The Tor Browser / file revision

 // Basic for-of test with Proxy.

 function iterableProxy(arr) {

     return Proxy.create({

         getPropertyDescriptor: function (name) {

             for (var obj = arr; obj; obj = Object.getPrototypeOf(obj)) {

                 var desc = Object.getOwnPropertyDescriptor(obj, name);

                 if (desc)

                     return desc;

             }

             return undefined;

         }

     });

 }

 var s = '';

 var arr = ['a', 'b', 'c', 'd'];

 var p = iterableProxy(arr);

 // Test the same proxy twice. Each time through the loop, the proxy handler's

 // getPropertyDescriptor method will be called 10 times (once for 'iterator',

 // five times for 'length', and once for each of the four elements).

 for (var i = 0; i < 2; i++) {

     var j = 0;

     for (var x of p)

         assertEq(x, arr[j++]);

     assertEq(j, arr.length);

 }