Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.javascript > #15607

Re: Semicolons and the do..while statement

Followups directed to: comp.lang.javascript

Show all headers | View raw

BGB wrote:

> On 8/29/2012 1:56 AM, RobG wrote:
>> The syntax for the do..while statement (ECMA-262 12.6.1[1]) is:
>>
>>      do /Statement/ while ( /Expression/ );
>>
>> Noting the trailing semicolon, I expected the following to throw an
>> error:
>>
>>      do {} while (false) var x = 3;
>>
>> since there is no semicolon to separate the two statements. However,
>> I've tested a number of browsers and no error is thrown. So either the
>> browsers I tested are all tolerant of the error (possible in one or
>> two but unlikely in half a dozen) or automatic semicolon insertion is
>> at work.
> 
> maybe?... (speculation here, based partly on my own implementation).

Did you mean _interpretation_?  If not, your implementation is borken.
 
> because some implementations don't actually "insert" semicolons, rather
> they "eat" them (check for semicolon, and if-seen, it is discarded).
> (this would be because it is faster and easier to treat the semicolon as
> a break and then eat it, than it is to figure out how to write logic to
> accurately figure out where to insert them, the "semicolon insertion"
> may then be generally interpreted as "if we may see a semicolon here,
> and see a newline or similar instead, interpret it as if it were a
> semicolon").
> 
> but, upon trying to eat the semicolon, there is none there for it to
> eat, so it continues on as normal (and since the do/while statement is
> otherwise self-terminating, it does not lead to an ambiguity).

I realize that you said "maybe?" and "speculation", but still…

Utter nonsense.

What actually happens is very simple:

,-[ECMA-262, 5.1 Edition]
| 
| 7.9 Automatic Semicolon Insertion
| 
| Certain ECMAScript statements (empty statement, variable statement,
| expression statement, do-while statement, continue statement, break
| statement, return statement, and throw statement) must be terminated with
| semicolons. Such semicolons may always appear explicitly in the source
                              ^^^        ^^^^^^^^^^^^^^^^^
| text. For convenience, however, such semicolons may be omitted from the
                                                  ^^^^^^^^^^^^^^
| source text in certain situations. These situations are described by
| saying that semicolons are automatically inserted into the source code
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^ (MUST)
| token stream in those situations.
               ^^^^^^^^^^^^^^^^^^^

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/.  It's not a bug, 
it's a feature, and a standards-compliant feature at that.

>> But my understanding of automatic semicolon insertion (ECMA-262
>> 7.9[2]) doesn't cover this case.
>>
>> Can someone please explain why no error is thrown and it seems that a
>> semicolon is inserted?
>>
>>
>>   1. http://es5.github.com/#x12.6.1
>>   2. http://es5.github.com/#x7.9
>>
> 
> standards are not always interpreted as an exact definition of the
> mechanisms, rather, of their externally perceived interface and behavior.
> 
> in edge cases though, one may find things where the behavior of actual
> implementations differs in minor ways from the mechanism which may be
> implied by the wording of a standard (though usually implementations
> will usually try to avoid this where possible).

Again, utter nonsense.  Instead, this particular standard gives its 
conforming implementations a wide latitude (see "Conformance"); in addition, 
some of its implementations are simply not conforming in some features.

See also: <http://PointedEars.de/es-matrix>


PointedEars
-- 
Danny Goodman's books are out of date and teach practices that are
positively harmful for cross-browser scripting.
  -- Richard Cornford, cljs, <cife6q$253$1$8300dec7@news.demon.co.uk> (2004)

Back to comp.lang.javascript | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

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

csiph-web