Develop Reference

Run a function for a specified amount of time and stop [duplicate]

I would like to repeat a text for 2 seconds in a while loop. How do I break the loop after 2 seconds?
This is what I have tried so far but it doesn't work:
var repeat = true;
setTimeout(function() { var repeat = false }, 2000)
while(repeat) {
console.log("Let's repeat for 2 seconds...");
}

Additionaly to the other answer you could just check the time instead:
const start = +new Date;
while(+new Date < start + 2000) {
console.log("Let's repeat for 2 seconds...");
}

JavaScript is single threaded. This means that as long as your loop runs, your timeout will never be fired.
Depending on what you want and whether or not you want to lock down your browser (by using an actual infinite loop), you can use the setInterval as a loop, and use the setTimeout to stop the interval after 2 seconds.
console.log("Starting loop");
var interval = setInterval(function () {
console.log("Let's repeat for 2 seconds...");
}, 0);
setTimeout(function() {
clearInterval(interval);
console.log("Finished loop");
}, 2000);

If you're concerned at all with performance of this loop, the proposed solution would be a problem.
Note that in most cases, you shouldn't be synchronously iterating a huge number of times in Javascript since that would completely block the browser or server during this time, but there are some cases where this might be needed. Just be aware that this is generally not a good thing to do.
What follows is my own experimentation with several options to reduce the overhead of checking a timeout, since I was using it to benchmark other code.
I added console.time logs to both solutions, and added a couple of optimizations that might be worth considering.
The accepted solution has the worst performance:
const label = 'interval';
console.time(label);
let i = 0;
const interval = setInterval(
() => {
i += 1;
},
0);
setTimeout(
() => {
clearInterval(interval);
console.timeEnd(label)
console.log(`${i.toExponential()} iterations`);
},
2000);
// interval: 2001.100ms
// 4.93e+2 iterations
The next answer has much better performance, but it's still performing unnecessary work, type conversions and addition on every loop run:
let i = 0;
let start = +new Date;
let label = '+new Date + 2000';
console.time(label);
while ((+new Date) < start + 2000) {
i += 1;
}
console.timeEnd(label)
console.log(`${i.toExponential()} iterations`);
// +new Date + 2000: 1999.800ms
// 1.0921121e+7 iterations
By using Date.now() instead of (+new Date) you get a performance increment of around 2.5x:
let label = 'Date.now()';
console.time(label);
let end = Date.now() + 2000;
let i = 0;
while (Date.now() < end) {
i += 1;
}
console.timeEnd(label)
console.log(`${i.toExponential()} iterations`);
// Date.now(): 1999.000ms
// 2.6477108e+7 iterations
If performance is much more important that the exact nanosecond when you stop, and the operations are super fast, you can reduce the number of checks for even more operations:
let label = 'fuzzy + 2000';
console.time(label);
let end = Date.now() + 2000;
let i = 0;
// Only check the timeout every 1000 operations thanks to lazy evaluation.
while (i % 1000 !== 0 || Date.now() < end) {
i += 1;
}
console.timeEnd(label)
console.log(`${i.toExponential()} iterations`);
// fuzzy + 2000: 1999.800ms
// 6.5632e+8 iterations
30x better! You will need to tweak the frequency of the checks based on both the average loop time and how exact you want your timeout to be.

Leave a loop after 2 seonds

I would like to repeat a text for 2 seconds in a while loop. How do I break the loop after 2 seconds?
This is what I have tried so far but it doesn't work:
var repeat = true;
setTimeout(function() { var repeat = false }, 2000)
while(repeat) {
console.log("Let's repeat for 2 seconds...");
}

Additionaly to the other answer you could just check the time instead:
const start = +new Date;
while(+new Date < start + 2000) {
console.log("Let's repeat for 2 seconds...");
}

JavaScript is single threaded. This means that as long as your loop runs, your timeout will never be fired.
Depending on what you want and whether or not you want to lock down your browser (by using an actual infinite loop), you can use the setInterval as a loop, and use the setTimeout to stop the interval after 2 seconds.
console.log("Starting loop");
var interval = setInterval(function () {
console.log("Let's repeat for 2 seconds...");
}, 0);
setTimeout(function() {
clearInterval(interval);
console.log("Finished loop");
}, 2000);

If you're concerned at all with performance of this loop, the proposed solution would be a problem.
Note that in most cases, you shouldn't be synchronously iterating a huge number of times in Javascript since that would completely block the browser or server during this time, but there are some cases where this might be needed. Just be aware that this is generally not a good thing to do.
What follows is my own experimentation with several options to reduce the overhead of checking a timeout, since I was using it to benchmark other code.
I added console.time logs to both solutions, and added a couple of optimizations that might be worth considering.
The accepted solution has the worst performance:
const label = 'interval';
console.time(label);
let i = 0;
const interval = setInterval(
() => {
i += 1;
},
0);
setTimeout(
() => {
clearInterval(interval);
console.timeEnd(label)
console.log(`${i.toExponential()} iterations`);
},
2000);
// interval: 2001.100ms
// 4.93e+2 iterations
The next answer has much better performance, but it's still performing unnecessary work, type conversions and addition on every loop run:
let i = 0;
let start = +new Date;
let label = '+new Date + 2000';
console.time(label);
while ((+new Date) < start + 2000) {
i += 1;
}
console.timeEnd(label)
console.log(`${i.toExponential()} iterations`);
// +new Date + 2000: 1999.800ms
// 1.0921121e+7 iterations
By using Date.now() instead of (+new Date) you get a performance increment of around 2.5x:
let label = 'Date.now()';
console.time(label);
let end = Date.now() + 2000;
let i = 0;
while (Date.now() < end) {
i += 1;
}
console.timeEnd(label)
console.log(`${i.toExponential()} iterations`);
// Date.now(): 1999.000ms
// 2.6477108e+7 iterations
If performance is much more important that the exact nanosecond when you stop, and the operations are super fast, you can reduce the number of checks for even more operations:
let label = 'fuzzy + 2000';
console.time(label);
let end = Date.now() + 2000;
let i = 0;
// Only check the timeout every 1000 operations thanks to lazy evaluation.
while (i % 1000 !== 0 || Date.now() < end) {
i += 1;
}
console.timeEnd(label)
console.log(`${i.toExponential()} iterations`);
// fuzzy + 2000: 1999.800ms
// 6.5632e+8 iterations
30x better! You will need to tweak the frequency of the checks based on both the average loop time and how exact you want your timeout to be.

