Can you modify the state directly in a slice reducer? Why does it work?

You're writing a slice reducer and you directly assign to state.items.push(newItem) — it works. Why does it work, and what would happen if you switched to vanilla Redux without RTK?

You see a teammate write state.user.name = 'Alice' in a reducer and it updates the UI. How would you explain to them why this isn't actually mutating the original state?

If you accidentally return state instead of modifying it in an RTK reducer, what behavior would you observe and why?

A feature where users edit a list of items breaks intermittently — sometimes the UI updates, sometimes it doesn’t. You notice the reducer is doing direct state mutations like state.items[0] = newItem. What’s likely going wrong, and how would you fix it?

Your team migrated from vanilla Redux to RTK and now reducers are mutating state directly. A new engineer says 'We don’t need immer anymore' — how do you respond, and what edge case might break if they start using this pattern outside RTK?

You’re debugging a bug where a nested object in state doesn’t update after a reducer runs. You suspect the mutation is shallow. How would you verify if Immer is properly handling the nested structure, and what’s the risk if it isn’t?

You’re optimizing a high-frequency state update path in RTK and considering bypassing Immer by using mutable operations in a custom middleware. What are the risks, and how would you validate correctness at scale?

Your team is migrating a legacy Redux app to RTK. Some reducers rely on direct state mutation for performance. How do you assess whether these mutations are safe under Immer, and what testing strategy would you design to prevent regressions?

A performance audit shows that RTK reducers are causing excessive re-renders due to object reference changes. You suspect the direct mutation pattern is creating deep copies unnecessarily. How would you investigate and optimize this without breaking immutability guarantees?

You’re designing a cross-team state management standard. Should you allow direct state mutation in reducers across all services, even if they use RTK? What long-term maintenance, onboarding, and debugging costs does this introduce, and how would you enforce consistency?

Your company is migrating from RTK to a custom state library that doesn’t use Immer. How do you handle the technical debt of hundreds of reducers that rely on direct mutation, and what’s your migration strategy to avoid silent bugs?

An external audit flags RTK’s mutable-style reducers as a 'hidden mutation anti-pattern' that could break if Immer is ever replaced. How do you defend or refactor this pattern at the architecture level, and what metrics would you track to measure the risk?