To launch it when the page finished loading you could add a body tag like the following one to your page:
This script is small and easy to understand. You can hack it to make it load multiple files.
<script data-main="//js/main-1.0.1" src="//js/require-2.0.6.min.js"></script>
the HTML5 async attribute
<script async src="//example.com/someScript.js"></script>
for more information about the async attribute you may want to read the w3c HTML5 specification chapter that covers the new async attribute. If you want to know which browsers support it, check out caniuse.com