private fields using closuers

private-methods.js

function Registry(defaultValue){
  const defaultValue = defaultValue;
  const values = Object.create(null);
  
  // These functions will always have access to the above variables. Javascript creates
  // a closure when functions are created, so the outer scope is available to it.
  // These first two functions are public
  this.register = function(name, value){
    values[name] = value;
  };

  this.getValue = function(name){
    let value;
    if (Object.prototype.hasOwnProperty.call(values, name)){
      value = values[name];
    } else {
      value = defaultValue;
    }
    return value;
  };
  
  function privateMethod() {
    // can't be called from outside this constructor (gets hoisted to the top so we can define it here)
  }
}

Register.prototype.publicPrototypeMethod() {};