Function group

doc/demo/underscore.js:325–333

(obj, value, context, behavior)

  // An internal function used for aggregate "group by" operations.
  var group = function(obj, value, context, behavior) {
    var result = {};
    var iterator = lookupIterator(value == null ? _.identity : value);
    each(obj, function(value, index) {
      var key = iterator.call(context, value, index, obj);
      behavior(result, key, value);
    });
    return result;
  };

  // Groups the object's values by a criterion. Pass either a string attribute
  // to group by, or a function that returns the criterion.