Develop Reference

Javascript : setTimeout and interface freezing

Context
I've got about 10 complex graphs which take 5sec each to refresh. If I do a loop on these 10 graphs, it takes about 50 seconds to refresh. During these 50 seconds, the user can move a scrollbar. If the scrollbar is moved, the refresh must stop and when the scrollbar stops to move, the refresh occurs again.
I'm using the setTimeout function inside the loop to let the interface refresh.
the algorithm is :
render the first graph
setTimeout(render the second graph, 200)
when the second graph is rendered, render the third one in 200ms, and so on
The setTimeout allows us to catch the scrollbar event and to clearTimeout the next refresh to avoid to wait 50sec before moving the scrollbar...
The problem is that it does not run anytime.
Take the simple following code (you can try it in this fiddle : http://jsfiddle.net/BwNca/5/) :
HTML :
<div id="test" style="width: 300px;height:300px; background-color: red;">
</div>
<input type="text" id="value" />
<input type="text" id="value2" />
Javascript :
var i = 0;
var j = 0;
var timeout;
var clicked = false;
// simulate the scrollbar update : each time mouse move is equivalent to a scrollbar move
document.getElementById("test").onmousemove = function() {
// ignore first move (because onclick send a mousemove event)
if (clicked) {
clicked = false;
return;
}
document.getElementById("value").value = i++;
clearTimeout(timeout);
}
// a click simulates the drawing of the graphs
document.getElementById("test").onclick = function() {
// ignore multiple click
if (clicked) return;
complexAlgorithm(1000);
clicked = true;
}
// simulate a complexe algorithm which takes some time to execute (the graph drawing)
function complexAlgorithm(milliseconds) {
var start = new Date().getTime();
for (var i = 0; i < 1e7; i++) {
if ((new Date().getTime() - start) > milliseconds){
break;
}
}
document.getElementById("value2").value = j++;
// launch the next graph drawing
timeout = setTimeout(function() {complexAlgorithm(1000);}, 1);
}
The code does :
when you move your mouse into the red div, it updates a counter
when you click on the red div, it simulates a big processing of 1sec (so it freezes the interface due to javascript mono thread)
after the freezing, wait 1ms, and resimulate the processing and so on until the mouse move again
when the mouse move again, it breaks the timeout to avoid infinite loop.
The problem
When you click one time and move the mouse during the freeze, I was thinking that the next code that will be executed when a setTimeout will occurs is the code of the mousemove event (and so it will cancel the timeout and the freeze) BUT sometimes the counter of click gains 2 or more points instead of gaining only 1 point due to the mouvemove event...
Conclusion of this test : the setTimeout function does not always release resource to execute a code during a mousemove event but sometimes kept the thread and execute the code inside the settimeout callback before executing another code.
The impact of this is that in our real example, the user can wait 10 sec (2 graphs are rendered) instead of waiting 5 seconds before using the scrollbar. This is very annoying and we need to avoid this and to be sure that only one graph is rendered (and other canceled) when the scrollbar is moved during a render phase.
How to be sure to break the timeout when the mouse move ?
PS: in the simple example below, if you update the timeout with 200ms, all runs perfectly but it is not an acceptable solution (the real problem is more complex and the problem occurs with a 200ms timer and a complex interface). Please do not provide a solution as "optimize the render of the graphs", this is not the problem here.
EDIT : cernunnos has a better explanation of the problem :
Also, by "blocking" the process on your loop you are ensuring no event can be handled until that loop has finished, so any event will only be handled (and the timeout cleared) inbetween the execution of each loop (hence why you sometimes have to wait for 2 or more full executions before interrupting).
The problem is exactly contains in bold words : I want to be sure to interrupt the execution when I want and not to wait 2 or more full executions before interrupting
Second EDIT :
In summary : takes this jsfiddle : http://jsfiddle.net/BwNca/5/ (the code above).
Update this jsfiddle and provide a solution to :
Mouse move on the red div. Then click and continue moving : the right counter must raise only once. But sometimes it raises 2 or 3 times before the first counter can run again... this is the problem, it must raise only once !

