Javascript Array first(predicate, def)

Array.prototype.first = function (predicate, def) {
 var l = this.length;
 if (!predicate) return l ? this[0] : def == null ? null : def;
 for (var i = 0; i < l; i++){
  if (predicate(this[i], i, this))
   return this[i];
 }

 return def == null ? null : def;
};