The best way to load external JavaScript
Though there’s been a lot of research on ways to load JavaScript without blocking, there really is just one way that I’d recommend as a best practice. There should really be no need to load anything more than two scripts to get your site initialize and interactive. Make the initial JavaScript file as small as possible and then load in the larger one dynamically to avoid blocking. This is the simplest, easiest way to get all of your JavaScript onto the page without affecting the user experience.