setTimeout executes faster than simultaneous setInterval

So, I have a setInterval and a setTimeout running simultaneously in this click-o-meter thing I'm doing: the user enters an specified number of seconds he/she wants the game to run for, and then it counts how many clicks you have done, what was the average time between each click, and the average amount of clicks per second you've made during the specified period of time.
<html>
<head></head>
<body>
<input type='text' id='timerInput'></input>
<button id='btn'>Click</button>
<script>
var before;
var now;
var clicks = 0;
var cts = 0; //Stands for 'Clicks This Second'
var intervals = new Array();
var cps = new Array();
var cpsCounter;
var timer;
var canContinue = true;
var timerInput = document.getElementById('timerInput');
var timerTime;
var wasBad = false;
document.getElementById('btn').onclick = function() {
if(canContinue) {
if(clicks <= 0) {
if(timerInput.value.replace(/\D/, '') === timerInput.value) {
wasBad = false;
timerTime = parseInt(timerInput.value.replace(/\D/, '')) * 1000;
before = new Date();
cpsCounter = window.setInterval(ctsFunction, 1000);
timer = window.setTimeout(finish, timerTime);
}else{
alert('Only numbers please!');
wasBad = true;
}
}else{
now = new Date();
console.log(now - before);
intervals.push(now - before);
before = new Date();
}
if(!wasBad){
clicks++;
cts++;
}
}else{console.log('Game ended');}
};
function ctsFunction() {
console.log('Clicks this second: ' + cts);
cps.push(cts);
cts = 0;
}
function finish() {
console.log('Clicks: ' + clicks);
console.log('Average Speed (ms): ' + Math.floor(intervals.reduce(function(a, b){return a + b;}) / (clicks - 1)));
console.log('Average Speed (clicks per second): ' + (cps.reduce(function(a, b){return a + b;}) / cps.length));
intervals = new Array();
console.log('cps.length: ' + cps.length);
cps = new Array();
clicks = 0;
cts = 0;
window.clearInterval(cpsCounter);
canContinue = false;
}
</script>
</body>
</html>
So, the problem is that when the gmae finishes, that is, when timer reaches the end, ctsFunction() is supposed to run once more at the last second, so it can register data from it; but finish() is executed faster, or prior to ctsFunction(), thus clearing the cpsCounter interval and not allowing it to do anything on the last second. I've tried adding some extra milliseconds to timer, but if you choose to run the game for enough seconds, the same problem will eventually happen (e.g. if you add 1ms, the problem will be solved for up to 2 seconds, but not for more).

I have a setInterval and a setTimeout running simultaneously
It will never happens because javascript is a single thread language. There is no matter what is in your code, javascript can't execute two commands simultaneously.
And one more:
timer delay is not guaranteed. JavaScript in a browser executes on a
single thread asynchronous events (such as mouse clicks and timers)
are only run when there’s been an opening in the execution.
Read this article to understand how javascript timers work.

Async foreach in javascript

I hope this will make sense:
I need to create a foreach function in javascript that will be used like this:
foreach(["A", "B", "C"], function(letter, done) {
// do something async with 'letter'
doSomthing(letter, done); // ***
}, function () {
// final callback that is called after all array has been visted.
// do some final work
});
So I was thinking about the following implementation:
var foreach = function(array, func, ready) {
if (!array.length)
ready();
var that = this;
func(array[0], function(){
that.foreach(array.slice(1, array.length), func, ready);
});
}
And it seems that it actually works! very cool.
But I was thinking if there is a solution that doesn't use recursion?
I couldn't think of one...

Your approach is technically correct but it is not good to do in such a way.
Pls implement using promise pattern in javasript .
I recommend you using when.js an open source js available on git for implementing promise pattern Pls refert to the below code
var service = {
fetch: function (query) {
// return a promise from the function
return when(["A", "B", "C"].forEach(function (name) {
alert(name);
}));
}
};
service.fetch("hello world").then(function () {
alert("work has been completed");
});

I just am here again for academic purposes at this point. Now, I recommend everyone to understand Aadit's approach in its elegance and terseness first because it was a nice learning experience for me in his technique of using bind as I did not know about that nor did I know about the additional arguments you can place after setTimeout.
After learning these things, I've reduced my code down to this:
var foreach = function(array,doSomething,onComplete) {
var i = 0, len = array.length, completeCount = 0;
for(;i < len; i++) {
window.setTimeout(function() {
doSomething(arguments[0]);
completeCount++;
if (completeCount === len) {
onComplete();
}
},0,array[i]);
}
};
I argue that you must have a "completeCount" because although Aadit's code is a great terse working solution that is reducing the array automatically, it is not truly asynchronous as "next()" is called after each method completes in the array linearly. The "completeCount" allows code to finish execution in any order which is the point of this I believe. In Aadit's code, there is also the side effect of modifying your input array as well as needing to alter the Function prototype in which I argue this is not necessary. "Hoisting" is also not practiced in his code which I think should be done as that style reduces errors.
Again, I respect Aadit's code very much and have taken the time to come back again to try to present a better solution based on what I have learned through other smart folks as well as Aadit. I welcome any critiques and corrections as I will try to learn from it.
FYI: here is one that is a general deferred method
var deferred = function(methods,onComplete) {
var i = 0, len = methods.length, completeCount = 0,
partialComplete = function() {
completeCount++;
if (completeCount === len) {
onComplete();
}
};
for(;i < len; i++) {
window.setTimeout(function() {
arguments[0](partialComplete);
},0,methods[i]);
}
};
// how to call it
deferred([
function (complete) {
// this could easily be ajax that calls "complete" when ready
complete();
},
function (complete) {
complete();
}
], function() {
alert('done');
});

Check out the async library. It has several different functions like that, and it's actively supported.

