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I need to create a simple but accurate timer.
This is my code:
var seconds = 0;
setInterval(function() {
timer.innerHTML = seconds++;
}, 1000);
After exactly 3600 seconds, it prints about 3500 seconds.
Why is it not accurate?
How can I create an accurate timer?
Why is it not accurate?
Because you are using setTimeout() or setInterval(). They cannot be trusted, there are no accuracy guarantees for them. They are allowed to lag arbitrarily, and they do not keep a constant pace but tend to drift (as you have observed).
How can I create an accurate timer?
Use the Date object instead to get the (millisecond-)accurate, current time. Then base your logic on the current time value, instead of counting how often your callback has been executed.
For a simple timer or clock, keep track of the time difference explicitly:
var start = Date.now();
setInterval(function() {
var delta = Date.now() - start; // milliseconds elapsed since start
…
output(Math.floor(delta / 1000)); // in seconds
// alternatively just show wall clock time:
output(new Date().toUTCString());
}, 1000); // update about every second
Now, that has the problem of possibly jumping values. When the interval lags a bit and executes your callback after 990, 1993, 2996, 3999, 5002 milliseconds, you will see the second count 0, 1, 2, 3, 5 (!). So it would be advisable to update more often, like about every 100ms, to avoid such jumps.
However, sometimes you really need a steady interval executing your callbacks without drifting. This requires a bit more advanced strategy (and code), though it pays out well (and registers less timeouts). Those are known as self-adjusting timers. Here the exact delay for each of the repeated timeouts is adapted to the actually elapsed time, compared to the expected intervals:
var interval = 1000; // ms
var expected = Date.now() + interval;
setTimeout(step, interval);
function step() {
var dt = Date.now() - expected; // the drift (positive for overshooting)
if (dt > interval) {
// something really bad happened. Maybe the browser (tab) was inactive?
// possibly special handling to avoid futile "catch up" run
}
… // do what is to be done
expected += interval;
setTimeout(step, Math.max(0, interval - dt)); // take into account drift
}
I'ma just build on Bergi's answer (specifically the second part) a little bit because I really liked the way it was done, but I want the option to stop the timer once it starts (like clearInterval() almost). Sooo... I've wrapped it up into a constructor function so we can do 'objecty' things with it.
1. Constructor
Alright, so you copy/paste that...
/**
* Self-adjusting interval to account for drifting
*
* #param {function} workFunc Callback containing the work to be done
* for each interval
* #param {int} interval Interval speed (in milliseconds)
* #param {function} errorFunc (Optional) Callback to run if the drift
* exceeds interval
*/
function AdjustingInterval(workFunc, interval, errorFunc) {
var that = this;
var expected, timeout;
this.interval = interval;
this.start = function() {
expected = Date.now() + this.interval;
timeout = setTimeout(step, this.interval);
}
this.stop = function() {
clearTimeout(timeout);
}
function step() {
var drift = Date.now() - expected;
if (drift > that.interval) {
// You could have some default stuff here too...
if (errorFunc) errorFunc();
}
workFunc();
expected += that.interval;
timeout = setTimeout(step, Math.max(0, that.interval-drift));
}
}
2. Instantiate
Tell it what to do and all that...
// For testing purposes, we'll just increment
// this and send it out to the console.
var justSomeNumber = 0;
// Define the work to be done
var doWork = function() {
console.log(++justSomeNumber);
};
// Define what to do if something goes wrong
var doError = function() {
console.warn('The drift exceeded the interval.');
};
// (The third argument is optional)
var ticker = new AdjustingInterval(doWork, 1000, doError);
3. Then do... stuff
// You can start or stop your timer at will
ticker.start();
ticker.stop();
// You can also change the interval while it's in progress
ticker.interval = 99;
I mean, it works for me anyway. If there's a better way, lemme know.
