Edit in JSFiddle

let handler = {
 get: function(target, name) {
  	return name in target ? target[name]*10 : "Key does not exist";
  }
}
 
let obj = {
  a: 1,
  b: 2
};
 
let proxyObj = new Proxy(obj, handler);
 
document.write(proxyObj.a, "<br>"); //Result -> 10
document.write(proxyObj.b, "<br>"); //Result -> 20
document.write(proxyObj.c); //Result -> "Key does not exist"