__{define,lookup}{Get,Set}ter__ don't work as they do in impls

[gsnedders]

Consider the following:

var x = {};
Object.defineProperty(x, "foo", {get: function(){return 1}});
print(x.__lookupGetter__("foo"));

Per the current spec, this should print undefined. Per every implementation, this should print the get property of the foo property descriptor on x (i.e., the function).

As far as I can tell, __lookupGetter__ (with __lookupSetter__ being a very slight variation on this with .get replaced by .set) should be:

Object.prototype.__lookupGetter__ = function(propertyName) {
  var obj = this;
  do {
    var desc = Object.getOwnPropertyDescriptor(obj, propertyName);
    if (desc !== undefined)
      return desc.get;
  } while((obj = Object.getPrototypeOf(obj)) !== null);
}

…where all the built-ins have their initial values (i.e., Object.prototype, Object.getOwnPropertyDescriptor, Object.getPrototypeOf, and undefined).

Now, to look at the define variants:

Set the internal [[Enumerable]] property of obj’s prop property to true.

What if obj doesn't have a prop property? In a case like var x = {}; x.__defineGetter__("foo", function(){return 1}); there is no foo property on x, as where would it come from?

Whenever a property with the name of prop is accessed on the obj object, call function with the this binding set to obj, and return the result.

What does this even mean? What should the following do:

var x = {};
x.__defineGetter__("foo", function(){return 1});
print(Object.getOwnPropertyDescriptor("foo"));
print(Object.getOwnPropertyDescriptor("foo").get);

Is this accessing the property? What on earth should the object that Object.getOwnPropertyDescriptor look like? You've not created a property descriptor for foo, you've just magically defined its behaviour!

As far as I can tell, the actual behaviour is the following (again, assuming all built-ins have their initial value, and the setter variant just being the set property and not the get property):

Object.prototype.__defineGetter__ = function(propertyName, func) {
  if (typeof func !== "function")
    throw TypeError("not a function");
  Object.defineProperty(this, propertyName, {get: func, enumerable: true});
}

As it is now, it is pretty much totally undefined how these functions relate to the ES object model (just some handwaving about "whenever a property with name of prop is accessed on the obj object").

[gsnedders]

(i.e., these operations should be defined in terms of [[Get]] and [[Put]], not black-box magic)

[anba]

Which alternative do you prefer? 😄

• SpiderMonkey
• JavaScriptCore
• V8

[anba]

Adding @bterlson, because he volunteered to write an initial spec draft for ES7.

[gsnedders]

Nice fix. :)