Correct me if I am wrong but from what I understand from your question I believe you want to take an array, serially call a function asynchronously on every member of that array, and then execute a callback function when every member of the array has been processed.
Now to execute a function asynchronously in a browser environment we would do something like this:
Function.prototype.async = function () {
setTimeout.bind(window, this, 0).apply(window, arguments);
};
alert.async(5);
alert(6);
In the above example the setTimeout function is used to call the given function asynchronously due to which we first see the value 6 and then the value 5 being alerted.
Next, to make your foreach function asynchronous we would do something as follows:
function forEach(array, funct, callback) {
if (array.length)
funct.async(array[0], forEach.bind(null, array.slice(1), funct, callback));
else callback.async();
}
The above solution doesn't use recusrion. Sure the forEach function is referenced within itself but it's only being called in the funct function which is called asynchronously. Hence the forEach function returns before it's called again in the funct function.
I have included links to JS fiddles before each snippet of code. If you have any more doubts I would be happy to answer them.
Edit:
If you do not like modifying the prototype of Function (#kitgui.com) then you may use this modified code:
var async = Function.prototype.call.bind(function () {
setTimeout.bind(null, this, 0).apply(null, arguments);
});
async(alert, 5);
alert(6);
Since I am not referencing window in the above code it will also work in non-browser environments.
Then we may rewrite the forEach function as follows:
function forEach(array, funct, callback) {
if (array.length)
async(funct, array[0], forEach.bind(null, array.slice(1), funct, callback));
else async(callback);
}
There we have it. No need to modify the prototype of Function. The function body of async is pretty much the same. We simple created an unbound wrapper for it using call.bind. You may see the live demo for yourself.
Bonus:
You can create errbacks using the above pattern as follows (see the live demo):
function forEach(array, funct, callback, error) {
if (array.length && !error)
async(funct, array[0], forEach.bind(null, array.slice(1), funct, callback));
else async(callback, error || null);
}
This is equivalent to the forEachSeries function in caolan's async library in under 10 lines of code.

