load them in the node_helper via require as single instance, and move the memory and load to the server side

add my dependencies to this new file, then npm install

Great!
Instead of manually adding dependencies to package.json you can as well install the module with a --saveflag

npm install axios --save

Automatically adds it as dependency to package.json.

npm uninstall axios --save

automatically removes it from package.json.

• adding a package.json file to my MM module (npm init --yes)
• add my dependencies to this new file, then npm install
• add a getScripts entry in my main js file, referencing the various dependencies included in my module

All scripts included are of course installed via console: npm install NODE_MODULE

An example of using them in both environments:

node_helper.js file

var NodeHelper = require('node_helper');
var request = require('request');

very easy, you can now use request as a function.

A bit different with the main MMM-module.js file:

  // Define required scripts. Chart.js needed for the graph.
  getScripts: function() {
    return [
      'modules/MMM-Buienalarm/node_modules/chart.js/dist/Chart.bundle.js',
    ];
  },

Basically, the script is appended to the one you’re writing. Now you can use the functions integrated in the referenced script.

var xxxx = require(‘nom_module_name’)

then see the services (methods) provided there…
see my sample module here

https://github.com/sdetweil/SampleModule