The BIG problem here is setTimeout is unpredictable once it started, and especially when it is doing some heavy lifiting.
You can see the demo here:
http://jsfiddle.net/wao20/C9WBg/
var secTmr = setTimeout(function(){
$('#display').append('Timeout Cleared > ');
clearTimeout(secTmr);
// this will always shown
$('#display').append('I\'m still here! ');
}, 100);
There are two things you can do to minimize the impact on the browser performance.
Store all the intances of the setTimeoutID, and loop through it when you want to stop
var timers = []
// When start the worker thread
timers.push( setTimeout(function () { sleep(1000);}, 1) );
// When you try to clear
while (timers.length > 0) {
clearTimeout(timers.pop());
}
Set a flag when you try to stop process and check that flag inside your worker thread just in case clearTimeout failed to stop the timer
// Your flag
var STOPForTheLoveOfGod = false;
// When you try to stop
STOPForTheLoveOfGod = true;
while (timers.length > 0) {
clearTimeout(timers.pop());
}
// Inside the for loop in the sleep function
function sleep(milliseconds) {
var start = new Date().getTime();
for (var i = 0; i < 1e7; i++) {
if (STOPForTheLoveOfGod) {
break;
}
// ...
}
}
You can try out this new script.
http://jsfiddle.net/wao20/7PPpS/4/

I may have understood the problem but assuming you are trying to block the interface after a click for a minimum of 1 second and unblocking it by moving the mouse (after that 1 second minimum):
This is not a good implementation of sleep, as you are keeping the process running the whole time (doing nothing != sleeping), this results in a waste of resources.
Why not create an overlay (a semi/fully transparent div), put it on top of the rest of the interface (position fixed, full width and full height) and use it to prevent any interaction with the underlying interface. Then destroy it when the conditions are right (a second has passed and the user moved the mouse).
This behaves more like a sleep (has some initial processing time but then releases the processor for a given amount of time) and should help you achieve the behavior you need (assuming i understood it right).
It has the added bonus of allowing you to give the user some visual cue that some processing is being done.
Edit:
Also, by "blocking" the process on your loop you are ensuring no event can be handled until that loop has finished, so any event will only be handled (and the timeout cleared) inbetween the execution of each loop (hence why you sometimes have to wait for 2 or more full executions before interrupting).

Surprising enough you have not figured out that, when you setTimeout(); you can input a check after that. A variable is true then trash the wait, or trash it. Now there is a method that you can check to scroll with a scroll bar. After you have checked it true inside a variabled using the means, then you will find this will repeat inifite times as they scroll the bar, making many executing times of 5 seconds. To fix this add a 1 second wait to make sure it doesn't over repeat. Your welcome :)

Any long-running function is going to tie up your browser window. Consider moving your complexAlgorithm() outside of your main javascript code using WebWorkers.

The answer is in your question
...the refresh must stop and when the scrollbar stops to move, the
refresh occurs again.
You should write complexAlgorithm in such way that you can almost instantly brake it in a middle (just when you know you will have to re run)
so main code should look something like
stopAllRefresh; //should instantly(or after completing small chunk) stop refresh
setTimeout(startRefresh, 100);
and render graph in small chunks (each runs < 1sec) in setTimeout
like
var curentGraph = 0;
var curentChunk = 0;
function renderGraphChunk(){
if (needToBreak) //check if break rendering
{exit};
// Render chunk here
render(curentGraph, curentChunk);
curentChunk +=1;
setTimeout(renderGraphChunk, 1);
}
this is just a idea sketch, real implementation can be completely different

What you want to do can not be done without web worker, that is only implemented in some latest browser specially Chrome.
Otherwise, you have to break your algorithm in queue. Just like jQuery UI puts every next animation calculation in queue. http://api.jquery.com/jQuery.queue/
It is a simple queue and next instruction set is queued with help of setTimeout.
for (i=0; i <1000; i++)
{
process (i) ;
}
Can be translated to
function queue(s,n, f)
{
this.i=s;
this.n=n;
this.f=f;
this.step = function(){
if ( this.i <this.n)
{
this.f(this.i);
this.i = this.i +1;
var t = this;
setTimeout( function ( ) { t.step(); } , 5);
}
}
this.step();
}
queue ( O, 1000, function(i){
process(i);
}) ;
This is just an example of how Synchronous for loop can be written to execute same logic asynchronously using smaller independent iteration.

Try and check out web workers. I think it will be useful in this situation.
http://en.wikipedia.org/wiki/Web_worker
http://www.html5rocks.com/en/tutorials/workers/basics/

javascript set interval run as separate thread?

