Declaring variable in loop or before loop in AS3?

Question

Should I declare the _mcContainer var before the loop or no? (performance increase?)

for(var i:uint = _startIndex; i <= _endIndex; ++i){    
    var _mcContainer:MovieClip = _mcParent["i_" + _position];
}

or

var _mcContainer:MovieClip;
for(var i:uint = _startIndex; i <= _endIndex; ++i){
    _mcContainer = _mcParent["i_" + _position];
}

?

second option is faster, have a look at this: rozengain.com/blog/2007/05/01/… — George Profenza
– George Profenza, Commented Nov 9, 2010 at 17:05
@george, which part of that page relates to this question? I'm having trouble finding anything. — spender
– spender, Commented Nov 9, 2010 at 17:09
@spender "We can also speed up the loop by storing the array’s length in a variable outside of the loop" before 'Constants from other classes'...u are right...not in plain sight — George Profenza
– George Profenza, Commented Nov 9, 2010 at 23:55
@george-profenza but array's length is different case cause there if you don't use a var to reference to the length each time your loop will call need to get the array length over and over again. But in my case I was wondering does it change something if I declare the variable and it's type before the loop so on looping it wouldn't declare new variable each time. — Rihards
– Rihards, Commented Nov 9, 2010 at 23:59
ok, I see...in your case, as @spender mentioned it shouldn't make a difference — George Profenza
– George Profenza, Commented Nov 10, 2010 at 0:25

spender · Accepted Answer · 2010-11-09 17:04:51Z

6

It's not hard to test...

...however according to the docs, it shouldn't make any difference because variable declarations are hoisted to the top of the method block anyway.

From the docs:

An interesting implication of the lack of block-level scope is that you can read or write to a variable before it is declared, as long as it is declared before the function ends. This is because of a technique called hoisting , which means that the compiler moves all variable declarations to the top of the function.

answered Nov 9, 2010 at 17:04

spender

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3 Comments

Courtney Christensen Over a year ago

Good point. This has bitten me several times when I was developing in AS3. Other compilers like C++ (and even Java) recognize using a variable before it is declared as a compile-time error.

Rihards Over a year ago

From the Flex coding conventions: opensource.adobe.com/wiki/display/flexsdk/… i thought that i've to declare variables only before they are used and that would increase performance, but thanks to your docs reference i see it does not. :] Thanks!

ixx Over a year ago

Having variables declared after they are used reduce code legibility, in my opinion. They should be declared before they are used - in worst case in the same line. But not later.

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