❗️Что будет на выходе:

const handler = {
  get(target, prop, receiver) {
    if (prop in target) {
      return Reflect.get(target, prop, receiver) * 2;
    }
    return Reflect.get(target, prop, receiver);
  },
  set(target, prop, value, receiver) {
    if (typeof value !== "number") return false;
    return Reflect.set(target, prop, value * 3, receiver);
  },
  has(target, prop) {
    return prop.startsWith("x") && Reflect.has(target, prop);
  },
};

const obj = new Proxy({ x1: 10, y1: 20 }, handler);
obj.x2 = 15;
obj.y2 = 40;

console.log(obj.x1);
console.log(obj.x2);
console.log("x1" in obj);
console.log("y1" in obj);
console.log(obj.y2);

Ответ: 20 90 true false 240

JavaScript test | #JavaScript & Max