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Editorial Team
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Asked: 2026-06-10T17:48:26+00:00

In the book JavaScript Patterns Stoyan Stefanov he claims that the common way of

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In the book JavaScript Patterns Stoyan Stefanov he claims that the common way of looping in JavaScript

for (i = 0, max = myarray.length; i < max; i++) {
// do something with myarray[i]
}

can be optimized by using this pattern instead

for (i = myarray.length; i--;) {
// do something with myarray[i]
}

I found that to be interesting so I decided to test it in the real world by applying the technique to a performance intensive loop showed in this blog post about doing pixel manipulation with canvas. The benchmarks comparing the regular code with the “optimized” code can be seen here.

The interesting thing is that the supposedly optimized loop is actually slower than the regular way of looping in both Opera and Firefox. Why is that?

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1 Answer

  1. Editorial Team

    Most of the ways to try to optimise a loop comes from C, and a time when compilers where simpler and processors executed one instruction after the other.

    Modern processors run the code very differently, so optimising specific instructions doesn’t have the same effect now.

    For Javascript the changes are quite rapid now. It has gone from being interpreted to being compiled, which makes a huge performance difference. The compilers are very different between browsers, and they change with each new browser version, so something that is faster in one browser today, may be slower tomorrow.

    I have tested some different ways of optimising loops, and currently there is very little difference in performance: http://jsperf.com/loopoptimisations

    One thing that can be said for certain though, is that the regular way of writing loops is the most common, so that is what all compilers will be focusing on optimising.

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