proxy.observable.js

const onChange = value => console.log(value);

const proxy = new Proxy({}, {
  set: function (target, propertyName, newValue) {
    const oldName = Reflect.get(target, propertyName);
    onChange({ propertyName, oldName, newValue });
    return Reflect.set(target, propertyName, newValue);
  },
  deleteProperty: function (target, propertyName) {
    onChange({ propertyName, oldValue: Reflect.get(target, propertyName) });
    return Reflect.deleteProperty(target, propertyName);
  } 
});

proxy.id = "foo"; // {propertyName: "id", oldName: undefined, newValue: "foo"}
delete proxy.id; // {propertyName: "id", oldValue: "foo"}