Bergi's answer pinpoints exactly why the timer from the question is not accurate. Here's my take on a simple JS timer with start, stop, reset and getTime methods:
class Timer {
constructor () {
this.isRunning = false;
this.startTime = 0;
this.overallTime = 0;
}
_getTimeElapsedSinceLastStart () {
if (!this.startTime) {
return 0;
}
return Date.now() - this.startTime;
}
start () {
if (this.isRunning) {
return console.error('Timer is already running');
}
this.isRunning = true;
this.startTime = Date.now();
}
stop () {
if (!this.isRunning) {
return console.error('Timer is already stopped');
}
this.isRunning = false;
this.overallTime = this.overallTime + this._getTimeElapsedSinceLastStart();
}
reset () {
this.overallTime = 0;
if (this.isRunning) {
this.startTime = Date.now();
return;
}
this.startTime = 0;
}
getTime () {
if (!this.startTime) {
return 0;
}
if (this.isRunning) {
return this.overallTime + this._getTimeElapsedSinceLastStart();
}
return this.overallTime;
}
}
const timer = new Timer();
timer.start();
setInterval(() => {
const timeInSeconds = Math.round(timer.getTime() / 1000);
document.getElementById('time').innerText = timeInSeconds;
}, 100)
<p>Elapsed time: <span id="time">0</span>s</p>
The snippet also includes a solution for your problem. So instead of incrementing seconds variable every 1000ms interval, we just start the timer and then every 100ms* we just read elapsed time from the timer and update the view accordingly.
* - makes it more accurate than 1000ms
To make your timer more accurate, you would have to round
Most of the timers in the answers here will linger behind the expected time because they set the "expected" value to the ideal and only account for the delay that the browser introduced before that point. This is fine if you just need accurate intervals, but if you are timing relative to other events then you will (nearly) always have this delay.
To correct it, you can keep track of the drift history and use it to predict future drift. By adding a secondary adjustment with this preemptive correction, the variance in the drift centers around the target time. For example, if you're always getting a drift of 20 to 40ms, this adjustment would shift it to -10 to +10ms around the target time.
Building on Bergi's answer, I've used a rolling median for my prediction algorithm. Taking just 10 samples with this method makes a reasonable difference.
var interval = 200; // ms
var expected = Date.now() + interval;
var drift_history = [];
var drift_history_samples = 10;
var drift_correction = 0;
function calc_drift(arr){
// Calculate drift correction.
/*
In this example I've used a simple median.
You can use other methods, but it's important not to use an average.
If the user switches tabs and back, an average would put far too much
weight on the outlier.
*/
var values = arr.concat(); // copy array so it isn't mutated
values.sort(function(a,b){
return a-b;
});
if(values.length ===0) return 0;
var half = Math.floor(values.length / 2);
if (values.length % 2) return values[half];
var median = (values[half - 1] + values[half]) / 2.0;
return median;
}
setTimeout(step, interval);
function step() {
var dt = Date.now() - expected; // the drift (positive for overshooting)
if (dt > interval) {
// something really bad happened. Maybe the browser (tab) was inactive?
// possibly special handling to avoid futile "catch up" run
}
// do what is to be done
// don't update the history for exceptionally large values
if (dt <= interval) {
// sample drift amount to history after removing current correction
// (add to remove because the correction is applied by subtraction)
drift_history.push(dt + drift_correction);
// predict new drift correction
drift_correction = calc_drift(drift_history);
// cap and refresh samples
if (drift_history.length >= drift_history_samples) {
drift_history.shift();
}
}
expected += interval;
// take into account drift with prediction
setTimeout(step, Math.max(0, interval - dt - drift_correction));
}
I agree with Bergi on using Date, but his solution was a bit of overkill for my use. I simply wanted my animated clock (digital and analog SVGs) to update on the second and not overrun or under run creating obvious jumps in the clock updates. Here is the snippet of code I put in my clock update functions:
var milliseconds = now.getMilliseconds();
var newTimeout = 1000 - milliseconds;
this.timeoutVariable = setTimeout((function(thisObj) { return function() { thisObj.update(); } })(this), newTimeout);
It simply calculates the delta time to the next even second, and sets the timeout to that delta. This syncs all of my clock objects to the second. Hope this is helpful.
