Inside the templates and the script file it is possible to write functions in JavaScript. The conventions of JavaScript will not be covered in this document. For that, please refer to documentation available online like:
https://developer.mozilla.org/en-US/docs/Web/JavaScript

Custom functions can be defined directly in the template (= local function) or in (exactly) one script file that can be passed in a CIB jsMerge call (= global function).

The necessary functions are defined in a script block, e.g.:

{# myGlobalFunc = function(){ … } }