What I've been doing lately is adding functions to my collection level pre-request script like so

collectionUtils = {
    clearEnvData: function (pm) {
        some useful code
    },
    // called after every request to ensure server coverage durring smoke testing.
    cycleCurrentServer: function (serverCount, pm) {
        some useful code
    }
}

Then wherever I want to use these methods I do something like this

collectionUtils.cycleCurrentServer(index, pm);

I think generally the best way is to externalize the code into a library. You'll then make changes to the library and those changes will be reflected everywhere. Now there are several ways to implement this method, I'll leave with with the two that make sense for your use case:

Load your library from a remote site

If you are using some sort of development workflow that publishes your changes upstream, and you have your library published somewhere, you can load it at runtime:

pm.sendRequest("https://cdnjs.cloudflare.com/ajax/libs/dayjs/1.11.0/dayjs.min.js", (err, res) => {
   //convert the response to text and save it as an environment variable
   pm.collectionVariables.set("dayjs_library", res.text());
 
   // eval will evaluate the JavaScript code and initialize the min.js
   eval(pm.collectionVariables.get("dayjs_library"));
 
   // you can call methods in the cdn file using this keyword
   let today = new Date();
   console.log("today=", today);
   console.log(this.dayjs(today).format())
})

Store your code on a collection variable and load it that way

A less pretty way to do it but more convenient to a lot of folks is just to drop the whole library into a collection variable like this:

Then you can load it when you need it:

​​const dayjs_code = pm.collectionVariables.get('dayjs_code');
 
// Invoke an anonymous function to get access to the dayjs library methods
(new Function(dayjs_code))();
 
let today = new Date();
console.log(dayjs(today).format())

In both cases when you update your library you either have to republish it or copy paste again to the collection variable. However that surely beats copy pasting a piece of code to 20 odd places and figuring out what's updated, what's not and fighting bugs while at it.