Here's a solution that pauses when the window is hidden, and can be cancelled with an abort controller.
function animationInterval(ms, signal, callback) {
const start = document.timeline.currentTime;
function frame(time) {
if (signal.aborted) return;
callback(time);
scheduleFrame(time);
}
function scheduleFrame(time) {
const elapsed = time - start;
const roundedElapsed = Math.round(elapsed / ms) * ms;
const targetNext = start + roundedElapsed + ms;
const delay = targetNext - performance.now();
setTimeout(() => requestAnimationFrame(frame), delay);
}
scheduleFrame(start);
}
Usage:
const controller = new AbortController();
// Create an animation callback every second:
animationInterval(1000, controller.signal, time => {
console.log('tick!', time);
});
// And stop it sometime later:
controller.abort();
Modern, Fully Programmable Timer
This timer takes a frequency in Hertz, and a callback that can take up to four arguments, the current frame index, the current time, the time that the current frame would have ideally occurred at, and a reference to the timer instance (so the caller and callback can both access its methods).
Note: All times are based on performance.now, and are relative to the moment that the page loaded.
Timer instances have three API methods:
stop: Takes no args. Kills the timer immediately (and permanently).
Returns the frame index for the next frame (the cancelled frame).
adapt: Takes a frequency in Hertz and adapts the timer to it, beginning
from the next frame. Returns the implied interval in milliseconds.
redefine: Takes a new callback function. Swaps it with the current
callback. Effects the next frame. Returns undefined.
Note: The tick method passes this around explicitly (as self) to work around the problem of this referencing window when the tick method is invoked via setTimeout.
class ProgrammableTimer {
constructor(hertz, callback) {
this.target = performance.now(); // target time for the next frame
this.interval = 1 / hertz * 1000; // the milliseconds between ticks
this.callback = callback;
this.stopped = false;
this.frame = 0;
this.tick(this);
}
tick(self) {
if (self.stopped) return;
const currentTime = performance.now();
const currentTarget = self.target;
const currentInterval = (self.target += self.interval) - currentTime;
setTimeout(self.tick, currentInterval, self);
self.callback(self.frame++, currentTime, currentTarget, self);
}
stop() { this.stopped = true; return this.frame }
adapt(hertz) { return this.interval = 1 / hertz * 1000 }
redefine(replacement) { this.callback = replacement }
}
Doesn't get much more accurate than this.
var seconds = new Date().getTime(), last = seconds,
intrvl = setInterval(function() {
var now = new Date().getTime();
if(now - last > 5){
if(confirm("Delay registered, terminate?")){
clearInterval(intrvl);
return;
}
}
last = now;
timer.innerHTML = now - seconds;
}, 333);
As to why it is not accurate, I would guess that the machine is busy doing other things, slowing down a little on each iteration adds up, as you see.
This is an old question but figured I'd share some code I use sometimes:
function Timer(func, delay, repeat, runAtStart)
{
this.func = func;
this.delay = delay;
this.repeat = repeat || 0;
this.runAtStart = runAtStart;
this.count = 0;
this.startTime = performance.now();
if (this.runAtStart)
this.tick();
else
{
var _this = this;
this.timeout = window.setTimeout( function(){ _this.tick(); }, this.delay);
}
}
Timer.prototype.tick = function()
{
this.func();
this.count++;
if (this.repeat === -1 || (this.repeat > 0 && this.count < this.repeat) )
{
var adjustedDelay = Math.max( 1, this.startTime + ( (this.count+(this.runAtStart ? 2 : 1)) * this.delay ) - performance.now() );
var _this = this;
this.timeout = window.setTimeout( function(){ _this.tick(); }, adjustedDelay);
}
}
Timer.prototype.stop = function()
{
window.clearTimeout(this.timeout);
}
Example:
time = 0;
this.gameTimer = new Timer( function() { time++; }, 1000, -1);
Self-corrects the setTimeout, can run it X number of times (-1 for infinite), can start running instantaneously, and has a counter if you ever need to see how many times the func() has been run. Comes in handy.
