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Re: <!--

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  Re: <!-- Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-12-07 15:01 +0100
    Re: <!-- Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-12-07 20:33 +0100
    Re: <!-- John Harris <niam@jghnorth.org.uk.invalid> - 2015-12-08 11:02 +0000

#28986 — Re: <!--

Stefan Ram wrote:

>   I tried the following with the Firefox console (one line of
>   input, one line of output, and one blank line is given below):
> 
> <
> SyntaxError: expected expression, got '<'
> 
> <!
> SyntaxError: expected expression, got '<'
> 
> <!-
> SyntaxError: expected expression, got '<'
> 
> <!--
> undefined
> 
>   So, when I input arbitrary symbols, it usually gives me a
>   syntax error.

Those are not arbitrary symbols; the first one is an operator that requires 
two operands.

>   But when I hit »<!--«, it does not say
>   »SyntaxError«, but »undefined«. Where is the production in
>   the grammar of JavaScript that allows this »<!--«?

I presume that “<!--”, at least at the beginning of a program, is parsed 
like “//” in Mozilla JavaScript was built into SpiderMonkey after people 
abused HTML comments to hide improperly escaped “script” element content 
from improperly working HTML parsers and the (properly working) W3C 
Validator.  That is one of the syntax extensions allowed by the 
“Conformance” section of ECMAScript Editions.

And so this program is empty; there is no remaining expression, the missing 
statement gets automatic semicolon insertion for the empty statement (ES 6, 
§ 13.4), and the program’s result is the “undefined” value.  See ES 5.1 for 
the /Program/ production; I could not find it in ES 6.

-- 
PointedEars
FAQ: <http://PointedEars.de/faq> | SVN: <http://PointedEars.de/wsvn/>
Twitter: @PointedEars2 | ES Matrix: <http://PointedEars.de/es-matrix>
Please do not cc me. / Bitte keine Kopien per E-Mail.

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#28987

Thomas 'PointedEars' Lahn wrote:

> Stefan Ram wrote:
>>   I tried the following with the Firefox console (one line of
>>   input, one line of output, and one blank line is given below):
>> […]
>>   But when I hit »<!--«, it does not say
>>   »SyntaxError«, but »undefined«. Where is the production in
>>   the grammar of JavaScript that allows this »<!--«?
> 
> I presume that “<!--”, at least at the beginning of a program, is parsed
> like “//” in Mozilla JavaScript was built into SpiderMonkey after people
> abused HTML comments to hide improperly escaped “script” element content
> from improperly working HTML parsers and the (properly working) W3C
> Validator.  That is one of the syntax extensions allowed by the
> “Conformance” section of ECMAScript Editions.
> […]

Apparently “<!--” is parsed exactly like “//” on *every* line of a program 
in Mozilla JavaScript as of Iceweasel 38.1.0 and Google V8 JavaScript as of 
Chromium 46.0.2490.71, both running under Debian GNU/Linux 8.2 64-bit 
(console tests).

BTW, it is probably not a good idea to develop and to test only with Mozilla 
JavaScript, as according to StatCounter.com the top 3 of world-wide market 
shares as of November 2015, all platforms, Chrome and Firefox 5+ combined 
each, are:

Chrome:       44.17 % (Google V8 JavaScript)
Firefox 5+:   10.63 % (Mozilla JavaScript 5+)
IE 11:         7.00 % (“Internet Explorer JavaScript”, i.e. JScript 11.x)

<http://gs.statcounter.com/#all-browser_version_partially_combined-ww-monthly-201411-201511-bar>

See also <http://PointedEars.de/es-matrix/#environments>
(whose display I need to improve).

-- 
PointedEars
FAQ: <http://PointedEars.de/faq> | SVN: <http://PointedEars.de/wsvn/>
Twitter: @PointedEars2 | ES Matrix: <http://PointedEars.de/es-matrix>
Please do not cc me. / Bitte keine Kopien per E-Mail.

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#28989

On Mon, 07 Dec 2015 15:01:45 +0100, Thomas 'PointedEars' Lahn
<PointedEars@web.de> wrote:


  <snip>
>See ES 5.1 for 
>the /Program/ production; I could not find it in ES 6.

In ECMAScript 2015 (its official name) the 'program' production has
been replaced by the two productions 'script' and 'module' (see
section 5.1.4).

  John

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