console.js

> _.split([1, 2, 3, 4], function(item) { return [item, item*2]; })
[1, 2, 2, 4, 3, 6, 4, 8]

some-lib.js

_.mixin({
  split: function(collection, func) {
    if (collection.length === 0) return [];
    return func(_.first(collection)).concat(_.split(_.rest(collection), func));
  }
}