This polyfills Map.prototype.forEach and Set.prototype.forEach.

The ordering of the iteration is done by storing a parallel object that maps the keys to an string key which is used on a plain JavaScript object which we do the iteration over.

var map = new Map;
map.set('a', 'A');
map.set(document.body, 'B');
map.forEach(function(value, key) {
  console.log(value, key);
});

In the case of Set the callback passes the value twice so that the callback function has the same signature as the callback used in Array.prototype.forEach.

var set = new Set;
set.add('a');
set.add(document.body);
set.forEach(function(value, valueAgain) {
  assert(value === valueAgain);
  console.log(value);
});

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This repository uses Git Submodules. You will need to run git submodule init and git submodule update from the top level directory of the project working tree before you can run tests, for example.