate entries in the result. Explain a possible cause related to array methods and how you'd fix it.
3. 3Your React component receives a prop that should be an array of objects, but occasionally gets `null`. How would you safely iterate over it without causing a runtime error?

5-8 years experience

1. 1Our service processes JSON payloads containing arrays up to 10 million items. What memory or performance concerns arise with native arrays, and what alternatives or patterns would you consider?
2. 2We need a sliding window over a large array of events to compute a moving average in real time. Describe an efficient algorithm and discuss the trade‑offs of mutating the original array versus using a circular buffer.
3. 3A legacy codebase frequently uses `Array.prototype.splice` to remove items, causing UI jank. How would you refactor the code to improve responsiveness while preserving behavior?

8+ years experience

1. 1Our public API returns large collections of data. We're debating between returning a plain JavaScript array versus a paginated cursor/stream. What architectural factors influence your recommendation?
2. 2Multiple teams share a utility library for array manipulation across micro‑frontends. How would you design a versioned, type‑safe helper module to avoid breaking changes?
3. 3We plan to migrate a performance‑critical component from JavaScript arrays to WebAssembly typed arrays. Outline the migration steps and the risks you would mitigate.