Why does Strict Mode run effects twice in development?
PICTURE THIS: REACT RENDER CYCLE
Simple meaning
React 18 Strict Mode mounts, unmounts, and remounts to surface missing cleanup.
WHY — React instead of guessing?
Why interviewers care about React:
on React.
the situation, the default choice, and one exception - that reads as experience.
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:
- 1React 18 Strict Mode
mounts, unmounts, and remounts to surface missing cleanup.
- 2If my effect is
not idempotent I will see double fetches or duplicate listeners.
- 3I fix cleanup rather
than disabling Strict Mode.
- 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
React 18 Strict Mode mounts, unmounts, and remounts to surface missing cleanup. If my effect is not idempotent I will see double fetches or duplicate listeners.