Once you have created a JavaScript (.js) file holding some functions you want to reuse, you need to import it into your tool scripts. The syntax for importing a javascript file is:

importScript("/ElixirWorkspace/MyScripts/Core.js");

To avoid repeated import of the same scripts, the import should be done in a script that is only executed once. For example the Function Definitions of a Report template, or the Script tab of a Composite DataSource.