I want to use a timer as a fallback in case I end up in an infinite loop. It seems that set interval is the right way to do this. However, it's not working for me.
From my research, it seems like setInterval should run in a separate thread in the background, but I don't see it.
Why is this behavior happening? And how do I solve this?
var time = 0;
window.setInterval(function(){time++;}, 1000);
while (true) {
//stuff done
if (time >= 5) {
break;
}
}

Browser javascript runs in a single thread. So if you perform something that takes too long - it will freeze browser.
See John Resig article for further details: http://ejohn.org/blog/how-javascript-timers-work/
After you read that article you'll get that your setInterval callback queued to be run in 1000ms after now but only after the current code is finished. It cannot finish though, because of the infinite loop.

zerkms has the correct answer. But I would add that web workers are a way to get some multi-threaded-ish behavior from client side javascript.
var worker = new Worker('my_task.js');
worker.onmessage = function(event) {
console.log("Called back by the worker!\n");
};
The worker runs in a background thread, and you can exchange messages and subscribe to events. It's pretty nifty.

As has been already said - the callback to setInterval doesn't run until the infinite loop finishes. To do what you are trying to achieve - without using web workers - you have to check the time from the loop itself:
var start = Date.now();
while((Date.now() - start) < 5000){
...
}

Using setTimeout to improve responsiveness

When looking to improve a page's performance, one technique I haven't heard mentioned before is using setTimeout to prevent javascript from holding up the rendering of a page.
For example, imagine we have a particularly time-consuming piece of jQuery inline with the html:
$('input').click(function () {
// Do stuff
});
If this code is inline, we are holding up the perceived completion of the page while the piece of jquery is busy attaching a click handler to every input on the page.
Would it be wise to spawn a new thread instead:
setTimeout(function() {
$('input').click(function () {
// Do stuff
})
}, 100);
The only downside I can see is that there is now a greater chance the user clicks on an element before the click handler is attached. However, this risk may be acceptable and we have a degree of this risk anyway, even without setTimeout.
Am I right, or am I wrong?

The actual technique is to use setTimeout with a time of 0.
This works because JavaScript is single-threaded. A timeout doesn't cause the browser to spawn another thread, nor does it guarantee that the code will execute in the specified time. However, the code will be executed when both:
The specified time has elapsed.
Execution control is handed back to the browser.
Therefore calling setTimeout with a time of 0 can be considered as temporarily yielding to the browser.
This means if you have long running code, you can simulate multi-threading by regularly yielding with a setTimeout. Your code may look something like this:
var batches = [...]; // Some array
var currentBatch = 0;
// Start long-running code, whenever browser is ready
setTimeout(doBatch, 0);
function doBatch() {
if (currentBatch < batches.length) {
// Do stuff with batches[currentBatch]
currentBatch++;
setTimeout(doBatch, 0);
}
}
Note: While it's useful to know this technique in some scenarios, I highly doubt you will need it in the situation you describe (assigning event handlers on DOM ready). If performance is indeed an issue, I would suggest looking into ways of improving the real performance by tweaking the selector.
For example if you only have one form on the page which contains <input>s, then give the <form> an ID, and use $('#someId input').

setTimeout() can be used to improve the "perceived" load time -- but not the way you've shown it. Using setTimeout() does not cause your code to run in a separate thread. Instead setTimeout() simply yields the thread back to the browser for (approximately) the specified amount of time. When it's time for your function to run, the browser will yield the thread back to the javascript engine. In javascript there is never more than one thread (unless you're using something like "Web Workers").
So, if you want to use setTimeout() to improve performance during a computation-intensive task, you must break that task into smaller chunks, and execute them in-order, chaining them together using setTimeout(). Something like this works well:
function runTasks( tasks, idx ) {
idx = idx || 0;
tasks[idx++]();
if( idx < tasks.length ) {
setTimeout( function(){ runTasks(tasks, idx); },1);
}
}
runTasks([
function() {
/* do first part */
},
function() {
/* do next part */
},
function() {
/* do final part */
}
]);
Note:
The functions are executed in order. There can be as many as you need.
When the first function returns, the next one is called via setTimeout().
The timeout value I've used is 1. This is sufficient to cause a yield, and the browser will take the thread if it needs it, or allow the next task to proceed if there's time. You can experiment with other values if you feel the need, but usually 1 is what you want for these purposes.

