Getter and Setter methods for object properties in JavaScript: https://medium.com/@maurobringolf/javascript-data-encapsulation-and-property-getter-and-setter-methods-a59cbddf2ef4#.qimkogc2u

restricted-javascript-setter.js

//Defining a restricted access property o.x using getters and setters
var o = {
  c: 0,
  get x() { return this.c; },
  set x(y) { this.c = Math.max(y,0); }
}

o.x = 3;
o.x;      //3

o.x = -4;
o.x;      //0

//can be broken
o.c = -4;
o.x;      //-4