es5-private-methods.js

var Service = (function () {
  var Service = function () {
    this.say = "Hello";
  }
  
  Service.prototype.publicMethod = function () {
    privateMethod.call(this);
  }

  var privateMethod = function () {
    console.log(this.say);
  }

  return Service;
})();

// Uncaught TypeError: (intermediate value).privateMethod is not a function(...)
new Service().privateMethod();

// Will successfully call privateMethod internally.
new Service().publicMethod();