Debugging a Node.js application

Model answer. From quickest to most powerful:

1. Logging: console.log/console.error for quick checks; in real services a structured logger (pino, winston) with levels and request ids, so problems can be traced in production.
2. Built-in inspector: start with node --inspect app.js (or --inspect-brk to pause on the first line), then open chrome://inspect, or attach VS Code's debugger. You get breakpoints, step over/into, the call stack, variable inspection and a console in context.
3. `debugger;` statements in code pause there when an inspector is attached.
4. Watch mode: node --watch restarts on file changes while you iterate.
5. Performance and memory: node --cpu-prof, heap snapshots in DevTools, or clinic.js to find slow functions, event-loop blocking and memory leaks.
6. Tests: a failing unit or integration test that reproduces the bug is the most reliable way to fix it and keep it fixed.

Also read stack traces carefully (enable source maps for TypeScript), and check environment variables and configuration, which cause a large share of "works on my machine" bugs.