ecmascript 5 and the script tag

Fri Apr 17 12:26:57 PDT 2009

Thanks, Brendan, for an excellent summary of these issues. The question of script versioning, going forward, is of considerable interest to Microsoft/IE so we agree it is something that needs to be discussed and address, probably in the context of TC-39.

I just had a couple of things to add:

As, Brendan pointed out to me sometime in the past, the script type and version can be globally defaulted for a page using the "Content-Script-Type" Meta tag (http://www.w3.org/TR/REC-html40/interact/scripts.html ).  We should probably also consider what, if anything, is globally implied about the global ECMAScript environment versions by the use of that tag.

>1. RFC 4329 (http://www.ietf.org/rfc/rfc4329.txt) mandates user agent
>parsing of version parameter, but does not specify legal values. Not
>all user agents comply (does IE8? I have not tested). This may become
>important over time, so bug-fixing soonest is best.

To the best of my knowledge, IE8 still ignores the version parameter on script tags. The issue, is what to do with the version request after it is parsed.  I think, that we would find considerable support within Microsoft for interpreting the version as an explicit opt-in requests for new functionality (or at least new syntax) where the new functionality is not accessible without an explicit opt-in. I think there would be considerable resistance to outright rejection of scripts that specify unknown or unsupported versions. The fear would be that we would reject too many unnecessarily over-specified version requests that might arise from copy and paste page authors.

>* Versioning to opt into new APIs on Object, or in the global object
>(JSON, conspicuously, in ES5) will not work as expected: precedent
>from ES1-3 and ongoing in Mozilla's JS number line, and in the
>prototype ES5 implementations I've seen (Mozilla has one in open
>source; there's a JScript DLL I've seen in action from Microsoft, not
>released) all expose new properties unconditionally.

IE8 does have a form of global object versioning.  In IE8 the JSON object and associated toJSON methods,  the methods Object.defineProperty and Object.getOwnPropertyDescriptor(they only supports DOM objects in IE8), and a few other minor standards/interp conformance fixes are only provided for pages/iframes operating in "IE8 standards mode" (the default mode).  This was a very late change in the IE development cycle a was primarily made in response to user feedback concerning the IE8 release candidate (http://blogs.msdn.com/ie/archive/2009/03/20/rtm-platform-changes.aspx ). We'll see how it works out...

Allen

>-----Original Message-----
>From: es-discuss-bounces at mozilla.org [mailto:es-discuss-
>bounces at mozilla.org] On Behalf Of Brendan Eich
>Sent: Thursday, April 16, 2009 3:17 PM
>To: Maciej Stachowiak
>Cc: es-discuss at mozilla.org
>Subject: Re: ecmascript 5 and the script tag
>
>On Apr 16, 2009, at 2:56 PM, Maciej Stachowiak wrote:
>
>> On Apr 16, 2009, at 2:37 PM, kevin curtis wrote:
>>
>>> Hi
>>>
>>> Has it been established how browsers will handle ecmascript 5? e.g.
>>> <script type="application/ecmascript;version=5"> ...
>>> The above is from the es4 proposal with 4 replaced with 5:
>>> http://wiki.ecmascript.org/doku.php?id=proposals:versioning
>>>
>>> Also, how will ecmascript shells process ecmascript 5 scripts?
>>> Spidermonkey has the -v flag (as well as the version() builtin):
>>> ./js -v 5 -e "print('version: ' + version())"
>>>
>>> Is it likely that the the c/c++ 'script' objects in the test
>>> implementations will have a property indicating the ecmascript
>>> version of
>>> the script?
>>
>> I don't think there is a need for out-of-band declared versioning in
>> ES5 as specified. Browsers can and should handle all scripts as ES5
>> once they implement the standard.
>
>A few things seem worth discussing:
>
>1. RFC 4329 (http://www.ietf.org/rfc/rfc4329.txt) mandates user agent
>parsing of version parameter, but does not specify legal values. Not
>all user agents comply (does IE8? I have not tested). This may become
>important over time, so bug-fixing soonest is best.
>
>2. Ecma TC39 is the likely standards body to define version parameter
>values for the standard scripting media types from RFC 4329. We should
>add this to the agenda. It came up at the last face-to-face.
>
>3. Before ES5 is supported by all user agents targeted by content
>authors, those authors will have to use a combination of object
>detection and user-agent sniffing to decide whether the ES5 features
>they want are supported. ES5 has little "new" syntax, but getters and
>setters targeting downrev IE are an obvious problem.
>
>The idea of in-band version selection, akin to strict mode's backward-
>compatible pragma syntax, has come up:
>
>"use version 5";  // wait, JS5? JScript5? Oh, ES5!
>
>but this raises several issues:
>
>* Versioning to "opt into" new keyword usage, e.g. let and yield,
>can't work this way, since the downrev engines will not respect those
>keywords and choke on 'let x = 42' and 'yield foo()' (and silently
>misinterpret 'yield(foo())').
>
>* Versioning to opt into new APIs on Object, or in the global object
>(JSON, conspicuously, in ES5) will not work as expected: precedent
>from ES1-3 and ongoing in Mozilla's JS number line, and in the
>prototype ES5 implementations I've seen (Mozilla has one in open
>source; there's a JScript DLL I've seen in action from Microsoft, not
>released) all expose new properties unconditionally.
>
>* How would you set the default for a "page" or "app" (graph of
>connected windows/frames)?
>
>* Whose version? With RFC4329's media type parameter, you know
>application/ecmascript;version=5 is ES5 while application/
>javascript;version=1.8 is Mozilla's (we respect this, always have --
>we gave up on version selection via <script language="JavaScript1.8">
>however, since IE ignores version suffix there). With "use version 5";
>there is greater distance between the version number and the media
>type used (or not used, rather defaulted) on the script tag -- the two
>may not be in the same file.
>
>So there's actually a lot to discuss!
>
>At the last TC39 meeting we realized that versioning script tags or
>other compilation units to opt into new keywords was a separate issue
>from versioning a shared global object, for which no mechanism exists
>(AFAIK) in browsers or extant web standards.
>
>Opt-in versioning for new keywords is unavoidable in Mozilla's
>experience: we tried adding 'let' and 'yield' by default in JS1.7 pre-
>releases, and had to back off to opt-in; on the up side, ES5 strict
>mode reserves these two along with other future reserved identifiers.
>
>Versioning the global object does not mean multiple version
>"namespaces". We have no goal or requirement for any such thing and
>strong requirements against if the only way to hide new APIs from some
>scripts, where those new APIs (properties) were exposed on the same
>global object to other scripts, amounted to ES5-like namespaces.
>
>The question is: do we need a single-global-object-version opt-in
>mechanism? No such thing has evolved so far (again AFAIK).
>
>/be
>
>>
>>
>> - Maciej
>>
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