main.js

const MyType = require('./MyType.js');

const t = new MyType();
t.id = 42;

t.publicMethod();
t.publicMethodThatCallsPrivate('hello', 'world');
t.privateMethod(); // won't work

MyType.js

function MyType() {
  this.id = null;
}

function privateMethod(foo, bar) {
  console.log('private work on instance: ' + this.id);
  console.log(foo, bar);
}

MyType.prototype.publicMethod = function () {
  console.log('public work on instance: ' + this.id);
};

MyType.prototype.publicMethodThatCallsPrivate = function () {
  console.log('gonna call private: ' + this.id);
  privateMethod.apply(this, arguments);
};