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From: Joao Rodrigues <groups_jr-1@yahoo.com.br>
Newsgroups: comp.lang.javascript
Subject: Re: Why does (a,b,c) return only c?
Date: Mon, 25 May 2015 19:24:18 -0300
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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