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Zach Leatherman

Problems with Looping through window.setInterval

23 December 2007 Read this in less than 1 minute.

Warning

This article is fairly old and probably contains information that is no longer relevant. 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.