Edit in JSFiddle

const cars = {
  merc: 's320',
  buggati: 'veyron',
};

const handler = {
  deleteProperty: function(target, prop) {
    if (prop in target) {
    	document.write(`${prop} has been removed <br>`); // Result -> merc has been removed
      delete target[prop];
    }
  }
};

document.write(cars.merc, "<br>"); // Result -> "s320"

const proxy = new Proxy(cars, handler);
delete proxy.merc;

document.write(cars.merc, "<br>"); // Result -> undefined