What happens if you use the array index as a key?
PICTURE THIS: ARRAY IN MEMORY
Index starts at 0. Scan once for max — O(n).
Simple meaning
Inserting or reordering makes React reuse the wrong component instance, so inputs keep the wrong text.
WHY — React instead of guessing?
Why interviewers care about React:
who only read docs from people who shipped.
and tied to Frontend work.
Name the idea, why it exists, then one short example.
End with when you use it and one common pitfall.
STEPS — What happens step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1Inserting or reordering makes
React reuse the wrong component instance, so inputs keep the wrong text.
- 2Index keys are acceptable
only for static lists that never change.
- 3Interviewers want to hear
a stable id from the data instead.
- 4Give an example
One tiny concrete case you can say aloud.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
Inserting or reordering makes React reuse the wrong component instance, so inputs keep the wrong text. Index keys are acceptable only for static lists that never change.