Node.js is an open-source runtime environment that runs JavaScript outside the browser, mainly on servers. It is neither a language nor a framework.
Created in 2009 by Ryan Dahl, it is built on V8, Google Chrome's JavaScript engine. Its distinctive feature is its asynchronous, non-blocking model: instead of waiting for a slow operation to finish (reading a file, calling a database), it moves on and handles the result when it arrives. This makes it efficient at handling many simultaneous connections.
It is mainly used to build APIs, real-time applications (chat, notifications), microservices and development tools. It ships with npm, the package manager that gives access to a huge ecosystem of libraries. The most common server-side frameworks are Express, Fastify and NestJS.
Its big advantage: a single language, JavaScript or TypeScript, across the whole product. Alternatives such as Deno and Bun follow the same principle.
Why it matters when hiring
A front-end JavaScript developer does not automatically make a good Node.js developer: back-end work requires mastering databases, security, error handling and performance. In the interview, check their understanding of the event loop and of what blocks a Node server. The right signal: APIs in production, ideally in TypeScript, with tests.