How do you group an array of objects by a key?

Answer

For a coding answer, state the input contract and complexity before implementation. Handle empty input and mutation rules, then use a few focused examples to validate the solution.

For How do you group an array of objects by a key?, start with the rule or behaviour, then anchor it in a small realistic example. Distinguish the default approach from the exceptions and name the observable outcome: correctness, maintainability, accessibility, performance, or security.

Example

function groupBy(items, key) {
  return items.reduce((groups, item) => {
    const value = item[key]
    ;(groups[value] ??= []).push(item)
    return groups
  }, {})
}

This is a linear-time grouping solution and does not mutate the input array.

How to structure your answer

1. Define the concept in one or two sentences.
2. Explain when you would use it and when you would choose an alternative.
3. Walk through a small example, including an edge case.
4. Close with how you would test or measure the result.

Common mistakes

• Repeating a definition without connecting it to real code.
• Treating an optimization or abstraction as a default rather than a trade-off.
• Omitting lifecycle, error, cleanup, accessibility, or testing considerations when they apply.

Follow-up prompts

• What failure mode would you expect if this were implemented incorrectly?
• How would you test this behaviour?
• What changes when the feature must scale to a larger application or team?

Interview tip

For this easy-level question, narrate your assumptions before coding. Interviewers can assess reasoning from a clear, bounded example much better than from a list of APIs.