Questions
2 of 5
Basics
JSX
Virtual DOM
Reconciliation
State and Props
Components
Class Components
Function Components
Lifecycle functions
Code Sharing
Hooks
useState
useReducer
useMemo
useContext
useCallback
useEffect
Custom Hooks
New Hooks
HOC
Refs
Data Sharing
Events
Context API
Redux
Router
CSR-SSR
React APIs
Performance Optimisation
ReactDOM
Strict Mode
Security
What happens once you wrap your app in strict mode?
Difficulty: 5/10
strict mode, lifecycle warnings, dev tooling
<StrictMode> lets you find common bugs in your components early during development process. Although the Strict Mode checks only run in development, they help you find bugs that already exist in your code but can be tricky to reliably reproduce in production. Strict Mode lets you fix bugs before your users report them.
Strict Mode enables the following checks in development:
- 1
Your components will re-render an extra time to find bugs caused by impure rendering.
- 2
Your components will re-run Effects an extra time to find bugs caused by missing Effect cleanup.
- 3
Your components will be checked for usage of deprecated APIs.
Scenario Questions
0-2 years experience
- 1If you wrap your root component in <React.StrictMode>, what immediate changes do you notice when you run the app locally?
- 2How would you check that a useEffect cleanup runs correctly after enabling StrictMode?
- 3What happens to console warnings for deprecated lifecycle methods when StrictMode is turned on?
2-5 years experience
- 1You added StrictMode to a feature branch and a third‑party UI library now throws errors about unsafe lifecycles. How would you approach fixing this?
- 2After enabling StrictMode, a component renders twice and triggers duplicate API calls. Walk me through how you'd debug and resolve the issue.
- 3Explain why a form component might lose state updates under StrictMode and how you'd adjust the code to prevent it.
5-8 years experience
- 1Our monorepo is being migrated to StrictMode incrementally. What rollout strategy would you use to avoid breaking production, and how would you measure success?
- 2StrictMode runs certain checks only in development. How would you ensure similar safety guarantees are enforced in production builds?
- 3Discuss the performance impact of StrictMode's double rendering during development and how you would mitigate developer friction.
8+ years experience
- 1As a staff engineer, you need to get multiple teams to adopt StrictMode across a codebase with legacy class components and many external packages. What migration plan and governance model would you propose?
- 2If a critical third‑party library cannot be upgraded to support StrictMode, how would you balance technical debt, release timelines, and team velocity?
- 3How would you design tooling or CI checks to enforce that new code complies with StrictMode constraints organization‑wide?
Follow-up Questions
- Can you show an example of a warning you saw after enabling StrictMode?
- What would you do if a third‑party library fails under StrictMode?
- How do you verify that your fix actually resolves the StrictMode issue?