function isIndex(n) {
  // This isn't right, but pretend that this checks if n is an index in the
  // way that SpiderMonkey checks in js_IdIsIndex.
  return Number(n) % 1 == 0;
}

function indices(obj) {
  var result = [];
  for (var i = 0; i < obj.length; i++)
    result.push(i);
  return result;
}

function paMaker(backing) {
  return {
    defineProperty: function(name, desc) {
      throw new Error("immutable");
    },
    delete: function(name) {
      throw new Error("immutable");
    },
    getOwnPropertyDescriptor: function(name) {
      if (isIndex(name)) {
        if (({}).hasOwnProperty.call(backing, name))
          return { enumerable: true, configurable: false, writable: false, value: backing[name] };
        // report out-of-range indices as not there
        return undefined;
      }
      return Object.getOwnPropertyDescriptor(backing, name); // or something like that
    },
    hasOwn: function(name) {
      return name in backing;
    },
    has: function(name) {
      // the "in" test for indexed properties ignores the prototype hierarchy
      if (isIndex(name))
        return name in backing;
      return name in backing || name in proto;
    },
    get: function(receiver, name) {
      if (isIndex(name)) {
        if (({}).hasOwnProperty.call(backing, name))
          return backing[name];
        return undefined;
      }
      return backing[name];
    },
    set: function(receiver, name, val) {
      throw new Error("immutable");
    },
    enumerate: function() {
      return indices(obj);
    },
    keys: function() {
      return indices(obj);
    }
  };
}

Array.prototype[0] = "foo";
Array.prototype[42] = "foo";
var proxy = Proxy.create(paMaker([,2]));