So if you want to reuse a script across many client scripts there's two ways I see it being able to be done easily. One, is to use a Script Include and a GlideAjax call to handle the logic server side. Two, is to create a UI Script and load it with ScriptLoader call described here or on the developer site.

Lets get to it. How to call this in a client script.

function onChange(control, oldValue, newValue, isLoading, isTemplate) {
if (isLoading || newValue === '') {
ScriptLoader.getScripts('x_8821_catalog.awesome.jsdbx', function() {
//Type appropriate comment here, and begin script below