Edit: Note, this doesn't do any input checking (like if delay and repeat are the correct type. And you'd probably want to add some kind of get/set function if you wanted to get the count or change the repeat value.
One of my simplest implementations is down below. It can even survive page reloads. :-
Code pen: https://codepen.io/shivabhusal/pen/abvmgaV
$(function() {
var TTimer = {
startedTime: new Date(),
restoredFromSession: false,
started: false,
minutes: 0,
seconds: 0,
tick: function tick() {
// Since setInterval is not reliable in inactive windows/tabs we are using date diff.
var diffInSeconds = Math.floor((new Date() - this.startedTime) / 1000);
this.minutes = Math.floor(diffInSeconds / 60);
this.seconds = diffInSeconds - this.minutes * 60;
this.render();
this.updateSession();
},
utilities: {
pad: function pad(number) {
return number < 10 ? '0' + number : number;
}
},
container: function container() {
return $(document);
},
render: function render() {
this.container().find('#timer-minutes').text(this.utilities.pad(this.minutes));
this.container().find('#timer-seconds').text(this.utilities.pad(this.seconds));
},
updateSession: function updateSession() {
sessionStorage.setItem('timerStartedTime', this.startedTime);
},
clearSession: function clearSession() {
sessionStorage.removeItem('timerStartedTime');
},
restoreFromSession: function restoreFromSession() {
// Using sessionsStorage to make the timer persistent
if (typeof Storage == "undefined") {
console.log('No sessionStorage Support');
return;
}
if (sessionStorage.getItem('timerStartedTime') !== null) {
this.restoredFromSession = true;
this.startedTime = new Date(sessionStorage.getItem('timerStartedTime'));
}
},
start: function start() {
this.restoreFromSession();
this.stop();
this.started = true;
this.tick();
this.timerId = setInterval(this.tick.bind(this), 1000);
},
stop: function stop() {
this.started = false;
clearInterval(this.timerId);
this.render();
}
};
TTimer.start();
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
<h1>
<span id="timer-minutes">00</span> :
<span id="timer-seconds">00</span>
</h1>
Inspired by Bergi's answer I created the following complete non drifting timer. What I wanted was a way to set a timer, stop it, and do this simply.
var perfectTimer = { // Set of functions designed to create nearly perfect timers that do not drift
timers: {}, // An object of timers by ID
nextID: 0, // Next available timer reference ID
set: (callback, interval) => { // Set a timer
var expected = Date.now() + interval; // Expected currect time when timeout fires
var ID = perfectTimer.nextID++; // Create reference to timer
function step() { // Adjusts the timeout to account for any drift since last timeout
callback(); // Call the callback
var dt = Date.now() - expected; // The drift (ms) (positive for overshooting) comparing the expected time to the current time
expected += interval; // Set the next expected currect time when timeout fires
perfectTimer.timers[ID] = setTimeout(step, Math.max(0, interval - dt)); // Take into account drift
}
perfectTimer.timers[ID] = setTimeout(step, interval); // Return reference to timer
return ID;
},
clear: (ID) => { // Clear & delete a timer by ID reference
if (perfectTimer.timers[ID] != undefined) { // Preventing errors when trying to clear a timer that no longer exists
console.log('clear timer:', ID);
console.log('timers before:', perfectTimer.timers);
clearTimeout(perfectTimer.timers[ID]); // Clear timer
delete perfectTimer.timers[ID]; // Delete timer reference
console.log('timers after:', perfectTimer.timers);
}
}
}
// Below are some tests
var timerOne = perfectTimer.set(() => {
console.log(new Date().toString(), Date.now(), 'timerOne', timerOne);
}, 1000);
console.log(timerOne);
setTimeout(() => {
perfectTimer.clear(timerOne);
}, 5000)
var timerTwo = perfectTimer.set(() => {
console.log(new Date().toString(), Date.now(), 'timerTwo', timerTwo);
}, 1000);
console.log(timerTwo);
setTimeout(() => {
perfectTimer.clear(timerTwo);
}, 8000)
driftless is a drop-in replacement for setInterval that mitigates drift. Makes life easy, import the npm package, then use it like setInterval / setTimeout:
setDriftlessInterval(() => {
this.counter--;
}, 1000);
setDriftlessInterval(() => {
this.refreshBounds();
}, 20000);
you can use a function called setTimeout that we can use to set the countdown.
