Subj : Re: "x = " vs. "var x = "
To   : Lau Lei Cheong
From : Brendan Eich
Date : Tue Sep 07 2004 10:01 am

Lau Lei Cheong wrote:
> I also have this kind of problem, but for most time I simply take away the
> "var" and then ignore it. This seems a good chance to make it clear.
> 
> Would you tell my why "var x=5" won't do it? To my understanding "initialize
> an variable with something" is the same as "declaring the variable, then
> assign its value with something".


Because of eval.  eval("for (var i=0; i<0; i++);") should have no eval 
result, meaning that eval returns undefined given that input, not false 
or 0.  The same goes for eval("var x=0").

Note the difference between no eval result and undefined eval result:

   eval("42; var x=43; 44") => 44

while

   eval("42; var x=43") => 42.

JS is a C-like language, so it has statements as well as expressions, 
and an expression can be a statement if started where a statement may 
start and followed by a semicolon (or a newline, with automatic 
semicolon insertion).  The expression statements have result values when 
eval'ed.  Other statements and their contained expression-parts do not 
set eval results.

In the case of a var statement, the decision may seem arbitrary, since 
one could cause any var contained in a for loop header to have no eval 
result, but still allow var statements to have eval result.

But in the interest of distinguishing declarations from expression 
statements, the ECMA spec makes the former have no eval result.  This 
may help to future-proof the language design; it also allows certain 
optimizations.

/be


> 
> Thank a lot. :)
> 
> "Georg Maaß" <georg@bioshop.de> ??? news:2q3rudFqq0fpU2@uni-berlin.de ???...
> 
>>Sterling Bates wrote:
>>
>>>Just a quick q: why does the expression "var x = 5" return "undefined"?
>>
>>This is not an expression. This is a statement, declaring a variable x
>>initializing it with 5.
>>
>>
>>> The expression "x = 5" returns 5, as I would expect.
>>
>>This is an expression, not an initialization.
>>
>>
>>-- 
>>Georg Maaß   -   bioshop.de    D-76227 Karlsruhe, Westmarkstraße 82
>>HTML, XML / JavaScript, C++, Java, PHP, VB / CGI, JSP, ASP, ASP.net
>>   - The ultimate DHTML engine: http://gml-modul.sourceforge.net -
>>                        http://sourceforge.net/projects/gml-modul
>>
> 
> 
>

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