You are correct, there is a greater chance of a "missed" click, but with a low timeout value, its pretty unlikely.

javascript, while loop

i'm trying to get my script to wait for user input (click of a button) before continuing, this is v feasible in other languages, but seems impossible in js. basically, i want the user to select an option within a given time frame, if the user selects the wrong option, they're told..script then conts...otherwise, if after a certain amount of time theres no response...script just continues again sowing them the correct ans, but there seems to be nothing in js to make the script wait for that user input! ive tried a while loop, but that is just a big no no in js, ive used settimeout but has no real effect because the script just continues like normal then performs an action after x amount of time, ive tried setting variables and letting the script cont only if it is of a particular value, which is set only if the user clicks...eg var proceed=false, this is only set to true if the user clicks a button, but it still doesn't work... ive tried sooo many other solutions but nothing actually seems to be working. i like the idea of a while loop, because it doeas exactly what i want it to so, but if completly freezes my browser, is there a more effecient type of loop that will will peroform in the same manner with crashing my browser?
heres my code below that compltely freezes my computer. this method is called within a for loop which calls another method after it.
function getUserResp(){
$("#countdown").countdown({seconds: 15});
setTimeout("proceed=true", 16000);
$("#ans1").click(function(){
ansStr=$(this).text();
checkAns(ansStr);
});
$("#ans2").click(function(){
ansStr=$(this).text();
checkAns(ansStr);
});
$("#ans3").click(function(){
ansStr=$(this).text();
checkAns(ansStr);
});
would like something like this.....or just some sort of loop to make the script wait before going ahead so at least it gives the user some time to respond rather than running straight though!
do{
$(".ans").mouseover(function(){
$(this).addClass("hilite").fadeIn(800);
});
$(".ans").mouseout(function(){
$(this).removeClass("hilite");
});
}while(proceed==false);
}

You're doing it wrong.
JavaScript in the browser uses an event-driven model. There's no main function, just callbacks that are called when an event happens (such as document ready or anchor clicked). If you want something to happen after a user clicks something, then put a listener on that thing.
What you've done just keeps adding an event listener every time round the loop.
If you want to wait for user input then just don't do anything - the browser waits for user input (it's got an internal event loop). The worst thing you can do is try to reimplement your own event loop on top of the browser's.
You need to learn JavaScript. Trying to write JavaScript like you would another language only leads to pain and suffering. Seriously.
Douglas Crockford said it best:
JavaScript is a language that most people don’t bother to learn before they use. You can’t do that with any other language, and you shouldn’t want to, and you shouldn’t do that with this language either. Programming is a serious business, and you should have good knowledge about what you’re doing, but most people feel that they ought to be able to program in this language without any knowledge at all, and it still works. It’s because the language has enormous expressive power, and that’s not by accident.

You can't block the Javascript from running in the same way that you can in some other imperative languages. There's only one thread for Javascript in the browser, so if you hang it in a loop, nothing else can happen.
You must use asynchronous, event-driven programming. Setting a click handler (or whatever) combined with a timeout is the right way to start. Start a 15 second setTimeout. Inside the click handler for the answers, cancel the timeout. This way the timeout's handler only happens if the user doesn't click an answer.
For example:
var mytimeout = setTimeout(15000, function() {
// This is an anonymous function that will be called when the timer goes off.
alert("You didn't answer in time.");
// Remove the answer so the user can't click it anymore, etc...
$('#ans').hide();
});
$('#ans').click(function() {
// Clear the timeout, so it will never fire the function above.
clearTimeout(mytimeout);
alert("You picked an answer!");
});
See how the code must be structured such that it's event-driven. There's no way to structure it to say "do this thing, and wait here for an answer."

