To create a local (unpublished) library package

Create a 'my-library' folder. Include source code, exporting any desired functions. Folder must include the 'package.json' file generated by npm init

cd into the folder of the project that needs to use your library. Run npm install --save local/path/to/my-library. The --save will add the package to your dependencies in the project's package.json file, as it does with 3rd party published packages. It will also add a copy of the source code to the node modules folder of the project, as always.

import/require the package as you would normally, from any project. For example import { myFunction } from "my-library"