How do you approach onboarding to an unfamiliar Node.js codebase?

Describe a repeatable method, because you will be doing exactly this in the first week of the job.

• Get it running first. Before reading anything, install, run, and hit one endpoint. What is missing from the README is itself the first useful contribution.
• Follow one request end to end. Pick a real route and trace it through routing, middleware, service layer, and database. One vertical slice teaches you more than reading every folder.
• Read the tests and the package.json. Tests document intent, and the dependency list tells you the architecture — an ORM, a queue library, and a validation library between them describe the shape of the system.
• Find out how it is deployed and observed. Where the logs go, what the alerts are, how a rollback works.
• Start with a small real change. A bug fix teaches you the workflow; reading teaches you the map.

Note: Mention writing down what confused you and turning it into documentation. It is a genuinely valued habit and easy to demonstrate.