Create and publish an NPM module
Code modularization, achieved in one way or another, is a technique a good developer must aim for because it helps keeping things small, well-tested and organized. And of course, it follows the DRY directive. So as a Node.js developer (and maybe contributor to the Open Source), creating and publishing an NPM module is one of those steps you will eventually face.
Probably if any Node.js developer would have to pick an indispensable tool of the ecosystem, npm would win by far. Its greatness resides in how much easy is to build applications using on of several modules, each of them providing one certain functionality, as if they were building bricks, picking the right tool for each task.