Firstly, create a javascript snippet and add it to your page as follows;
var remainingTime = 30;
var elem = document.getElementById('countdown_div');
var timer = setInterval(countdown, 1000); //set the countdown to every second
function countdown() {
if (remainingTime == -1) {
clearTimeout(timer);
doSomething();
} else {
elem.innerHTML = remainingTime + ' left';
remainingTime--; //we subtract the second each iteration
}
}
Source + more details -> https://www.growthsnippets.com/30-second-countdown-timer-javascript/
Many of these answers here are great, but they typically their code examples are pages and pages of code (the good ones even have instructions on the best way to copy/paste it all). I just wanted to understand this problem with a very simple example.
Working Demo
var lastpause = 0;
var totaltime = 0;
function goFunction(e) {
if(this.innerText == 'Off - Timer Not Going') {
this.innerText = 'On - Timer Going';
} else {
totaltime += Date.now() - lastpause;
this.innerText = 'Off - Timer Not Going';
}
lastpause = Date.now();
document.getElementById('count').innerText = totaltime + ' milliseconds.';
}
document.getElementById('button').addEventListener('click', goFunction);
<button id="button">Off - Timer Not Going</button> <br>
Seconds: <span id="count">0 milliseconds.</span>
Explanation of Demo
totaltime — This is the total time calculated.
lastpause — This is the only real temporary variable we have. Whenever someone hits pause, we set lastpause to Date.now(). When someone unpauses, and re-pauses again, we calculate the time diff of Date.now() subtracted from the last pause.
We only need those two variables: Our total and the last time we stopped the timer. The other answers seem to use this approach, but I wanted a compact explanation.
I gave up building my own and ended up using this neat library.
This is my script:
var find = setInterval(function() {
if (document.getElementsByClassName('RDlrG Inn9w iWO5td')[0]) {
if (document.getElementsByClassName('w1OTme')[0]) {
window.open(document.getElementsByClassName('w1OTme')[0].href);
//here I call the setTimeout function for my SetInterval
}
}
}, 2000);
This is a Tampermonkey script I am developing for Google Calendar.
I want to set a timeout function on my find function aka setInterval function so it doesn't spam the window.open function.
In short:
Is there a way I could set a Timeout function on setInterval function which is called from my setInterval function?
If yes, how so?
You can't pause the interval of a setInterval, but you can stop it and start it again after some time.
let find = null;
function intervalFunc() {
if (condition) {
// Do some operations which should not be repeated for the next 30 seconds
// Clear current interval
clearInterval(find);
// Schedule to start the setInterval after 30 seconds.
setTimeout(function() {
find = setInterval(intervalFunc, 2000);
}, 30000 - 2000);
// ^
// Subtracting the interval dalay to cancel out the delay for the first invocation.
// (Because the first invocation will also wait for 2 seconds, so the pause would be 32 seconds instead of 30)
}
}
// Start the initial setInterval
find = setInterval(intervalFunc, 2000);
Here is a working example:
let count = 0;
const intervalDelay = 200;
const pauseDelay = 3000;
let find = null;
function intervalFunc() {
count++;
console.log('check', count);
if (count >= 5) {
count = 0;
console.log('Pausing for ' + (pauseDelay / 1000) + ' seconds');
clearInterval(find);
setTimeout(function() {
find = setInterval(intervalFunc, intervalDelay);
}, pauseDelay - intervalDelay);
}
}
find = setInterval(intervalFunc, intervalDelay);
Code is as follows：
/**
* this is calculate Timer
* #param delay Timing steps
* #returns object Provide calling interface
*/
function countTime(delay = 20) {
let millseconds = 0,
seconds = 0,
timer = null;
// calculate
function calculate() {
millseconds += delay;
if (millseconds === 1000) {
seconds += 1;
millseconds = 0;
}
}
// start the calculate
function start() {
timer = setInterval(calculate, delay);
}
// end the calculate
function end() {
clearInterval(timer);
timer = null;
}
return {
start,
end,
seconds
};
}
const obj = countTime();
obj.start();
// Wait for a while to execute
obj.end();
// execute output
console.log(obj.seconds); // always output 0
Has anyone encountered a similar issue?