You're looking at client-side javascript as if it wasn't already in an event-driven loop. All you need to do is wait for the appropriate event to happen, and if it hasn't happened yet, continue to wait, or else perform some default action.
You don't need to:
create main loop: // All
wait for user input // Of
timer = start_timer() // This
// Is done for you
if [user has input data]:
process_data()
else if [timer > allowed_time]:
process_no_data()
else:
wait() // By the Browser
You only need the middle part. All you need to do is (Actual javascript follows, not pseudo-code):
// First, store all of the answer sections,
// so you're not grabbing them every time
// you need to check them.
var answers = {};
answers.ans1 = $("#ans1");
answers.ans2 = $("#ans2");
answers.ans3 = $("#ans3");
// This is a flag. We'll use it to check whether we:
// A. Have waited for 16 seconds
// B. Have correct user input
var clear_to_proceed = false;
var timer_id;
// Now we need to set up a function to check the answers.
function check_answers() {
if ( ! clear_to_proceed ) {
clear_to_proceed = checkAns(answers.ans1.text());
clear_to_proceed = checkAns(answers.ans2.text());
clear_to_proceed = checkAns(answers.ans3.text());
// I assume checkAns returns
// true if the answer is correct
// and false if it is wrong
}
if ( clear_to_proceed ) {
clearTimeout(timer_id);
return true; // Or do whatever needs be done,
// as the client has answered correctly
} else {
// If we haven't set a timer yet, set one
if ( typeof timer_id === 'undefined' ) {
timer_id = setTimeout(function(){
// After 16 seconds have passed we'll check their
// answers one more time and then force the default.
check_answers();
clear_to_proceed = true;
check_answers();
}, 16000);
}
return false; // We're just waiting for now.
}
}
// Finally, we check the answers any time the user interact
// with the answer elements.
$("#ans1,#ans2,#ans3").bind("focus blur", function() {
check_answers();
});

Javascript Poll server. Will this cause a stack overflow?

I am not too familiar with the specifics of every javascript implementation on each browser. I do know however that using setTimeout, the method passed in gets called on a separate thread. So would using a setTimeout recursively inside of a method cause its stack to grow indefinitely until it causes a Stack Overflow? Or would it create a separate callstack and destroy the current frame once it goes out of focus? Here is the code that I'm wondering about.
function pollServer()
{
$.getJSON("poll.php", {}, function(data){
window.setTimeout(pollServer, 1000);
});
}
window.setTimeout(pollServer, 0);
I want to poll the server every second or so, but do not want to waste CPU cycles with a 'blocking loop' - also I do not want to set a timelimit on how long a user can access a page either before their browser dies.
EDIT
Using firebug, I set a few breakpoints and by viewing the "Script -> Stack" panel saw that the call stack is literally just "pollServer" and it doesn't grow per call. This is good - however, do any other implementations of JS act differently?

I am not sure if it would create a stack overflow, but I suggest you use setInterval if the period is constant.
This is how prototype implements its PeriodicalExecuter.
// Taken from Prototype (www.prototypejs.org)
var PeriodicalExecuter = Class.create({
initialize: function(callback, frequency) {
this.callback = callback;
this.frequency = frequency;
this.currentlyExecuting = false;
this.registerCallback();
},
registerCallback: function() {
this.timer = setInterval(this.onTimerEvent.bind(this), this.frequency * 1000);
},
execute: function() {
this.callback(this);
},
stop: function() {
if (!this.timer) return;
clearInterval(this.timer);
this.timer = null;
},
onTimerEvent: function() {
if (!this.currentlyExecuting) {
try {
this.currentlyExecuting = true;
this.execute();
} finally {
this.currentlyExecuting = false;
}
}
}
});

setTimeout executes sometime later in the future in the event pump loop. Functions passed to setTimeout are not continuations.
If you stop and think about it, what useful purpose or evidencec is there that the call stack is shared by the timeout function.
If they were shared what stack would be shared from the setter to the timeout function ?
Given the setter can do a few returns and pop some frames - what would be passed ?
Does the timeout function block the original thread ?
Does the statement after the setTimeout function execute after the timeout executes ?
Once you answer those questions it clearly becomes evident the answerr is NO.

setTimeout does not grow the callstack, because it returns immediately. As for whether your code will run indefinitely in any browser, I'm not sure, but it seems likely.

take a look at the jQuery "SmartUpdater" plugin.
http://plugins.jquery.com/project/smartupdater
Following features are available:
stop() - to stop updating.
restart() - to start updating after pause with resetting time interval to minTimeout.
continue() - to start updating after pause without resetting time interval.
status attribute - shows current status ( running | stopping | undefined )
updates only if new data is different from the old one.
multiplies time interval each time when data is not changed.
handle ajax failures by stopping to request data after "maxFailedRequests".