SQL-like "join" function as an underscore.js mixin

underscore.join.js

(function () {

  /*
  TODO: Support more join types?

  TODO: Support single key?
  */

  var crossProduct = function(objects1, objects2) {
    var objects = [];
    for (var i = 0; i < objects1.length; i++) {
      for (var j = 0; j < objects2.length; j++) {
        objects.push(_.extend({}, objects1[i], objects2[j]));
      }
    }
    return objects;
  };

  var leftJoin = function(collectionA, collectionB, keyA, keyB) {
    var indexedA = _.groupBy(collectionA, keyA),
        indexedB = _.groupBy(collectionB, keyB),
        keys = _.keys(indexedA);
    return _.reduce(keys, function(acc, x) {
      var objsA = indexedA[x],
          objsB = indexedB[x] || [{}];
      return acc.concat(crossProduct(objsA, objsB));
    }, []);
  };

  _.mixin({
    leftJoin: leftJoin
  });
})();