I really want to know what mechanism is causing javascript, Or is there something missing in my own writing?
There are 2 problems with the code. First one is that when your function gets executed it returns object with "seconds" property. Unfortunately, that property is not being calculated each time you check against it. So basically it's a stale value. To fix that you can create a new function in your code `getSeconds:
function countTime(delay = 20) {
function getSeconds() {
return seconds;
}
...
return {
start,
end,
getSeconds
}
Another issue is that you are not waiting enough time before checking for the seconds. So you should have something like that:
const obj = countTime();
obj.start();
setTimeout(() => {
obj.end();
console.log(obj.getSeconds());
}, 3000)
I'm having a problem.
I want to make a counter that counts from 1 to 9 and repeats.
The time between the counts should be variable (1 to 10 seconds in the same series).
Sometimes, it should add 1, sometimes 2 (so sometimes skip one number).
Is that possible with javascript?
Thank you in advance.
This is the code I have, but is only counts, does not skip a number sometimes and the time to count is fixed at 500 ms.
<div id="value">1</div>
<script>
function animateValue(id){
var obj = document.getElementById(id);
var current = obj.innerHTML;
setInterval(function(){
obj.innerHTML = current++;
},500);
}
animateValue('value');
</script>
</html>````
First, a JSFiddle:
http://jsfiddle.net/5k0xsrj6/embedded/result/
JSFiddle with larger, stylized text:
https://jsfiddle.net/9f4vgLbx/embedded/result
Edit: I see you're not familiar with JavaScript. I've included non-ES6 JavaScript as well.
The biggest issue you'll face with your code is the use of setInterval, as you want a variable timer.
Instead of setInterval, consider a function that calls itself and sets a timer. Once the setTimeout is called, it will invoke the function again to set another timeout, effectively creating an interval.
Non ES6 Script:
var el = document.body;
var max_count = 9;
var current_count = 1;
// Function which sets our timer
function timer(delay) {
// Set a timeout with our passed `delay` arg
setTimeout(function () {
// Adds either 1 or 2 based on the return value of getIteration
current_count += getIteration();
// As we have a max, reset to 1 if we're over
if (current_count > max_count) {
current_count = 1;
}
// Update innerHTML
writer();
// Call next iteration
loop();
}, delay);
}
// Writes our innerHTML
function writer() {
el.innerHTML = current_count;
}
// Returns 1000 through 10000
function getDelay() {
return Math.ceil(Math.random() * 10) * 1000;
}
// Returns either 1 or 2
function getIteration() {
return Math.ceil(Math.random() * 2);
}
// Our main function to loop
function loop() {
// getDelay will return a value between 1000 - 10000
timer(getDelay());
}
// Sets Initial Value
writer();
// Main
loop();
Original:
Here's an example of the code on the JSFiddle. I've included comments to hopefully explain the logic.
{
const el = document.body;
const max_count = 9;
let current_count = 1;
// Function which sets our timer
const timer = delay => {
setTimeout(() => {
current_count += getIteration();
if (current_count > max_count) {
current_count = 1;
}
// Update innerHTML
writer();
// Call next iteration
main();
}, delay);
}
// Writes our innerHTML
const writer = (str, log) => {
if (log) {
console.log(str);
} else {
el.innerHTML = `Current count: ${current_count}`;
}
}
// Returns 1000 through 10000
const getDelay = () => {
return Math.ceil(Math.random() * 10) * 1000;
}
// Returns either 1 or 2
const getIteration = () => {
return Math.ceil(Math.random() * 2);
}
// Our main function to loop
const main = () => {
const delay = getDelay();
writer(`Next delay is ${delay}ms`, true);
timer(delay);
}
// Set Initial Value
writer();
// Main
main();
}
Hope this helps! If you have any questions, feel free to ask!
