There isn’t a prepare field on the JS side of things. To be honest, I’m not really sure what you’re trying to accomplish using JS. Do you want to set the field to something based on another field?
The ACF documentation for custom JS is here https://www.advancedcustomfields.com/resources/adding-custom-javascript-fields/, other than this you need to look for similar questions on this forum or look for examples. I have a few examples of some custom JS for dynamic fields here that might also give you some ideas https://github.com/Hube2/acf-dynamic-ajax-select-example
