MyType.js

function MyType() {
  // this is the constructor: init shit here
  this.foo = 'bar';
  this.id = 42;
}

// define MyType prototype (available methods for each instances with this bound to proper context)
MyType.prototype = {
  init: function init() {
    console.log('init');
    console.log('foo', this.foo);
    console.log('id', this.id);
  },
  
  doAsyncWork: function (callback) {
    // faking async work with setTimeout()
    var self = this;
    setTimeout(function () {
      // here you cannot access your instance properties with "this"
      // because it is bound to the new anonymous function you just created
      console.log('logging', self.foo);
    }, 1000);
  }
};