Getter / Setter

define.js

var obj = {
  _a: 0
};

Object.defineProperty(obj, 'a', {
  get: function () {
    return this._a;
  },
  set: function (val) {
    this._a = val;
    console.log('Updated a to', this._a)
  }
});

simple.js

var obj = {
  _a: 7,
  get a() { 
    return this._a;
  },
  set a(val) {
    this._a = val;
    console.log('Updated a to', this._a)
  }
};