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Re: Semicolons and the do..while statement

Followups directed to: comp.lang.javascript

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RobG wrote:

> Thomas 'PointedEars' Lahn wrote:
> 
> [breakdown of ASI]

That appears to be correct.

>> IOW,
>>
>> do {} while (false) var x = 3;
>>
>> *is* equivalent to
>>
>> do {} while (false); var x = 3;
>>
>> because `var' delimits a /VariableStatement/ and the parser is allowed,
>> in order to produce
>>
>> /Program/
>> → /SourceElements_opt/
>> → /SourceElements/     /SourceElement/
>> → /SourceElement/      /SourceElement/
>> → /Statement/          /Statement/
>> → /IterationStatement/ /VariableStatement/
>> → do /Statement/ while ( /Expression/ ); /VariableStatement/
>> → do /Block/ while ( /Expression/ );     /VariableStatement/
>> → …
>> → do {} while ( true );                  var x = 3;
>>
>> to insert the semicolon before the /VariableStatement/.

To be precise, after the closing parenthesis of a supposed do-while 
statement.  Function serialization, although implementation-dependent, can 
show this:

$ js
Rhino 1.7 release 2 2010 02 06
js> (function () { do {} while (true) var x = 3; }).toString()

function () {
    do {
    } while (true);
    var x = 3;
}

>> It's not a bug,>it's a feature, and a standards-compliant feature at
>> that.
> 
> I suppose I'm still missing something because my understanding of this
> explanation is that a semicolon should be inserted for
> 
>      /VariableStatement/ /VariableStatement/
> 
> as it's one of the "certain statements", but it isn't. If the newline
> is removed from the variable statement example above:
> 
>     var x = 3 var y = 4
> 
> then a syntax error is thrown.

ACK:

js> (function () { var x = 3 var y = 4; }).toString()
js: "<stdin>", line 3: missing ; before statement
js: (function () { var x = 3 var y = 4; }).toString()
js: ............................^

> I can't see how the second "var" can be interpreted as anything other than
> the start of a variable statement, and therefore that a semicolon can (or
> should) be inserted before it (per your logic above).

Good "question".  I do not have a good answer to it yet.  However:
 
> The only explanation I can think of is that the "v" at the start of
> the second "var" is interpreted as the start of an identifier, so
> either a semicolon, comma[1] or newline (which would force semicolon
> insertion) is required. Because of the ambiguity, an error results, so
> the whole "var" token is not read and the second variable statement
> isn't identified.

I think we can exclude that possibility.  `var' is a /ReservedWord/, in 
particular a /Keyword/, so explicitly _not_ an /Identifier/:

| 7.6.1 Reserved Words
| 
| A reserved word is an IdentifierName that cannot be used as an Identifier.
| 
| Syntax
| 
| ReservedWord ::
|   Keyword
|   FutureReservedWord
|   NullLiteral
|   BooleanLiteral
| 
| 7.6.1.1 Keywords
| 
| The following tokens are ECMAScript keywords and may not be used as
| Identifiers in ECMAScript programs.
| 
| Syntax
| 
| Keyword :: one of
|   break     do        instanceof  typeof
|   case      else      new         var
|   catch     finally   return      void
|   continue  for       switch      while
|   debugger  function  this        with
|   default   if        throw       delete
|   in        try


PointedEars
-- 
var bugRiddenCrashPronePieceOfJunk = (
    navigator.userAgent.indexOf('MSIE 5') != -1
    && navigator.userAgent.indexOf('Mac') != -1
)  // Plone, register_function.js:16

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Thread

Re: Semicolons and the do..while statement BGB <cr88192@hotmail.com> - 2012-08-29 13:55 -0500
  Re: Semicolons and the do..while statement Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2012-08-29 22:20 +0200
    Re: Semicolons and the do..while statement BGB <cr88192@hotmail.com> - 2012-08-29 20:02 -0500
    Re: Semicolons and the do..while statement RobG <rgqld@iinet.net.au> - 2012-08-29 18:27 -0700
      Re: Semicolons and the do..while statement Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2012-08-30 12:37 +0200
    Re: Semicolons and the do..while statement John G Harris <john@nospam.demon.co.uk> - 2012-08-30 14:24 +0100
      Re: Semicolons and the do..while statement Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2012-08-30 15:59 +0200
        Re: Semicolons and the do..while statement John G Harris <john@nospam.demon.co.uk> - 2012-08-30 16:26 +0100
          Re: Semicolons and the do..while statement Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2012-08-30 17:47 +0200
            Re: Semicolons and the do..while statement John G Harris <john@nospam.demon.co.uk> - 2012-09-01 11:40 +0100
              Re: Semicolons and the do..while statement Dr J R Stockton <reply1235@merlyn.demon.co.uk.invalid> - 2012-09-02 19:54 +0100
          Re: Semicolons and the do..while statement BGB <cr88192@hotmail.com> - 2012-08-30 11:17 -0500
      Re: Semicolons and the do..while statement Gene Wirchenko <genew@ocis.net> - 2012-08-30 10:25 -0700
        Re: Semicolons and the do..while statement Dr J R Stockton <reply1235@merlyn.demon.co.uk.invalid> - 2012-08-31 23:20 +0100
          Re: Semicolons and the do..while statement Gene Wirchenko <genew@ocis.net> - 2012-08-31 16:03 -0700
  Re: Semicolons and the do..while statement Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2012-08-29 22:30 +0200

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