__noSuchProperty__

__noSuchProperty__.js

var myObject = {
  __noSuchProperty__: function(name) {
    return name;
  }
};
 
var myProxyObject = new Proxy(myObject, {
  get: function(target, name) {  
    return name in target? 
            target[name]:
            target.__noSuchProperty__(name);
  }
});
 
myProxyObject.foo; // "foo"
myProxyObject.foo = "bar";
myProxyObject.foo; // "bar