Underscore mixin which rejects items from a list if too many of them match the iterator function. Keeps all other list items.

limitQuantity.js

/**
 * Rejects items which match the iterator, beyond the threshold. Accepts all other items.
 *
 * Example: _([1, 1, 1, 1, 2]).limitQuantity(2, function(n) {return n == 1;}); // [1, 1, 2]
 *
 * @param list The list to be filtered. Omitted in OO-style (i.e. _(list).limitQuantity).
 * @param n The number of matching items to be allowed.
 * @param f The filter function against which list items will be matched.
 */
_.limitQuantity || _.mixin({
  limitQuantity: function(list, n, f) {
    var observed = 0;

    return _(list).reduce(function(result, item) {
      if (f(item)) {
        observed += 1;

        if (observed <= n) {
          result.push(item);
        }
      } else {
        result.push(item);
      }

      return result;
    }, []);
  }
});