What is the difference between a mock and a stub?
PICTURE THIS: DATA SPLIT
Fit on train, tune on val, report on test once.
Simple meaning
A stub returns prepared data so the unit under test can run.
WHY — Testing instead of guessing?
Why interviewers care about Testing:
contrast on Testing, not two memorised paragraphs.
the developer, then one case where picking wrong hurts.
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:
- 1A stub returns prepared
data so the unit under test can run.
- 2A mock also asserts
that certain calls happened, such as an email being sent once.
- 3Overusing mocks couples tests
to implementation and makes refactors painful.
- 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
A stub returns prepared data so the unit under test can run. A mock also asserts that certain calls happened, such as an email being sent once.