What is declaration merging and when does it bite you?
PICTURE THIS: GIT FLOW
Simple meaning
Interfaces with the same name in the same scope merge their members, which is useful for extending libraries.
WHY — TypeScript instead of guessing?
Why interviewers care about TypeScript:
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:
- 1Interfaces with the same
name in the same scope merge their members, which is useful for extending libraries.
- 2Accidental merges from duplicate
names create extra required fields and confusing errors.
- 3I prefer unique names
and type aliases when I do not want merging.
- 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
Interfaces with the same name in the same scope merge their members, which is useful for extending libraries. Accidental merges from duplicate names create extra required fields and confusing errors.