Assuming you want to do raw computation, and you want it asynchronous so it does not block the browser.
I have been using the "setTimeout(Func,0);" trick for about year. Here is some recent research i wrote up to explain how to speed it up a bit. If you just want the answer, skip to Step 4. Step 1 2 and 3 explain the reasoning and mechanics;
// In Depth Analysis of the setTimeout(Func,0) trick.
//////// setTimeout(Func,0) Step 1 ////////////
// setTimeout and setInterval impose a minimum
// time limit of about 2 to 10 milliseconds.
console.log("start");
var workCounter=0;
var WorkHard = function()
{
if(workCounter>=2000) {console.log("done"); return;}
workCounter++;
setTimeout(WorkHard,0);
};
// this take about 9 seconds
// that works out to be about 4.5ms per iteration
// Now there is a subtle rule here that you can tweak
// This minimum is counted from the time the setTimeout was executed.
// THEREFORE:
console.log("start");
var workCounter=0;
var WorkHard = function()
{
if(workCounter>=2000) {console.log("done"); return;}
setTimeout(WorkHard,0);
workCounter++;
};
// This code is slightly faster because we register the setTimeout
// a line of code earlier. Actually, the speed difference is immesurable
// in this case, but the concept is true. Step 2 shows a measurable example.
///////////////////////////////////////////////
//////// setTimeout(Func,0) Step 2 ////////////
// Here is a measurable example of the concept covered in Step 1.
var StartWork = function()
{
console.log("start");
var startTime = new Date();
var workCounter=0;
var sum=0;
var WorkHard = function()
{
if(workCounter>=2000)
{
var ms = (new Date()).getTime() - startTime.getTime();
console.log("done: sum=" + sum + " time=" + ms + "ms");
return;
}
for(var i=0; i<1500000; i++) {sum++;}
workCounter++;
setTimeout(WorkHard,0);
};
WorkHard();
};
// This adds some difficulty to the work instead of just incrementing a number
// This prints "done: sum=3000000000 time=18809ms".
// So it took 18.8 seconds.
var StartWork = function()
{
console.log("start");
var startTime = new Date();
var workCounter=0;
var sum=0;
var WorkHard = function()
{
if(workCounter>=2000)
{
var ms = (new Date()).getTime() - startTime.getTime();
console.log("done: sum=" + sum + " time=" + ms + "ms");
return;
}
setTimeout(WorkHard,0);
for(var i=0; i<1500000; i++) {sum++;}
workCounter++;
};
WorkHard();
};
// Now, as we planned, we move the setTimeout to before the difficult part
// This prints: "done: sum=3000000000 time=12680ms"
// So it took 12.6 seconds. With a little math, (18.8-12.6)/2000 = 3.1ms
// We have effectively shaved off 3.1ms of the original 4.5ms of dead time.
// Assuming some of that time may be attributed to function calls and variable
// instantiations, we have eliminated the wait time imposed by setTimeout.
// LESSON LEARNED: If you want to use the setTimeout(Func,0) trick with high
// performance in mind, make sure your function takes more than 4.5ms, and set
// the next timeout at the start of your function, instead of the end.
///////////////////////////////////////////////
//////// setTimeout(Func,0) Step 3 ////////////
// The results of Step 2 are very educational, but it doesn't really tell us how to apply the
// concept to the real world. Step 2 says "make sure your function takes more than 4.5ms".
// No one makes functions that take 4.5ms. Functions either take a few microseconds,
// or several seconds, or several minutes. This magic 4.5ms is unattainable.
// To solve the problem, we introduce the concept of "Burn Time".
// Lets assume that you can break up your difficult function into pieces that take
// a few milliseconds or less to complete. Then the concept of Burn Time says,
// "crunch several of the individual pieces until we reach 4.5ms, then exit"
// Step 1 shows a function that is asyncronous, but takes 9 seconds to run. In reality
// we could have easilly incremented workCounter 2000 times in under a millisecond.
// So, duh, that should not be made asyncronous, its horrible. But what if you don't know
// how many times you need to increment the number, maybe you need to run the loop 20 times,
// maybe you need to run the loop 2 billion times.
console.log("start");
var startTime = new Date();
var workCounter=0;
for(var i=0; i<2000000000; i++) // 2 billion
{
workCounter++;
}
var ms = (new Date()).getTime() - startTime.getTime();
console.log("done: workCounter=" + workCounter + " time=" + ms + "ms");
// prints: "done: workCounter=2000000000 time=7214ms"
// So it took 7.2 seconds. Can we break this up into smaller pieces? Yes.
// I know, this is a retarded example, bear with me.
console.log("start");
var startTime = new Date();
var workCounter=0;
var each = function()
{
workCounter++;
};
for(var i=0; i<20000000; i++) // 20 million
{
each();
}
var ms = (new Date()).getTime() - startTime.getTime();
console.log("done: workCounter=" + workCounter + " time=" + ms + "ms");
// The easiest way is to break it up into 2 billion smaller pieces, each of which take
// only several picoseconds to run. Ok, actually, I am reducing the number from 2 billion
// to 20 million (100x less). Just adding a function call increases the complexity of the loop
// 100 fold. Good lesson for some other topic.
// prints: "done: workCounter=20000000 time=7648ms"
// So it took 7.6 seconds, thats a good starting point.
