Difference between require and import.

0-2 years experience

1. 1You need to add a new utility function to an existing Node.js project that uses CommonJS. How would you import that function using require, and what would you need to change if the project switched to ES modules?
2. 2If you write `import fs from 'fs'` in a file that is currently executed with Node's default CommonJS mode, what error do you see and why?
3. 3When you see `module.exports = { foo }` in a file, how would you import `foo` using both require and import syntax?

2-5 years experience

1. 1Your team upgraded Node to version 14 and started seeing that some of our internal libraries that were written with `module.exports` no longer work when imported with `import`. Walk me through why this happens and how you would fix it.
2. 2During a code review you notice a mix of `require` and `import` statements in the same file, and the build is failing. Explain the root cause and how you would resolve the issue.
3. 3We have a feature flag that conditionally loads a heavy module only when needed. Show how you would implement lazy loading with both require and dynamic import, and discuss the trade‑offs.

5-8 years experience

1. 1Our microservice loads configuration files at startup. We want to move from CommonJS to ES modules to enable tree‑shaking, but we also need to support runtime `require` of plugins. How would you design the module loading layer to handle both patterns without breaking existing plugins?
2. 2A performance regression appeared after we switched a core utility from `require` to `import`. Explain possible reasons related to module caching and initialization order, and how you would investigate and mitigate.
3. 3We need to expose a library that can be consumed by both CommonJS and ESM consumers. Describe the packaging strategy (e.g., dual package, .cjs/.mjs) and the pitfalls you must watch for.

8+ years experience

1. 1Our organization is planning a phased migration of a large monorepo from CommonJS to ES modules over two years. Outline the high‑level migration plan, including versioning, CI checks, and how you would handle cross‑team dependencies that still use `require`.
2. 2Several legacy services import a shared utility using `require`, but new services are built with ESM and use `import`. Discuss the long‑term maintenance implications and propose an architectural approach to unify module consumption while minimizing disruption.
3. 3Explain how the Node.js module resolution algorithm differs between `require` and `import` when dealing with package.json `exports` and `type` fields, and how that influences our decision to adopt a single module system across all products.