Useful functions

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setTimeout with a shorter delay
Wednesday, 2010-03-09, 13:45 -0800

On Sunday, somebody with the nickname {g} was on asking about the behavior of setTimeout. In particular, he wanted to divide up work into a bunch of pieces in a way that allowed the user to interact with the page while the work was happening, and was doing this by doing a piece of the work, and then making a setTimeout call to continue the work. (In some cases, this could also be done using workers.) Unfortunately for him, setTimeout in most browsers doesn't allow a delay less than about 10 milliseconds (it forces any smaller delays to be longer), so the work wasn't finishing as fast as it could. (Chrome has changed this to 2 milliseconds, though, and apparently had some problems with it.)

A while ago, Jeff Walden suggested to me that Web pages could get the equivalent of setTimeout, with a real zero delay, using postMessage. This turns out to be relatively straightforward:

    // Only add setZeroTimeout to the window object, and hide everything
    // else in a closure.
    (function() {
        var timeouts = [];
        var messageName = "zero-timeout-message";

        // Like setTimeout, but only takes a function argument.  There's
        // no time argument (always zero) and no arguments (you have to
        // use a closure).
        function setZeroTimeout(fn) {
            window.postMessage(messageName, "*");

        function handleMessage(event) {
            if (event.source == window && == messageName) {
                if (timeouts.length > 0) {
                    var fn = timeouts.shift();

        window.addEventListener("message", handleMessage, true);

        // Add the one thing we want added to the window object.
        window.setZeroTimeout = setZeroTimeout;

I wrote a demo page that demonstrates that this is significantly faster than setTimeout(0). On a Firefox nightly 100 iterations of setZeroTimeout take about 10-20 milliseconds most of the time, but occasionally longer; on a WebKit build I have it takes about 4-6 milliseconds, but occasionally a bit longer. (We should probably investigate the performance difference here.) In comparison, in Firefox and on non-Chromium-based WebKit, the setTimeout version takes about a second (though perhaps even longer on Windows).

Update (2010-03-12): My numbers were on Linux. Boris tells me that on Mac, it's the opposite: Gecko is faster than Safari or Chrome.

function doCalculation() 
   //do your thing for a short time 
   //figure out how complete you are 
   var percent_complete=.... 
   return percent_complete; 
function pump() 
   var percent_complete=doCalculation(); 
   //maybe update a progress meter here! 
   //carry on pumping? 
   if (percent_complete<100) 
      setTimeout(pump, 50); 
//start the calculation 
arr.sort(function(e1,e2) {
    return e1>e2 ? 1:(e1<e2 ? -1:0)

arr.sort(function(e1,e2) {
    return e1>e2 ? -1:(e1<e2 ? 1:0)
<meta http-equiv="Content-type" content="text/html;charset=UTF-8">
<title>Show File Data</title>
<style type='text/css'>
body {
    font-family: sans-serif;
<script type='text/javascript'>

    function loadFile() {
        var input, file, fr;

        if (typeof window.FileReader !== 'function') {
            bodyAppend("p", "The file API isn't supported on this browser yet.");

        input = document.getElementById('fileinput');
        if (!input) {
            bodyAppend("p", "Um, couldn't find the fileinput element.");
        else if (!input.files) {
            bodyAppend("p", "This browser doesn't seem to support the `files` property of file inputs.");
        else if (!input.files[0]) {
            bodyAppend("p", "Please select a file before clicking 'Load'");
        else {
            file = input.files[0];
            fr = new FileReader();
            fr.onload = receivedText;

        function receivedText() {
            showResult(fr, "Text");

            fr = new FileReader();
            fr.onload = receivedBinary;

        function receivedBinary() {
            showResult(fr, "Binary");

    function showResult(fr, label) {
        var markup, result, n, aByte, byteStr;

        markup = [];
        result = fr.result;
        for (n = 0; n < result.length; ++n) {
            aByte = result.charCodeAt(n);
            byteStr = aByte.toString(16);
            if (byteStr.length < 2) {
                byteStr = "0" + byteStr;
        bodyAppend("p", label + " (" + result.length + "):");
        bodyAppend("pre", markup.join(" "));

    function bodyAppend(tagName, innerHTML) {
        var elm;

        elm = document.createElement(tagName);
        elm.innerHTML = innerHTML;

<form action='#' onsubmit="return false;">
<input type='file' id='fileinput'>
<input type='button' id='btnLoad' value='Load' onclick='loadFile();'>