According to this blog post, Netbeans's supposed to support Javascript type inference. And Javascript support should be built-in to the Netbeans editor.
However when I add an HTML file to a simple Java project, and include the Raphael javascript library using <script src="..."/> it seems that Netbeans does not recognize the library. Even very simple completions, like detecting the new Raphael function in the window object are not working.
For example:
<html>
<head>
<title>Raphael Play</title>
<script type="text/javascript" src="raphael.js"></script>
<script type="text/javascript">
window.onload = function() {
var paper = new Raphael(document.getElementById('canvas_container'),
500, 500);
// no ctrl+space for autocomp Ra -> Raphael
var candy = paper.set();
// and of course paper is recognized as Object, no autocompletions for it.
</script>
</head>
<body>
<div id="canvas_container"></div>
</body>
</html>