Perfomance of for loop

Introduction

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.

Background

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 http://www.w3schools.com/js/js_loop_for.asp:

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.

Conclusion

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.