Is <!-- valid javascript?

Question

I recently had to fix a bug that manifested sometimes in Internet Explorer. The bug was that, sometimes, the parser choked on code like

<script type="text/javascript"> <!-- // comments --> /*...*/ </script>

Which we fixed by correcting the comment. My question is: is "<!--" valid javascript code, or not? I tried testing it with firebug, and it only says" Undefined". JSFiddle didn't faze. IE only choked on it some of the times (reloading the page in question would show up the result of the script involved).

While knowing that, for historical reasons, an HTML comment inside js could be valid depending on its exact position and the phase of the moon is indeed useful, I was more interested in answers like "var <!-- foo is valid js code, but <!-- on its own is not. Here's why:..."

I did some analysis in firebug:

var x = 2; var y = 3; var z = 0; console.log(x); console.log(y); y<!--x; console.log(x); console.log(y); z = y<! --x; console.log(x); console.log(y); console.log(z);

resulting in the following:

2 3 2 3 1 3 false

The difference between the first and second tries is interesting.

I then tried

z = (y <!--x); console.log(z);

Which failed with

SyntaxError: missing ) in parenthetical

It might be because of the extra space before the HTML comment, if it is not a typo. Some Javascript parsers may only ignore HTML comments right at the beginnning of a script. — Frédéric Hamidi
– Frédéric Hamidi, Commented Jan 20, 2014 at 19:22
Of course. That doesn't mean it isn't valid JavaScript. It just means you didn't define foo, and probably not bar either. — cookie monster
– cookie monster, Commented Jan 20, 2014 at 19:42
@MackieeE, As stated, I was looking forward to an examination of <!-- as js code, not as relic. I think it's a different question. — Adriano Varoli Piazza
– Adriano Varoli Piazza, Commented Jan 20, 2014 at 19:52

Koen Peters · Accepted Answer · 2014-01-20 21:03:05Z

First to answer your question on Is <!-- valid JavaScript: No, it is not, in none of the forms you gave in your question. This is because it is not valid according to the JavaScript BNF grammar which you can find here: http://tomcopeland.blogs.com/EcmaScript.html

If you are interested, here's why you do see it inside script blocks: It is the HTML comment character. You do see it very often within script tags like this:

<script> <!-- .. JavaScript code... // --> </script>

The reason is that old browsers (and with "old" I mean "stone age" like Netscape 1.0) that do not even support JavaScript and would otherwise just show the code on the screen. By doing it this way those older browsers treat the JavaScript as HTML comments and do not show it. Newer browsers ignore this and just run the JavaScript.

This is how it actually works (from http://www.w3.org/TR/html401/interact/scripts.html#h-18.3.2): The JavaScript engine allows the string  from the JavaScript parser.

Because all browsers nowadays support JavaScript (even if it is turned off) you do not need to do this anymore. It is actually bad practice to do so because of these reasons (from http://www.javascripttoolbox.com/bestpractices/#comments):

Within XHTML documents, the source will actually be hidden from all browsers and rendered useless
-- is not allowed within HTML comments, so any decrement operations in script are invalid

An even deeper explanation and all the cons and pro's can be found here: http://lachy.id.au/log/2005/05/script-comments

I know it is an HTML comment starter, yes. Still, <, !, -- are valid js operators. I want to know if perchance the combination is somewhat valid, or exactly why not, in Javascript.
@AdrianoVaroliPiazza: It's not valid because there is no operand before <. var x = 10; 5<!(--x) would be valid though. It also depends depends on the context. Inside a script tag, the <!-- sequence would always indicate a HTML comment and you you would get syntax errors. If the code is in it's own file, this doesn't have to be the case.
Two things: this is new code, and the times I've seen similar code was with the HTML comment preceded by //, I think.
@AdrianoVaroliPiazza: Actually, outside of an HTML context (e.g. the console), 5 <!--x is valid as well, but it returns 5 and x is still 10 which somehow indicates that something weird is going there. In any case, <!-- alone is invalid because it misses operands.
@Adriano, actually the JS comment was before the closing tag of the HTML comment: . That's because script parsers only ignore the start tag of the comment, the end tag still has to be special-cased.

ddavison · Accepted Answer · 2014-01-20 19:20:12Z

In older browsers, it was required to have these comments inserted because the browsers couldn't parse the javascript properly. it would try to literally parse the javascript as html which caused script execution failures.

Today, browsers don't need this.

eis · Accepted Answer · 2014-01-20 20:45:44Z

It's not.

If you go to the standard, you'll see that a < can only exist in a RelationalExpression, which <!-- is not as it hasn't got anything on the left-hand side.

Dan H · Accepted Answer · 2014-01-20 19:19:42Z

0

Comments in Javascript are like other C-type languages (// for single line, /* */ for blocks.

answered Jan 20, 2014 at 19:19

Dan H

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1 Comment

Dan H Over a year ago

Let me try again then, but it's clearly been answered by this point. Expression wise it is not valid javascript, but it is ignored by a lot of interpreters today due to it's previous (hacked) usage as described above. < is a numeric comparison operator, ! is the 'not' operator, and -- is an arithmetic operator. If I were to theorize, if this hack was never used and thus ignored by parsers today, just writing <!-- in javascript would cause a syntax or operator error...

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