// Now, lets sprinkle in the async part with the burn concept
console.log("start");
var startTime = new Date();
var workCounter=0;
var index=0;
var end = 20000000;
var each = function()
{
workCounter++;
};
var Work = function()
{
var burnTimeout = new Date();
burnTimeout.setTime(burnTimeout.getTime() + 4.5); // burnTimeout set to 4.5ms in the future
while((new Date()) < burnTimeout)
{
if(index>=end)
{
var ms = (new Date()).getTime() - startTime.getTime();
console.log("done: workCounter=" + workCounter + " time=" + ms + "ms");
return;
}
each();
index++;
}
setTimeout(Work,0);
};
// prints "done: workCounter=20000000 time=107119ms"
// Sweet Jesus, I increased my 7.6 second function to 107.1 seconds.
// But it does prevent the browser from locking up, So i guess thats a plus.
// Again, the actual objective here is just to increment workCounter, so the overhead of all
// the async garbage is huge in comparison.
// Anyway, Lets start by taking advice from Step 2 and move the setTimeout above the hard part.
console.log("start");
var startTime = new Date();
var workCounter=0;
var index=0;
var end = 20000000;
var each = function()
{
workCounter++;
};
var Work = function()
{
if(index>=end) {return;}
setTimeout(Work,0);
var burnTimeout = new Date();
burnTimeout.setTime(burnTimeout.getTime() + 4.5); // burnTimeout set to 4.5ms in the future
while((new Date()) < burnTimeout)
{
if(index>=end)
{
var ms = (new Date()).getTime() - startTime.getTime();
console.log("done: workCounter=" + workCounter + " time=" + ms + "ms");
return;
}
each();
index++;
}
};
// This means we also have to check index right away because the last iteration will have nothing to do
// prints "done: workCounter=20000000 time=52892ms"
// So, it took 52.8 seconds. Improvement, but way slower than the native 7.6 seconds.
// The Burn Time is the number you tweak to get a nice balance between native loop speed
// and browser responsiveness. Lets change it from 4.5ms to 50ms, because we don't really need faster
// than 50ms gui response.
console.log("start");
var startTime = new Date();
var workCounter=0;
var index=0;
var end = 20000000;
var each = function()
{
workCounter++;
};
var Work = function()
{
if(index>=end) {return;}
setTimeout(Work,0);
var burnTimeout = new Date();
burnTimeout.setTime(burnTimeout.getTime() + 50); // burnTimeout set to 50ms in the future
while((new Date()) < burnTimeout)
{
if(index>=end)
{
var ms = (new Date()).getTime() - startTime.getTime();
console.log("done: workCounter=" + workCounter + " time=" + ms + "ms");
return;
}
each();
index++;
}
};
// prints "done: workCounter=20000000 time=52272ms"
// So it took 52.2 seconds. No real improvement here which proves that the imposed limits of setTimeout
// have been eliminated as long as the burn time is anything over 4.5ms
///////////////////////////////////////////////
//////// setTimeout(Func,0) Step 4 ////////////
// The performance numbers from Step 3 seem pretty grim, but GUI responsiveness is often worth it.
// Here is a short library that embodies these concepts and gives a descent interface.
var WilkesAsyncBurn = function()
{
var Now = function() {return (new Date());};
var CreateFutureDate = function(milliseconds)
{
var t = Now();
t.setTime(t.getTime() + milliseconds);
return t;
};
var For = function(start, end, eachCallback, finalCallback, msBurnTime)
{
var i = start;
var Each = function()
{
if(i==-1) {return;} //always does one last each with nothing to do
setTimeout(Each,0);
var burnTimeout = CreateFutureDate(msBurnTime);
while(Now() < burnTimeout)
{
if(i>=end) {i=-1; finalCallback(); return;}
eachCallback(i);
i++;
}
};
Each();
};
var ForEach = function(array, eachCallback, finalCallback, msBurnTime)
{
var i = 0;
var len = array.length;
var Each = function()
{
if(i==-1) {return;}
setTimeout(Each,0);
var burnTimeout = CreateFutureDate(msBurnTime);
while(Now() < burnTimeout)
{
if(i>=len) {i=-1; finalCallback(array); return;}
eachCallback(i, array[i]);
i++;
}
};
Each();
};
var pub = {};
pub.For = For; //eachCallback(index); finalCallback();
pub.ForEach = ForEach; //eachCallback(index,value); finalCallback(array);
WilkesAsyncBurn = pub;
};
///////////////////////////////////////////////
//////// setTimeout(Func,0) Step 5 ////////////
// Here is an examples of how to use the library from Step 4.
WilkesAsyncBurn(); // Init the library
console.log("start");
var startTime = new Date();
var workCounter=0;
var FuncEach = function()
{
if(workCounter%1000==0)
{
var s = "<div></div>";
var div = jQuery("*[class~=r1]");
div.append(s);
}
workCounter++;
};
var FuncFinal = function()
{
var ms = (new Date()).getTime() - startTime.getTime();
console.log("done: workCounter=" + workCounter + " time=" + ms + "ms");
};
WilkesAsyncBurn.For(0,2000000,FuncEach,FuncFinal,50);
// prints: "done: workCounter=20000000 time=149303ms"
// Also appends a few thousand divs to the html page, about 20 at a time.
// The browser is responsive the entire time, mission accomplished
// LESSON LEARNED: If your code pieces are super tiny, like incrementing a number, or walking through
// an array summing the numbers, then just putting it in an "each" function is going to kill you.
// You can still use the concept here, but your "each" function should also have a for loop in it
// where you burn a few hundred items manually.
///////////////////////////////////////////////

