Each report has a Function Definitions editor (on the Functions tab) in the Report Designer. Any code here will be run before the report is rendered.
In order to share code across reports, create a JavaScript file in the repository (“Add” > “JavaScript” is on the repository folder popup menu) and put the common code here.
Then go to the Function Definitions for each report and add a line:
=Code Snippet=
importScript("/ElixirSamples/path/to/my.js");
with the appropriate path to your .js file.