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Re: !0 versus true

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Scott Sauyet wrote:

> Thomas 'PointedEars' Lahn wrote:
>>                                                     An *expression* is
>> /equivalent/ to another *expression* if both expressions are evaluated to
>> the same value under the same conditions; one expression can be exchanged
>> with the other without affecting the outcome of expressions in which they
>> are used. [ ... ]
> 
> But this is less clear to me, at least as to its practical
> implications.  Because of the ability to retrieve (at least an
> approximation of) the source of a function as a string from within
> running code, expressions that are not character-for-character
> identical can cause different behaviors in a program, even though
> meeting your definition above.  This makes me suspect that your
> definition doesn't really capture what one would want it to capture:
> 
>     var f=function(){return 0;};
>     alert(("" + f).substring(19, 20));
> 
> versus:
> 
>     var f=function(){return +!1;};
>     alert(("" + f).substring(19, 20));
> 
> Although the expressions `0` and `+!1` should be considered equivalent
> by such a definition, these two short programs have different
> behaviors in many modern ES implementation, the first alerting "0",
> the second alerting "+".

If that.  The string representation of functions, that is, the return value 
of Function.prototype.toString() for a Function instance as its “this” 
value, is *implementation-dependent* (ES 5.1, §15.3.4.2).

Your confusion stems from the fact that you are proceeding from a false 
assumption.  It is _not_ the function that is used in your example, but a 
string representation of the function.

If you were to consider the function instead, you would find that

  f() === 0

in both cases.  That is, the expressions “0” and “+!1” *are* *equivalent* 
because they are evaluated to the same (*identical*) value, (+)0 under the 
same (here: all) conditions.  As a result, the functions returning that 
value are equivalent, i.e. one can be replaced with the other in the same 
expression without affecting the outcome (“true”) of evaluating the 
expression in which it is used.  (It is that equivalence why refactoring and 
minimizers work.)

HTH

-- 
PointedEars

Twitter: @PointedEars2
Please do not Cc: me. / Bitte keine Kopien per E-Mail.

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Thread

!0 versus true Andrew Poulos <ap_prog@hotmail.com> - 2013-05-23 23:31 +1000
  Re: !0 versus true Scott Sauyet <scott.sauyet@gmail.com> - 2013-05-23 07:58 -0700
  Re: !0 versus true Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-05-23 17:53 +0200
    Re: !0 versus true Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2013-05-24 12:17 -0700
      Re: !0 versus true Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-05-24 21:56 +0200
        Re: !0 versus true Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2013-05-24 13:13 -0700
          Re: !0 versus true Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-05-24 22:23 +0200
      Re: !0 versus true Denis McMahon <denismfmcmahon@gmail.com> - 2013-05-24 22:08 +0000
        Re: !0 versus true Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-05-25 07:00 +0200
          Re: !0 versus true Scott Sauyet <scott.sauyet@gmail.com> - 2013-05-25 20:04 -0700
            Re: !0 versus true "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2013-05-26 11:07 +0200
            Re: !0 versus true Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-05-26 13:11 +0200
              Re: !0 versus true Scott Sauyet <scott.sauyet@gmail.com> - 2013-05-26 13:28 -0700
                Re: !0 versus true Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-05-27 12:07 +0200
                Re: !0 versus true Scott Sauyet <scott.sauyet@gmail.com> - 2013-05-27 11:27 -0700
                Re: !0 versus true Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-05-27 23:30 +0200
                Re: !0 versus true Scott Sauyet <scott.sauyet@gmail.com> - 2013-05-27 17:12 -0700
                Re: !0 versus true Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-05-28 02:40 +0200
                Re: !0 versus true Scott Sauyet <scott.sauyet@gmail.com> - 2013-05-27 20:29 -0700
                Re: !0 versus true John Harris <niam@jghnorth.org.uk.invalid> - 2013-05-28 15:38 +0100
                Re: !0 versus true Scott Sauyet <scott.sauyet@gmail.com> - 2013-05-28 08:32 -0700
                Re: !0 versus true Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-05-28 21:14 +0200
                Re: !0 versus true Scott Sauyet <scott.sauyet@gmail.com> - 2013-05-28 15:38 -0700
                Re: !0 versus true Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-05-29 13:20 +0200
                Re: !0 versus true Scott Sauyet <scott.sauyet@gmail.com> - 2013-05-29 06:37 -0700
                Re: !0 versus true Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-05-29 16:28 +0200
                Re: !0 versus true Scott Sauyet <scott.sauyet@gmail.com> - 2013-05-29 13:32 -0700
                Re: !0 versus true Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-05-29 23:34 +0200
                Re: !0 versus true Scott Sauyet <scott.sauyet@gmail.com> - 2013-05-30 06:30 -0700
                Re: !0 versus true John Harris <niam@jghnorth.org.uk.invalid> - 2013-05-29 17:33 +0100
                Re: !0 versus true Scott Sauyet <scott.sauyet@gmail.com> - 2013-05-29 11:34 -0700
                Re: !0 versus true John Harris <niam@jghnorth.org.uk.invalid> - 2013-05-31 10:22 +0100
                Re: !0 versus true Christoph Becker <cmbecker69@gmx.de> - 2013-05-31 13:04 +0200
                Re: !0 versus true Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-05-31 18:13 +0200
                Re: !0 versus true Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-05-31 18:16 +0200
                Re: !0 versus true Scott Sauyet <scott.sauyet@gmail.com> - 2013-05-31 07:25 -0700
  Re: !0 versus true imma harper <immaterial_uk@yahoo.co.uk> - 2013-05-31 09:46 -0700
    Re: !0 versus true Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-05-31 21:38 +0200
    Re: !0 versus true John Harris <niam@jghnorth.org.uk.invalid> - 2013-06-01 17:37 +0100
  Re: !0 versus true Dr J R Stockton <reply1300@merlyn.demon.co.uk.invalid> - 2013-06-02 21:03 +0100
    Re: !0 versus true Scott Sauyet <scott.sauyet@gmail.com> - 2013-06-03 04:45 -0700
      Re: !0 versus true Dr J R Stockton <reply1300@merlyn.demon.co.uk.invalid> - 2013-06-04 17:23 +0100

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