How do you decide what to test in a new feature, and how do you balance unit tests against integration tests?

What the interviewer is looking for

• This question tests testing & code quality. Explains what deserves tests and why, distinguishes unit from integration tests, and treats readability and refactoring as part of the job.
• Strong answers: Focusing on business logic, edge cases and risky paths, fast unit tests for logic and fewer integration tests for wiring, and avoiding brittle tests of implementation details.
• Likely follow-up: Tell me about a test that caught a real bug before it reached users.

Another way it may be asked: You just built a checkout discount feature. Which parts would you write tests for, and what kinds of tests would they be?

A structure to follow: Answer, explain, example, limits

1. Step 1

   Answer

   Give your direct answer in the first sentence so the interviewer knows where you are going.

2. Step 2

   Explain

   Say how it works or why you would do it that way, in plain words before jargon.

3. Step 3

   Example

   Anchor it in something you have actually done or seen, with the detail that shows hands-on experience.

4. Step 4

   Limits

   Say when your approach would not be the right one. It shows judgement.

This is guidance on shape, not a script. Use your own real experience and facts; never invent details to fill a gap. See the STAR guide and answer frameworks.

Common slips

• Reciting a definition without showing you have applied it.
• Using terms you cannot explain if asked a level deeper.
• Claiming experience beyond what you can discuss in detail.

How to practise it

Say the answer in 90 seconds, then answer the follow-up as if it were a new question.

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