Question

<script type="text/javascript" language="JavaScript">
  <!--
    alert('foo');
  //-->
</script>

It’s used all over in my company’s grails app, but I know < is an illegal javascript character…

Should the <!-- be like //<!-- instead?

Answer 1

No, it is not illegal. From the specification:

The JavaScript engine allows the string “<!--” to occur at the start of a SCRIPT element, and ignores further characters until the end of the line.

It is, however, pointless. It is designed to stop browsers that do not recognise the script element rending the contents as text. The script element has been supported since Netscape 2! This makes the use of that syntax completely pointless today.

If, however, you are using XHTML then it is actively harmful. Since script elements don’t contain intrinsic CDATA, markup inside them is treated as markup and not CDATA, so it would actually comment the script out (if the document was processed as XHTML and not tag soup, which would require an application/xhtml+xml content-type).

Should the <!-- be like //<!-- instead?

No, that would (partially) defeat the object.