Yes there is. There's a documentation page on the subject.
https://developers.monogatari.io/documentation/script-actions/javascript
Basically, Monogatari's script labels are arrays that run each item one at a time. If, over the course of the loop, Monogatari comes to Start[4] is a function(){} then it will run it. Additionally, if you need Monogatari to maintain a consistent state should the player use the rewind (back button) function, then monogatari also accepts objects that have both an apply and revert key. Read the "Reversible Functions" section for more information on that.