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Some years ago, I was taught that JavaScript code blocks embedded inside HTML should always be capsulated inside HTML comments as following:

<script type="text/javascript">
<!--
var hello = "world";
-->
</script>

I was told to do this, but I never kind of fully figured out why. It kind of seems hacky to use HTML comments, so nowadays I have started using writing my JavaScript code inside the script block without the HTML comments:

<script type="text/javascript">
var hello = "world";
</script>

So my question is: should I use HTML comments to capsulate JavaScript code blocks? Is it safe to just write the script without the comments? I mean am I risking something when I leave out the comment tags?

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2 Answers 2

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The HTML comment was intended to hide the JavaScript from ancient browsers that didn't understand the <script> element and instead render its contents on the page. That was way back in the mid-90es, iirc. Nowadays you can safely assume that browsers from that era are no longer present on the web and omit the comments.

Some nice history on that can be found here:

The general rule regarding HTML tags that browsers do not understand is that the browser should ignore the tag completely and treat the content of the page as if that tag were not there. This means that when Netscape 2 first introduced JavaScript (or LiveScript as it was called then), it was necessary to place an HTML comment around the actual script in order to hide the code from other browsers that didn't understand the script tag and which therefore would have displayed the code rather than running it.

The JavaScript language was specifically written to accept the start of an HTML comment as the very first thing in the script and to ignore it in order that the HTML comment could be used to hide the script from people using Netscape 1, Mozaic, Internet Explorer 1, Internet Explorer 2, and other browsers of similar vintage none of which anyone uses any more. It is these prehistoric browsers (in JavaScript terms) that are meant when you see references in antiquated JavaScript tutorials to wrapping your JavaScript inside an HTML comment in order to hide it from "older" browsers.

With Internet Explorer 3, Microsoft introduced their own equivalent to JavaScript which they call JScript. Since then all browsers have at least recognised the script tag and more modern browsers (Netscape 2+, IE3+ etc) the HTML comment is no longer required.So once all your visitors have upgraded to use either Netscape 2, Internet Explorer 3, or a more recent browser than either of those two the commenting out of the script becomes redundant code.

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3 Comments

Thanks! This has kind of been my hunch as well. I haven't met a single client who has had problems with a page with commentless JavaScript block. Also do you happen to know in which specific browsers this unwanted behaviour would happen?
So it really really is an ancient requirement. Case closed. Thanks!
For anyone reading this answer, please see the excellent answer from Marko as well providing much more detail on this question.
7

Straight from the source

18.3.2 Hiding script data from user agents

User agents that don't recognize the SCRIPT element will likely render that element's contents as text. Some scripting engines, including those for languages JavaScript, VBScript, and Tcl allow the script statements to be enclosed in an SGML comment. User agents that don't recognize the SCRIPT element will thus ignore the comment while smart scripting engines will understand that the script in comments should be executed.

Another solution to the problem is to keep scripts in external documents and refer to them with the src attribute.

Commenting scripts in JavaScript 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. JavaScript interprets "//" as starting a comment extending to the end of the current line. This is needed to hide the string "-->" from the JavaScript parser.

<SCRIPT type="text/javascript">
<!--  to hide script contents from old browsers
  function square(i) {
    document.write("The call passed ", i ," to the function.","<BR>")
    return i * i
  }
  document.write("The function returned ",square(5),".")
// end hiding contents from old browsers  -->
</SCRIPT>

Furthermore, if you really want to understand what all of this means, read this excellent article. It's lengthy, but worth it.

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5 Comments

Can you specify a little more: how real is this risk? Do you know some specific browsers that behave like this?
No browser that supports a semi-recent version of HTML will render the <script> element contents.
That should really only bother Mosaic users on Amigas. And maybe a few users who haven't upgraded Lynx in 15 years.
@jsalonen, see my update and the comments by @Stan and @Joey.
Thanks! You gave good points here. Note that HTML 4.01 spec was written 1999. So unless I really want to design for legacy browsers, it seems that I should not bother using the comment tags. I do acknowledge the occasional need to escape HTML entities within JavaScript code and for this task, an escape mechanism is of course needed. Escaping JavaScript however was out of the scope of my question.

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