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Re: Why does (a,b,c) return only c?

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On 25/05/2015 16:57, Evertjan. wrote:

> for (var i = 0, j = 9; i <= 9; i++, j--)
>   document.write( i + " - " + j + "<br>");
> 
> "Note that the comma in the var statement is not the comma operator, because 
> it doesn't exist within an expression. Rather, it is a special character in 
> var statements to combine multiple of them into one."

Commas may be either separators such as in variable statements (ES5.1,
§12.2), or operators (ES5.1, §11.14).

> 
> After reading that 3 times, I could sort of understand why this is not 
> allowed:
> 
> for (var i = 0, var j = 9; i <= 9; i++, j--)

The for statement (ES5.1, §12.6.3) has three _optional_ parts:

for ( var VariableDeclarationListNoIn ; Expressionopt ; Expressionopt )

In the first part, we can declare variables separated by commas. In the
last two parts (expressions), we can use comma operators if we wish, but
I think the code becomes cryptic in doing so.

But why on Earth would someone use comma operators? Because they let you
specify more than one expression where JavaScript expects only one. See
examples at:
<https://javascriptweblog.wordpress.com/2011/04/04/the-javascript-comma-operator/>


-- 
Joao Rodrigues

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Thread

Why does (a,b,c) return only c? Steven D'Aprano <steve@pearwood.info> - 2015-05-25 21:43 +1000
  Re: Why does (a,b,c) return only c? "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2015-05-25 14:17 +0200
    Re: Why does (a,b,c) return only c? Luuk <luuk@invalid.lan> - 2015-05-25 14:42 +0200
      Re: Why does (a,b,c) return only c? "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2015-05-25 14:58 +0200
    Re: Why does (a,b,c) return only c? "Christoph M. Becker" <cmbecker69@arcor.de> - 2015-05-25 14:57 +0200
      Re: Why does (a,b,c) return only c? "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2015-05-25 15:07 +0200
        Re: Why does (a,b,c) return only c? "Christoph M. Becker" <cmbecker69@arcor.de> - 2015-05-25 15:32 +0200
          Re: Why does (a,b,c) return only c? "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2015-05-25 17:50 +0200
        Re: Why does (a,b,c) return only c? Luuk <luuk@invalid.lan> - 2015-05-25 19:53 +0200
          Re: Why does (a,b,c) return only c? "D. Aaron Sawyer" <aaron@110.net> - 2015-05-25 19:49 -0300
  Re: Why does (a,b,c) return only c? John-Paul Stewart <jpstewart@sympatico.ca> - 2015-05-25 13:58 -0400
    Re: Why does (a,b,c) return only c? "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2015-05-25 21:57 +0200
      Re: Why does (a,b,c) return only c? Joao Rodrigues <groups_jr-1@yahoo.com.br> - 2015-05-25 19:24 -0300
        Re: Why does (a,b,c) return only c? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-05-26 12:26 +0200
          Re: Why does (a,b,c) return only c? Joao Rodrigues <groups_jr-1@yahoo.com.br> - 2015-05-26 09:36 -0300
            Re: Why does (a,b,c) return only c? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-05-26 14:42 +0200
              Re: Why does (a,b,c) return only c? Joao Rodrigues <groups_jr-1@yahoo.com.br> - 2015-05-26 09:53 -0300
                Re: Why does (a,b,c) return only c? "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2015-05-26 15:15 +0200
                Re: Why does (a,b,c) return only c? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-05-26 15:17 +0200
          Re: Why does (a,b,c) return only c? Scott Sauyet <scott.sauyet@gmail.com> - 2015-05-26 08:59 -0700

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