JavaScript is a client-side scripting language. It can be embedded into HTML documents to create and add dynamic features to static pages.
Classic uses of JavaScript include embedding Flash using the SWFObject, using Google analytics, adding Lightbox onto a page etc. In terms of SEO, JavaScript should ideally be kept in a separate document and simply referenced from the HTML page. If the JavaScript has to go on the page, then it should be added at the bottom of the code to give priority to the content.