I have found this tutorial that explain exactly what I needed.
I hope this will also help somebody else.
http://nodetuts.com/tutorials/19-asynchronous-iteration-patterns.html#video

Asynchronous for cycle in JavaScript

I need a loop that waits for an async call before continuing. Something like:
for ( /* ... */ ) {
someFunction(param1, praram2, function(result) {
// Okay, for cycle could continue
})
}
alert("For cycle ended");
How could I do this? Do you have any ideas?

You can't mix synchronous and asynchronous in JavaScript if you block the script, you block the Browser.
You need to go the full event driven way here, luckily we can hide the ugly stuff away.
EDIT: Updated the code.
function asyncLoop(iterations, func, callback) {
var index = 0;
var done = false;
var loop = {
next: function() {
if (done) {
return;
}
if (index < iterations) {
index++;
func(loop);
} else {
done = true;
callback();
}
},
iteration: function() {
return index - 1;
},
break: function() {
done = true;
callback();
}
};
loop.next();
return loop;
}
This will provide us an asynchronous loop, you can of course modify it even further to take for example a function to check the loop condition etc.
Now on to the test:
function someFunction(a, b, callback) {
console.log('Hey doing some stuff!');
callback();
}
asyncLoop(10, function(loop) {
someFunction(1, 2, function(result) {
// log the iteration
console.log(loop.iteration());
// Okay, for cycle could continue
loop.next();
})},
function(){console.log('cycle ended')}
);
And the output:
Hey doing some stuff!
0
Hey doing some stuff!
1
Hey doing some stuff!
2
Hey doing some stuff!
3
Hey doing some stuff!
4
Hey doing some stuff!
5
Hey doing some stuff!
6
Hey doing some stuff!
7
Hey doing some stuff!
8
Hey doing some stuff!
9
cycle ended

I simplified this:
FUNCTION:
var asyncLoop = function(o){
var i=-1;
var loop = function(){
i++;
if(i==o.length){o.callback(); return;}
o.functionToLoop(loop, i);
}
loop();//init
}
USAGE:
asyncLoop({
length : 5,
functionToLoop : function(loop, i){
setTimeout(function(){
document.write('Iteration ' + i + ' <br>');
loop();
},1000);
},
callback : function(){
document.write('All done!');
}
});
EXAMPLE: http://jsfiddle.net/NXTv7/8/

A cleaner alternative to what #Ivo has suggested would be an Asynchronous Method Queue, assuming that you only need to make one async call for the collection.
(See this post by Dustin Diaz for a more detailed explanation)
function Queue() {
this._methods = [];
this._response = null;
this._flushed = false;
}
(function(Q){
Q.add = function (fn) {
if (this._flushed) fn(this._response);
else this._methods.push(fn);
}
Q.flush = function (response) {
if (this._flushed) return;
this._response = response;
while (this._methods[0]) {
this._methods.shift()(response);
}
this._flushed = true;
}
})(Queue.prototype);
You simply create a new instance of Queue, add the callbacks you need, and then flush the queue with the async response.
var queue = new Queue();
queue.add(function(results){
for (var result in results) {
// normal loop operation here
}
});
someFunction(param1, param2, function(results) {
queue.flush(results);
}
An added benefit of this pattern is that you can add multiple functions to the queue instead of just one.
If you have an object which contains iterator functions, you can add support for this queue behind the scenes and write code which looks synchronous, but isn't:
MyClass.each(function(result){ ... })
simply write each to put the anonymous function into the queue instead of executing it immediately, and then flush the queue when your async call is complete. This is a very simple and powerful design pattern.
P.S. If you're using jQuery, you already have an async method queue at your disposal called jQuery.Deferred.

Also look at this splendid library caolan / async. Your for loop can easily be accomplished using mapSeries or series.
I could post some sample code if your example had more details in it.

We can also use help of jquery.Deferred. in this case asyncLoop function would look like this:
asyncLoop = function(array, callback) {
var nextElement, thisIteration;
if (array.length > 0) nextElement = array.pop();
thisIteration = callback(nextElement);
$.when(thisIteration).done(function(response) {
// here we can check value of response in order to break or whatever
if (array.length > 0) asyncLoop(array, collection, callback);
});
};
the callback function will look like this:
addEntry = function(newEntry) {
var deferred, duplicateEntry;
// on the next line we can perform some check, which may cause async response.
duplicateEntry = someCheckHere();
if (duplicateEntry === true) {
deferred = $.Deferred();
// here we launch some other function (e.g. $.ajax or popup window)
// which based on result must call deferred.resolve([opt args - response])
// when deferred.resolve is called "asyncLoop" will start new iteration
// example function:
exampleFunction(duplicateEntry, deferred);
return deferred;
} else {
return someActionIfNotDuplicate();
}
};
example function that resolves deferred:
function exampleFunction(entry, deffered){
openModal({
title: "what should we do with duplicate"
options: [
{name:"Replace", action: function(){replace(entry);deffered.resolve(replace:true)}},
{name: "Keep Existing", action: function(){deffered.resolve(replace:false)}}
]
})
}

