Just Jake Just Jake - 2 years ago 103
Javascript Question

find the time left in a setTimeout()?

I'm writing some Javascript that interacts with library code that I don't own, and can't (reasonably) change. It creates Javascript timeouts used for showing the next question in a series of time-limited questions. This isn't real code because it is obfuscated beyond all hope. Here's what the library is doing:

// setup a timeout to go to the next question based on user-supplied time
var t = questionTime * 1000
test.currentTimeout = setTimeout( showNextQuestion(questions[i+1]), t );

I want to put a progress bar onscreen that fills towards
questionTime * 1000
by interrogating the timer created by
. The only problem is, there seems to be no way to do this. Is there a
function that I'm missing? The only information on Javascript timeouts that I can find is related only to creation via
setTimeout( function, time)
and deletion via
clearTimeout( id )

I'm looking for a function that returns either the time remaining before a timeout fires, or the time elapsed after a timeout has been called. My progress bar code looks like this:

var timeleft = getTimeout( test.currentTimeout ); // I don't know how to do this
var $bar = $('.control .bar');
while ( timeleft > 1 ) {
$bar.width(timeleft / test.defaultQuestionTime * 1000);

tl;dr: How do I find the time remaining before a javascript setTimeout()?

Here's the solution I'm using now. I went through the library section that's in charge of tests, and unscrambled the code (terrible, and against my permissions).

// setup a timeout to go to the next question based on user-supplied time
var t = questionTime * 1000
test.currentTimeout = mySetTimeout( showNextQuestion(questions[i+1]), t );

and here's my code:

// wrapper for setTimeout
function mySetTimeout( func, timeout ) {
timeouts[ n = setTimeout( func, timeout ) ] = {
start: new Date().getTime(),
end: new Date().getTime() + timeout
t: timeout
return n;

This works pretty spot-on in any browser that isn't IE 6. Even the original iPhone, where I expected things to get asynchronous.

Answer Source

If you can't modify the library code, you'll need to redefine setTimeout to suit your purposes. Here's an example of what you could do:

(function () {
var nativeSetTimeout = window.setTimeout;

window.bindTimeout = function (listener, interval) {
    function setTimeout(code, delay) {
        var elapsed = 0,

        h = window.setInterval(function () {
                elapsed += interval;
                if (elapsed < delay) {
                    listener(delay - elapsed);
                } else {
            }, interval);
        return nativeSetTimeout(code, delay);

    window.setTimeout = setTimeout;
    setTimeout._native = nativeSetTimeout;
window.bindTimeout(function (t) {console.log(t + "ms remaining");}, 100);
window.setTimeout(function () {console.log("All done.");}, 1000);

This is not production code, but it should put you on the right track. Note that you can only bind one listener per timeout. I haven't done extensive testing with this, but it works in Firebug.

A more robust solution would use the same technique of wrapping setTimeout, but instead use a map from the returned timeoutId to listeners to handle multiple listeners per timeout. You might also consider wrapping clearTimeout so you can detach your listener if the timeout is cleared.

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