Map using tuples or objects

Ok, I’ve raised the issue on esdiscuss now and I got an answer from Mozilla’s Jason Orendorff:

1. This is a problem with ES6 maps.
2. The solution will come in the form of ES7 value objects for keys instead of objects.
3. It was considered before to let people specify .equals and .hashCode but it was rejected in favor of value objects. (for good reasons in my opinion).
4. The only solution as of now is to roll your own collection.

A basic such collection (concept, don’t use in production code) was offered by Bradley on the ESDiscuss thread and might look something like this:

function HashMap(hash) {
  var map = new Map;
  var _set = map.set;
  var _get = map.get;
  var _has = map.has;
  var _delete = map.delete;
  map.set = function (k,v) {
    return _set.call(map, hash(k), v);
  }
  map.get = function (k) {
    return _get.call(map, hash(k));
  }
  map.has = function (k) {
    return _has.call(map, hash(k));
  }
  map.delete = function (k) {
    return _delete.call(map, hash(k));
  }
  return map;
}

function TupleMap() {
  return new HashMap(function (tuple) {
    var keys = Object.keys(tuple).sort();
    return keys.map(function (tupleKey) { // hash based on JSON stringification
               return JSON.stringify(tupleKey) + JSON.stringify(tuple[tupleKey]);
    }).join('\n');
    return hashed;
  });
}

A better solution is to use something like MontageJS/Collections which allows for specification of hash/equals functions.

You can see the API docs here.