How does React reconciliation use keys, and what goes wrong with unstable keys?
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Simple meaning
Reconciliation matches children between renders using position and key.
WHY — React instead of guessing?
Why interviewers care about React:
question about React.
trade-offs, and what you would actually do on a Full Stack project - not buzzwords.
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:
- 1Reconciliation matches children between
renders using position and key.
- 2Unstable keys remount components,
reset local state, and retrigger effects.
- 3That is why random
keys or index keys on a sortable list cause lost input state.
- 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
Reconciliation matches children between renders using position and key. Unstable keys remount components, reset local state, and retrigger effects.