If you want to develop your own npm package, you first have to test it locally.

I had this need with a project that I wanted to modularize.

I had a package I called, as an example, flaviocopes-common-database.

I prepended flaviocopes- to give it a unique namespace.

Inside the package I added a package.json file with the module name in the name property and a few dependencies:

{
  "name": "flaviocopes-common-database",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "dependencies": {
    "pg": "^8.0.2",
    "sequelize": "^5.21.6"
  }
}

Then I ran

npm link

This created a symbolic link in the /usr/local/lib/node_modules/ folder, that contains the global npm packages in the system, the ones installed using npm -g, to be clear.

I had

/usr/local/lib/node_modules/flaviocopes-common-database

Pointing to the local file I had in

/Users/flavio/dev/code/flaviocopes-common-database

Now in another project I wanted to use this module, so I ran

npm link flaviocopes-common-database

and I was able to import it in the Node.js code using the usual require() syntax:

const database = require('flaviocopes-common-database')