What is an array in JavaScript?

1
An array is a data structure that holds a list of elements.
2
Each element is identified by its index, starting from 0. You can create an array using square brackets []

javascript

Difficulty: 2/10

Topics: indexing, mutation, performance

0-2 years experience

1
You 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
Given let nums = [1, 2, 3];, what does nums[5] evaluate to and why?
3
How would you add a new element to the end of an array without mutating the original array?

2-5 years experience

1
A 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
When 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
Your 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
Our 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
We 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
A 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
Our 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
Multiple 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
We plan to migrate a performance‑critical component from JavaScript arrays to WebAssembly typed arrays. Outline the migration steps and the risks you would mitigate.