Perfios Software Software Development Engineer SDE II interview questions

Practice reading and changing existing code (not just writing from scratch). Refactor small Node/Express modules to be more testable; write unit tests (Jest) and small integration tests. Fix/extend React components for performance, accessibility, and maintainability; add unit tests (React Testing Library). MongoDB/Schema changes — write migration-safe updates and tests for queries (edge cases). Be comfortable explaining trade-offs (why a refactor helps, cost of change, backward compatibility). Prepare common MERN pitfalls: callback/Promise leaks, request-timeouts, memory leaks in Node, N+1 DB queries, unhandled rejections, React re-render causes. (General MERN interview topics and best practices can be reviewed in MERN preparation guides).