What points to note and take care of while writing custom hooks?

1
Custom Hooks lets you share stateful logic, not the state itself. Each call to a Hook is completely independent from every other call to the same Hook.
2
The code inside your custom Hooks will re-run during every re-render of your component. This is why, like components, custom Hooks need to be pure. Think of custom Hooks’ code as part of your component’s body!
3
Because custom Hooks re-render with your component, they always receive the latest props and state.
4
A good custom Hook makes the calling code more declarative by constraining what it does.

Difficulty: 6/10

Topics: dependency arrays, cleanup effects, state sharing

0-2 years experience

1
Imagine you need to fetch data in a component and you decide to extract the logic into a custom hook. Walk me through how you'd implement that hook and what you need to watch out for regarding dependencies and cleanup.
2
If you forget to include a value in the dependency array of a useEffect inside a custom hook, what could go wrong and how would you notice it?
3
How would you make sure a custom hook you write doesn't cause unnecessary re‑renders in components that consume it?

2-5 years experience

1
You have a custom hook that subscribes to a WebSocket and updates state. During a code review a teammate points out that the hook sometimes leaks listeners after the component unmounts. How would you debug and fix it?
2
When you compose several custom hooks together, you notice the order of hook calls changes the behavior. Explain why order matters and how you'd structure them to avoid bugs.
3
Suppose you need to share a piece of state across several unrelated components using a custom hook. What patterns would you consider and what trade‑offs do they have?

5-8 years experience

1
Our app has dozens of components using a custom hook that caches API responses, and under heavy load we see memory usage growing. How would you redesign the hook to be more memory‑efficient while preserving its public API?
2
We want to expose a library of custom hooks to other teams. What guidelines would you set for naming, error handling, and testing to ensure consistency and maintainability?
3
A custom hook uses useRef to store mutable values, but a bug appears when the ref is accessed before it's initialized. How would you safeguard against this in the hook's implementation?

8+ years experience

1
We are planning to replace several duplicated data‑fetching hooks across multiple services with a unified data layer. How would you approach the migration and what architectural considerations would you keep in mind for the custom hook design?
2
Our organization wants to enforce a policy that all side‑effectful logic lives in custom hooks. What cross‑team standards, lint rules, and runtime checks would you propose to ensure compliance and avoid performance regressions?
3
When scaling to server‑side rendering and streaming, what challenges do custom hooks present and how would you adapt our hook implementations to work correctly in both client and SSR environments?

❓How would you test that the cleanup logic in your hook actually runs?
❓What steps would you take if a consumer reports a memory leak caused by your hook?
❓How do you surface errors from a custom hook back to the component?