object prop watcher

Object.defineProperty(Object.prototype, "watch", {
  writable: false,
  enumerable: false,
  configurable: true,
  value: function(prop, handler){
    var propVal = this[prop];
    Object.defineProperty(this, prop, {
      get: () => propVal,
      set: (val) => {
        handler(this, val, propVal);
        propVal = val;
        return propVal;
      },
      enumerable: true,
      configurable: true,
    });
  }
});

Object.defineProperty(Object.prototype, "unwatch", {
  writable: false,
  enumerable: false,
  configurable: true,
  value: function(prop){
    var val = this[prop];
    delete this[prop]; // remove accessors
    this[prop] = val;
  }
});

var test = {};
test.watch("hello", (obj, newval, oldval)=>{
  console.log('modified hello from', oldval, 'to', newval);
});

test.hello = "world"; // triggers the watch handler