proxy.handler.js

const proxy = new Proxy(
  { id: "id-0" }, 
  {
    set: function (target, property, value, receiver) {
      if (property.charAt(0) === "_") return false;
      target[property] = value;
      return true;
    }
  ...
  }
);

console.log(proxy);

proxy._id = "_id-1";