I have been using the "setTimeout(Func,0);" trick for about year. Here is some recent research i wrote up to explain how to speed it up a bit. If you just want the answer, skip to Step 4. Step 1 2 and 3 explain the reasoning and mechanics;
// In Depth Analysis of the setTimeout(Func,0) trick.
//////// setTimeout(Func,0) Step 1 ////////////
// setTimeout and setInterval impose a minimum
// time limit of about 2 to 10 milliseconds.
console.log("start");
var workCounter=0;
var WorkHard = function()
{
if(workCounter>=2000) {console.log("done"); return;}
workCounter++;
setTimeout(WorkHard,0);
};
// this take about 9 seconds
// that works out to be about 4.5ms per iteration
// Now there is a subtle rule here that you can tweak
// This minimum is counted from the time the setTimeout was executed.
// THEREFORE:
console.log("start");
var workCounter=0;
var WorkHard = function()
{
if(workCounter>=2000) {console.log("done"); return;}
setTimeout(WorkHard,0);
workCounter++;
};
// This code is slightly faster because we register the setTimeout
// a line of code earlier. Actually, the speed difference is immesurable
// in this case, but the concept is true. Step 2 shows a measurable example.
///////////////////////////////////////////////
//////// setTimeout(Func,0) Step 2 ////////////
// Here is a measurable example of the concept covered in Step 1.
var StartWork = function()
{
console.log("start");
var startTime = new Date();
var workCounter=0;
var sum=0;
var WorkHard = function()
{
if(workCounter>=2000)
{
var ms = (new Date()).getTime() - startTime.getTime();
console.log("done: sum=" + sum + " time=" + ms + "ms");
return;
}
for(var i=0; i<1500000; i++) {sum++;}
workCounter++;
setTimeout(WorkHard,0);
};
WorkHard();
};
// This adds some difficulty to the work instead of just incrementing a number
// This prints "done: sum=3000000000 time=18809ms".
// So it took 18.8 seconds.
var StartWork = function()
{
console.log("start");
var startTime = new Date();
var workCounter=0;
var sum=0;
var WorkHard = function()
{
if(workCounter>=2000)
{
var ms = (new Date()).getTime() - startTime.getTime();
console.log("done: sum=" + sum + " time=" + ms + "ms");
return;
}
setTimeout(WorkHard,0);
for(var i=0; i<1500000; i++) {sum++;}
workCounter++;
};
WorkHard();
};
// Now, as we planned, we move the setTimeout to before the difficult part
// This prints: "done: sum=3000000000 time=12680ms"
// So it took 12.6 seconds. With a little math, (18.8-12.6)/2000 = 3.1ms
// We have effectively shaved off 3.1ms of the original 4.5ms of dead time.
// Assuming some of that time may be attributed to function calls and variable
// instantiations, we have eliminated the wait time imposed by setTimeout.
// LESSON LEARNED: If you want to use the setTimeout(Func,0) trick with high
// performance in mind, make sure your function takes more than 4.5ms, and set
// the next timeout at the start of your function, instead of the end.
///////////////////////////////////////////////
//////// setTimeout(Func,0) Step 3 ////////////
// The results of Step 2 are very educational, but it doesn't really tell us how to apply the
// concept to the real world. Step 2 says "make sure your function takes more than 4.5ms".
// No one makes functions that take 4.5ms. Functions either take a few microseconds,
// or several seconds, or several minutes. This magic 4.5ms is unattainable.
// To solve the problem, we introduce the concept of "Burn Time".
// Lets assume that you can break up your difficult function into pieces that take
// a few milliseconds or less to complete. Then the concept of Burn Time says,
// "crunch several of the individual pieces until we reach 4.5ms, then exit"
// Step 1 shows a function that is asyncronous, but takes 9 seconds to run. In reality
// we could have easilly incremented workCounter 2000 times in under a millisecond.
// So, duh, that should not be made asyncronous, its horrible. But what if you don't know
// how many times you need to increment the number, maybe you need to run the loop 20 times,
// maybe you need to run the loop 2 billion times.
console.log("start");
var startTime = new Date();
var workCounter=0;
for(var i=0; i<2000000000; i++) // 2 billion
{
workCounter++;
}
var ms = (new Date()).getTime() - startTime.getTime();
console.log("done: workCounter=" + workCounter + " time=" + ms + "ms");
// prints: "done: workCounter=2000000000 time=7214ms"
// So it took 7.2 seconds. Can we break this up into smaller pieces? Yes.
// I know, this is a retarded example, bear with me.
console.log("start");
var startTime = new Date();
var workCounter=0;
var each = function()
{
workCounter++;
};
for(var i=0; i<20000000; i++) // 20 million
{
each();
}
var ms = (new Date()).getTime() - startTime.getTime();
console.log("done: workCounter=" + workCounter + " time=" + ms + "ms");
// The easiest way is to break it up into 2 billion smaller pieces, each of which take
// only several picoseconds to run. Ok, actually, I am reducing the number from 2 billion
// to 20 million (100x less). Just adding a function call increases the complexity of the loop
// 100 fold. Good lesson for some other topic.
// prints: "done: workCounter=20000000 time=7648ms"
// So it took 7.6 seconds, thats a good starting point.
// Now, lets sprinkle in the async part with the burn concept
console.log("start");
var startTime = new Date();
var workCounter=0;
var index=0;
var end = 20000000;
var each = function()
{
workCounter++;
};
var Work = function()
{
var burnTimeout = new Date();
burnTimeout.setTime(burnTimeout.getTime() + 4.5); // burnTimeout set to 4.5ms in the future
while((new Date()) < burnTimeout)
{
if(index>=end)
{
var ms = (new Date()).getTime() - startTime.getTime();
console.log("done: workCounter=" + workCounter + " time=" + ms + "ms");
return;
}
each();
index++;
}
setTimeout(Work,0);
};
// prints "done: workCounter=20000000 time=107119ms"
// Sweet Jesus, I increased my 7.6 second function to 107.1 seconds.
// But it does prevent the browser from locking up, So i guess thats a plus.
// Again, the actual objective here is just to increment workCounter, so the overhead of all
// the async garbage is huge in comparison.
// Anyway, Lets start by taking advice from Step 2 and move the setTimeout above the hard part.
console.log("start");
var startTime = new Date();
var workCounter=0;
var index=0;
var end = 20000000;
var each = function()
{
workCounter++;
};
var Work = function()
{
if(index>=end) {return;}
setTimeout(Work,0);
var burnTimeout = new Date();
burnTimeout.setTime(burnTimeout.getTime() + 4.5); // burnTimeout set to 4.5ms in the future
while((new Date()) < burnTimeout)
{
if(index>=end)
{
var ms = (new Date()).getTime() - startTime.getTime();
console.log("done: workCounter=" + workCounter + " time=" + ms + "ms");
return;
}
each();
index++;
}
};
// This means we also have to check index right away because the last iteration will have nothing to do
// prints "done: workCounter=20000000 time=52892ms"
// So, it took 52.8 seconds. Improvement, but way slower than the native 7.6 seconds.
// The Burn Time is the number you tweak to get a nice balance between native loop speed
// and browser responsiveness. Lets change it from 4.5ms to 50ms, because we don't really need faster
// than 50ms gui response.
console.log("start");
var startTime = new Date();
var workCounter=0;
var index=0;
var end = 20000000;
var each = function()
{
workCounter++;
};
var Work = function()
{
if(index>=end) {return;}
setTimeout(Work,0);
var burnTimeout = new Date();
burnTimeout.setTime(burnTimeout.getTime() + 50); // burnTimeout set to 50ms in the future
while((new Date()) < burnTimeout)
{
if(index>=end)
{
var ms = (new Date()).getTime() - startTime.getTime();
console.log("done: workCounter=" + workCounter + " time=" + ms + "ms");
return;
}
each();
index++;
}
};
// prints "done: workCounter=20000000 time=52272ms"
// So it took 52.2 seconds. No real improvement here which proves that the imposed limits of setTimeout
// have been eliminated as long as the burn time is anything over 4.5ms
///////////////////////////////////////////////
//////// setTimeout(Func,0) Step 4 ////////////
// The performance numbers from Step 3 seem pretty grim, but GUI responsiveness is often worth it.
// Here is a short library that embodies these concepts and gives a descent interface.
var WilkesAsyncBurn = function()
{
var Now = function() {return (new Date());};
var CreateFutureDate = function(milliseconds)
{
var t = Now();
t.setTime(t.getTime() + milliseconds);
return t;
};
var For = function(start, end, eachCallback, finalCallback, msBurnTime)
{
var i = start;
var Each = function()
{
if(i==-1) {return;} //always does one last each with nothing to do
setTimeout(Each,0);
var burnTimeout = CreateFutureDate(msBurnTime);
while(Now() < burnTimeout)
{
if(i>=end) {i=-1; finalCallback(); return;}
eachCallback(i);
i++;
}
};
Each();
};
var ForEach = function(array, eachCallback, finalCallback, msBurnTime)
{
var i = 0;
var len = array.length;
var Each = function()
{
if(i==-1) {return;}
setTimeout(Each,0);
var burnTimeout = CreateFutureDate(msBurnTime);
while(Now() < burnTimeout)
{
if(i>=len) {i=-1; finalCallback(array); return;}
eachCallback(i, array[i]);
i++;
}
};
Each();
};
var pub = {};
pub.For = For; //eachCallback(index); finalCallback();
pub.ForEach = ForEach; //eachCallback(index,value); finalCallback(array);
WilkesAsyncBurn = pub;
};
///////////////////////////////////////////////
//////// setTimeout(Func,0) Step 5 ////////////
// Here is an examples of how to use the library from Step 4.
WilkesAsyncBurn(); // Init the library
console.log("start");
var startTime = new Date();
var workCounter=0;
var FuncEach = function()
{
if(workCounter%1000==0)
{
var s = "<div></div>";
var div = jQuery("*[class~=r1]");
div.append(s);
}
workCounter++;
};
var FuncFinal = function()
{
var ms = (new Date()).getTime() - startTime.getTime();
console.log("done: workCounter=" + workCounter + " time=" + ms + "ms");
};
WilkesAsyncBurn.For(0,2000000,FuncEach,FuncFinal,50);
// prints: "done: workCounter=20000000 time=149303ms"
// Also appends a few thousand divs to the html page, about 20 at a time.
// The browser is responsive the entire time, mission accomplished
// LESSON LEARNED: If your code pieces are super tiny, like incrementing a number, or walking through
// an array summing the numbers, then just putting it in an "each" function is going to kill you.
// You can still use the concept here, but your "each" function should also have a for loop in it
// where you burn a few hundred items manually.
///////////////////////////////////////////////

