Edit in JSFiddle

let handler = {
 set: function(target, name, value) {
  	target[name] = value*10;
  }
}
 
let obj = {
  a: 1,
  b: 2
};
 
let proxyObj = new Proxy(obj, handler);

proxyObj.c = 3;
document.write(proxyObj.a, "<br>"); //Result -> 1
document.write(proxyObj.b, "<br>"); //Result -> 2
document.write(proxyObj.c); //Result -> 30