What is a reducer and why must it be pure?
PICTURE THIS: 1, 2, 2, 8
Simple meaning
A reducer is a function of previous state and action that returns the next state.
WHY — Redux instead of guessing?
Why interviewers care about Redux:
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:
- 1A reducer is a
function of previous state and action that returns the next state.
- 2Purity means no fetch,
no random, and no mutation, so time-travel and tests work.
- 3Immer in Redux Toolkit
lets me write mutating syntax while keeping immutability underneath.
- 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 reducer is a function of previous state and action that returns the next state. Purity means no fetch, no random, and no mutation, so time-travel and tests work.