Given an asynchronous worker function someFunction that will call back a result function with a result argument saying whether or not the loop should continue:
// having:
// function someFunction(param1, praram2, resultfunc))
// function done() { alert("For cycle ended"); }
(function(f){ f(f) })(function(f){
someFunction("param1", "praram2", function(result){
if (result)
f(f); // loop continues
else
done(); // loop ends
});
})
In order to check whether or not to end the loop, the worker function someFunction can forward the result function to other asynchronous operations. Also, the whole expression can be encapsulated into an asynchronous function by taking a function done as callback.

If you like wilsonpage's answer but are more accustomed to using async.js's syntax, here is a variation:
function asyncEach(iterableList, callback, done) {
var i = -1,
length = iterableList.length;
function loop() {
i++;
if (i === length) {
done();
return;
}
callback(iterableList[i], loop);
}
loop();
}
asyncEach(['A', 'B', 'C'], function(item, callback) {
setTimeout(function(){
document.write('Iteration ' + item + ' <br>');
callback();
}, 1000);
}, function() {
document.write('All done!');
});
Demo can be found here - http://jsfiddle.net/NXTv7/8/

Here's another example which I think is more readable than others, where you wrap your async function inside a function that takes in a done function, the current loop index, and the result (if any) of the previous async call:
function (done, i, prevResult) {
// perform async stuff
// call "done(result)" in async callback
// or after promise resolves
}
Once done() is invoked, it triggers the next async call, again passing in the done function, current index and previous result. Once the entire loop is completed, the provided loop callback will be invoked.
Here's a snippet you can run:
asyncLoop({
limit: 25,
asyncLoopFunction: function(done, i, prevResult) {
setTimeout(function() {
console.log("Starting Iteration: ", i);
console.log("Previous Result: ", prevResult);
var result = i * 100;
done(result);
}, 1000);
},
initialArgs: 'Hello',
callback: function(result) {
console.log('All Done. Final result: ', result);
}
});
function asyncLoop(obj) {
var limit = obj.limit,
asyncLoopFunction = obj.asyncLoopFunction,
initialArgs = obj.initialArgs || {},
callback = obj.callback,
i = 0;
function done(result) {
i++;
if (i < limit) {
triggerAsync(result);
} else {
callback(result);
}
}
function triggerAsync(prevResult) {
asyncLoopFunction(done, i, prevResult);
}
triggerAsync(initialArgs); // init
}

You can use async await introduced in ES7:
for ( /* ... */ ) {
let result = await someFunction(param1, param2);
}
alert("For cycle ended");
This works only if someFunction is returning a Promise!
If someFunction is not returning a Promise, then you can make it return a Promise by yourself like this:
function asyncSomeFunction(param1,praram2) {
return new Promise((resolve, reject) => {
someFunction(praram1,praram2,(result)=>{
resolve(result);
})
})
}
Then replace this line await someFunction(param1, param2); by await asynSomeFunction(param1, param2);
Please understand Promises before writing async await code!

http://cuzztuts.blogspot.ro/2011/12/js-async-for-very-cool.html
EDIT:
link from github: https://github.com/cuzzea/lib_repo/blob/master/cuzzea/js/functions/core/async_for.js
function async_for_each(object,settings){
var l=object.length;
settings.limit = settings.limit || Math.round(l/100);
settings.start = settings.start || 0;
settings.timeout = settings.timeout || 1;
for(var i=settings.start;i<l;i++){
if(i-settings.start>=settings.limit){
setTimeout(function(){
settings.start = i;
async_for_each(object,settings)
},settings.timeout);
settings.limit_callback ? settings.limit_callback(i,l) : null;
return false;
}else{
settings.cbk ? settings.cbk(i,object[i]) : null;
}
}
settings.end_cbk?settings.end_cbk():null;
return true;
}
This function allows you to to create a percent break in the for loop using settings.limit. The limit property is just a integer, but when set as array.length * 0.1, this will make the settings.limit_callback to be called every 10%.
/*
* params:
* object: the array to parse
* settings_object:
* cbk: function to call whenwhen object is found in array
* params: i,object[i]
* limit_calback: function to call when limit is reached
* params: i, object_length
* end_cbk: function to call when loop is finished
* params: none
* limit: number of iteration before breacking the for loop
* default: object.length/100
* timeout: time until start of the for loop(ms)
* default: 1
* start: the index from where to start the for loop
* default: 0
*/
exemple:
var a = [];
a.length = 1000;
async_for_each(a,{
limit_callback:function(i,l){console.log("loading %s/%s - %s%",i,l,Math.round(i*100/l))}
});

A promise library based solution:
/*
Since this is an open question for JS I have used Kris Kowal's Q promises for the same
*/
var Q = require('q');
/*
Your LOOP body
#success is a parameter(s) you might pass
*/
var loopBody = function(success) {
var d = Q.defer(); /* OR use your favorite promise library like $q in angular */
/*
'setTimeout' will ideally be your node-like callback with this signature ... (err, data) {}
as shown, on success you should resolve
on failure you should reject (as always ...)
*/
setTimeout(function(err, data) {
if (!err) {
d.resolve('success');
} else {
d.reject('failure');
}
}, 100); //100 ms used for illustration only
return d.promise;
};
/*
function to call your loop body
*/
function loop(itr, fn) {
var def = Q.defer();
if (itr <= 0) {
def.reject({ status: "un-successful " });
} else {
var next = loop.bind(undefined, itr - 1, fn); // 'next' is all there is to this
var callback = fn.bind(undefined /*, a, b, c.... */ ); // in case you want to pass some parameters into your loop body
def.promise = callback().then(def.resolve, next);
}
return def.promise;
}
/*
USAGE: loop(iterations, function(){})
the second argument has to be thenable (in other words return a promise)
NOTE: this loop will stop when loop body resolves to a success
Example: Try to upload file 3 times. HURRAY (if successful) or log failed
*/
loop(4, loopBody).then(function() {
//success handler
console.log('HURRAY')
}, function() {
//failed
console.log('failed');
});

I needed to call some asynchronous function X times, each iteration must have happened after the previous one was done, so I wrote a litte library that can be used like this:
// https://codepen.io/anon/pen/MOvxaX?editors=0012
var loop = AsyncLoop(function(iteration, value){
console.log("Loop called with iteration and value set to: ", iteration, value);
var random = Math.random()*500;
if(random < 200)
return false;
return new Promise(function(resolve){
setTimeout(resolve.bind(null, random), random);
});
})
.finished(function(){
console.log("Loop has ended");
});
Each time user defined loop function is called, it has two arguments, iteration index and previous call return value.
This is an example of output:
"Loop called with iteration and value set to: " 0 null
"Loop called with iteration and value set to: " 1 496.4137048207333
"Loop called with iteration and value set to: " 2 259.6020382449663
"Loop called with iteration and value set to: " 3 485.5400568702862
"Loop has ended"