Problems with Looping through window.setInterval

December 23, 2007

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This article is old and may contain information that is outdated, irrelevant, or—dare I say it—no longer accurate. Read with care!

Look at this code. What do you expect to be the outcome?

var d = new Date(); var r = []; for(var j=0,k=2;j<k ;j++) { window.setInterval(function() { var next = new Date(); r.push(next.getTime() - d.getTime()); },1000); }

Every 1000 milliseconds (1 second), the r array should have three delta timestamps added into it (one for each interval set in the for loop). In FireFox, however, there are exactly nine delta timestamps added per second. If you set k=10, the result is one hundred delta timestamps per second. If anyone has the answer, I am curious, even if the result is that I'm an idiot. It seems to work correctly in Internet Explorer 7. Thanks for your help, internet.

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Zach Leatherman is a builder for the web at Font Awesome and the creator/maintainer of Eleventy (11ty), an award-winning open source site generator. At one point he became entirely too fixated on web fonts. He has given 84 talks in nine different countries at events like Beyond Tellerrand, Smashing Conference, Jamstack Conf, CSSConf, and The White House. Formerly part of CloudCannon, Netlify, Filament Group, NEJS CONF, and NebraskaJS. Learn more about Zach »

2 Comments

Jeff Watkins Disqus
16 Jan 2008

Um... do you mean to be using window.setTimeout rather than window.setInterval? Using setInterval will cause your internal function to execute once every 100 milliseconds. And will add lots of time stamps to the array.
Zach Leatherman Disqus
17 Jan 2008

I meant to use setInterval. Using the for loop above, I should have my internal payload function execute in 3 separate 1 second interval timers. So every second, I should get 3 entries added into the r array. But, I'm getting 9 entries added every second, which is confusing to me. There should be only 3 timers, not 9.

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Problems with Looping through window.setInterval

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Jeff Watkins Disqus

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