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Perfomance of for loop

, 6 Dec 2012 CPOL
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Lots of for loops are coded wrong causing a potential performance issue.


I like looking at the code behind of websites I admire. In doing so I think I've seen one of the most common mistakes when it comes to JavaScript performance. This is not something that would popup quickly because the effect is minimal. But I like coding, so I also like doing right.


The type of loop that I often see coded wrong and which I also used to code wrong in the past is the for loop. And I believe the reason for doing it wrong is, not fully understanding what the syntax does.

This is the example from

for (statement 1; statement 2; statement 3)
//  the code block to be executed

This goes on stating;

Statement 1 is executed before the loop (the code block) starts.

Statement 2 defines the condition for running the loop (the code block).

Statement 3 is executed each time after the loop (the code block) has been executed.

Although this is essentially right, they forget to say that Statement 2 will be executed every time the loop ends a code block.

The wrong example

for(var i=0; i < myarray.length; i++) {
 //do something

Why is this wrong?

myarray.length will be executed after every code block. Now if this array is small, the JavaScript engine will answer this question quickly. But what if the condition would be some function that could take a while?

The right example

for(var i=0, j=myarray.length; i < j; i++) {
 //do something

Now the length of myarray will only be executed at the start of the loop.


This might seem like a very little win if it comes to performance. But I believe people should always do things right. If myarray.length would be something like calculateSomeVariableLengthByRunningLengthyFunction(), this could end up as a performance penalty.

And even the w3schools tutorial doesn't explain this the right way.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Sebastiaan Meijerink
Software Developer (Senior) none
Netherlands Netherlands
I'm a developer with 12+ years of experience. Starting of on a MVS mainframe, moving to building big multi-tier ERP systems with unix backends, to building web-based BI-Portals. I've seen a lot of different languages, environments and paradigmes.
At this point my main interest is webdevelopment. Mainly javascript and ASP.NET. But I also like getting my hands dirty on some PHP.
I've been a member of CodeProject for many years, but only recently started writing some articles. I hope someone will enjoy them.

Comments and Discussions

GeneralMy vote of 5 PinmemberS. M. Ahasan Habib13-Feb-13 20:00 
GeneralMy vote of 5 PinmemberSamrat Banerjee9-Dec-12 20:30 
QuestionOnly applies circumstantially... Pinmembernich_critic7-Dec-12 8:20 
GeneralMy vote of 5 PinmemberJohn Isaiah Carmona6-Dec-12 16:50 
GeneralMy vote of 5 PinmemberSk. Tajbir6-Dec-12 13:05 
GeneralMy vote of 5 PinmemberHezek1ah8-Nov-12 10:48 
SuggestionYou should present your workings PinmemberJohn Brett8-Nov-12 1:25 
GeneralMy vote of 5 PinmemberUsm@n7-Nov-12 21:34 
GeneralMy vote of 5 Pinmemberdeepuam6-Nov-12 17:53 
QuestionReverse while is faster in most common situations Pinmembernschonni6-Nov-12 14:49 
AnswerRe: Reverse while is faster in most common situations PinmemberSebastiaan Meijerink6-Nov-12 21:10 
GeneralRe: Reverse while is faster in most common situations Pinmembernschonni7-Nov-12 6:25 
GeneralRe: Reverse while is faster in most common situations PinmemberSebastiaan Meijerink7-Nov-12 21:30 
GeneralGreat PinmemberRicardo Lynch6-Nov-12 8:04 
QuestionFor loop method not taught Pinmembersatovey6-Nov-12 6:57 
GeneralRe: For loop method not taught Pinmemberelijah.manor6-Dec-12 8:56 
GeneralCompiler optimization? Pinmemberpeterboulton6-Nov-12 6:11 
GeneralRe: Compiler optimization? PinmemberSebastiaan Meijerink6-Nov-12 6:49 
GeneralRe: Compiler optimization? Pinmemberpeterboulton6-Nov-12 7:35 
GeneralMy vote of 5 PinmemberNeeraj_Maurya6-Nov-12 4:10 
I like this things. we used to write this code quite often but ignore the right way of doing this.thanks

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