The Tor Browser / file revision

 // Removing a Map entry already visited by an iterator does not cause any

 // entries to be skipped.

 var map = Map();

 for (var i = 0; i < 20; i++)

     map.set(String.fromCharCode('A'.charCodeAt(0) + i), i);

 var log = '';

 for (var [k, v] of map) {

     log += k;

     if (v % 5 === 4) {

         // Delete all entries preceding this one.

         for (let [k1, v1] of map) {

             if (k1 === k)

                 break;

             map.delete(k1);

         }

     }

 }

 assertEq(log, 'ABCDEFGHIJKLMNOPQRST');

 assertEq(map.size, 1);  // Only the last entry remains.

 assertEq(map.get('T'), 19);