In the same way that you can add custom HTML to a calendar title, footer, or event, you can add custom JavaScript as well. Just make sure to wrap your code in between some <script> and </script> tags.
In this extremely annoying example, here is how to make a calendar display a JavaScript alert box every time the page is loaded.
Step 1: Copy the Code
The JavaScript for this example is below:
<script>alert("Greetings, visitor!");</script>
Step 2: Paste the code
For this exercise, we paste it into the calendar Title.
The Title and Footer fields don't let you paste multi-line text. You might need to paste your JavaScript someplace else (like Notepad) first and remove the line breaks before pasting it into localendar.
Success!
This is a simple example, but there are almost limitless applications for using your own JavaScript (usage tracking, advanced UI customizations, browser detection, Google Analytics tracking, etc.)