An idea to extend the functionality of Proxy objects.

Ranando King kingmph at gmail.com
Fri Jan 12 22:38:23 UTC 2018


I have an idea I’d like to propose with regards to Proxy objects. I would
like to make a change to spec section 6.1.7.3 (as seen in the latest
draft). The change is as follows:

The Internal Methods of Objects of an ECMAScript engine must conform to the
list of invariants specified below. Ordinary ECMAScript Objects as well as
all standard exotic objects in this specification maintain these
invariants. ECMAScript Proxy objects maintain these invariants by means of
runtime checks on the result of traps invoked on the [[ProxyHandler]]
object using either the corresponding traps on the [[ProxyHandler]] should
the needed trap be defined, or the default internal method otherwise.

Put simply, the change that I’m requesting would have the runtime checks
verify the selfconsistency of the results returned by the [[ProxyHandler]]
object instead of testing the [[ProxyHandler]] results against the target.
The rationale behind this change is to allow Proxy objects the freedom to
behave in a manner that is inconsistent with the behavior that would have
been should the target have been accessed directly, while still requiring
that the consistency of the behavior of the essential internal methods be
upheld. In this way, a Proxy object would be able to project new properties
for a proxied target even if the target object is not extensible, or even
completely hide non-configurable properties. The requirement to do so would
be implementation of the appropriate [[ProxyHandler]] methods so as to
satisfy all of the corresponding invariants for all implemented handlers.
The ECMAScript engine would then see results consistent with expectations,
despite the fact that the results are inconsistent with the actual nature
of the proxied target object.

An example might be the case of a library to mock objects.

/* SourceFile.js */
var obj = { bar: "The real bar. Accept no imitations!", fubar: "Always has
been." };
obj.seal();

export default obj;

/* TestFile.js */
import testObj from "SourceFile";

var mock = new Proxy(testObj, {
  has: function(target, key) {
    var retval = false;
    if (key == "foo") { //add the foo property
      retval = true;
    }
    else if (key != "bar") { //hide the bar property
      retval = (key in target);
    }

    return retval;
  },
  ownKeys: function(target) {
    var retval = Reflect.ownKeys(target);
    var barIndex = retval.indexOf(bar);

    if (barIndex != -1)
      retval.splice(barIndex, 1);

    retval.push("foo");
    return retval;
  },
  defineProperty: function(target, key, descriptor) {
    var retval = true;
    if ((key == "foo") || (key == "bar")) {
      retval = false;
    }
    else {
      Reflect.defineProperty(target, key, descriptor);
    }

    return retval;
  },
  get: function(target, key) {
    var retval = undefined;

    if (key == "foo") {
      retval = "You got the fake property!"
    }
    else if (key != "bar") {
      retval = Reflect.deleteProperty(target, key);
    }

    return retval;
  },
  set: function(target, key, value) {
    var retval = false;
    if ((key != "foo") && (key != "bar"))
      retval = Reflect.set(target, key);
    }
    return retval;
  },
  deleteProperty: function(target, key) {
    var retval = false;
    if ((key != "foo") && (key != "bar"))
      retval = Reflect.deleteProperty(target, key);
    }
    return retval;
  },
  getOwnPropertyDescriptor: function(target, key) {
    var retval;
    if (key == "foo") {
      retval = {
        enumerable: true,
        writable: false,
        configurable: false,
        value: "You got the fake property!"
      };
    }
    else if (key != "bar") {
      retval = Reflect.getOwnPropertyDescriptor(target, key);
    }
    return retval;
  }
});

console.log(mock.fubar); // "Always has been"
console.log(mock.foo);   // #1
console.log(mock.bar);   // #2


Currently, if the above code were run, an error would be thrown at comment
#1. Even if that line was commented out, an error would be thrown at
comment #2. With my proposed change, the code would run successfully.
Comment #1 would be "You got the fake property!" and comment #2 would be
undefined. As a matter of completeness, if the handler only contained the
"get" and "set" methods, this code would throw an error just as it
currently would. The reason that it works with all of the handlers this
that these handlers ensure that a consistent description of the is being
presented to the interpreter since the interpreter would rely on the
methods of the proxy object's handler to supply the data needed to validate
the response of the original call.
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