I need to create a simple but accurate timer.
This is my code:
var seconds = 0;
setInterval(function() {
timer.innerHTML = seconds++;
}, 1000);
After exactly 3600 seconds, it prints about 3500 seconds.
Why is it not accurate?
How can I create an accurate timer?
Why is it not accurate?
Because you are using setTimeout() or setInterval(). They cannot be trusted, there are no accuracy guarantees for them. They are allowed to lag arbitrarily, and they do not keep a constant pace but tend to drift (as you have observed).
How can I create an accurate timer?
Use the Date object instead to get the (millisecond-)accurate, current time. Then base your logic on the current time value, instead of counting how often your callback has been executed.
For a simple timer or clock, keep track of the time difference explicitly:
var start = Date.now();
setInterval(function() {
var delta = Date.now() - start; // milliseconds elapsed since start
…
output(Math.floor(delta / 1000)); // in seconds
// alternatively just show wall clock time:
output(new Date().toUTCString());
}, 1000); // update about every second
Now, that has the problem of possibly jumping values. When the interval lags a bit and executes your callback after 990, 1993, 2996, 3999, 5002 milliseconds, you will see the second count 0, 1, 2, 3, 5 (!). So it would be advisable to update more often, like about every 100ms, to avoid such jumps.
However, sometimes you really need a steady interval executing your callbacks without drifting. This requires a bit more advanced strategy (and code), though it pays out well (and registers less timeouts). Those are known as self-adjusting timers. Here the exact delay for each of the repeated timeouts is adapted to the actually elapsed time, compared to the expected intervals:
var interval = 1000; // ms
var expected = Date.now() + interval;
setTimeout(step, interval);
function step() {
var dt = Date.now() - expected; // the drift (positive for overshooting)
if (dt > interval) {
// something really bad happened. Maybe the browser (tab) was inactive?
// possibly special handling to avoid futile "catch up" run
}
… // do what is to be done
expected += interval;
setTimeout(step, Math.max(0, interval - dt)); // take into account drift
}
I'ma just build on Bergi's answer (specifically the second part) a little bit because I really liked the way it was done, but I want the option to stop the timer once it starts (like clearInterval() almost). Sooo... I've wrapped it up into a constructor function so we can do 'objecty' things with it.
1. Constructor
Alright, so you copy/paste that...
/**
* Self-adjusting interval to account for drifting
*
* #param {function} workFunc Callback containing the work to be done
* for each interval
* #param {int} interval Interval speed (in milliseconds)
* #param {function} errorFunc (Optional) Callback to run if the drift
* exceeds interval
*/
function AdjustingInterval(workFunc, interval, errorFunc) {
var that = this;
var expected, timeout;
this.interval = interval;
this.start = function() {
expected = Date.now() + this.interval;
timeout = setTimeout(step, this.interval);
}
this.stop = function() {
clearTimeout(timeout);
}
function step() {
var drift = Date.now() - expected;
if (drift > that.interval) {
// You could have some default stuff here too...
if (errorFunc) errorFunc();
}
workFunc();
expected += that.interval;
timeout = setTimeout(step, Math.max(0, that.interval-drift));
}
}
2. Instantiate
Tell it what to do and all that...
// For testing purposes, we'll just increment
// this and send it out to the console.
var justSomeNumber = 0;
// Define the work to be done
var doWork = function() {
console.log(++justSomeNumber);
};
// Define what to do if something goes wrong
var doError = function() {
console.warn('The drift exceeded the interval.');
};
// (The third argument is optional)
var ticker = new AdjustingInterval(doWork, 1000, doError);
3. Then do... stuff
// You can start or stop your timer at will
ticker.start();
ticker.stop();
// You can also change the interval while it's in progress
ticker.interval = 99;
I mean, it works for me anyway. If there's a better way, lemme know.
Bergi's answer pinpoints exactly why the timer from the question is not accurate. Here's my take on a simple JS timer with start, stop, reset and getTime methods:
class Timer {
constructor () {
this.isRunning = false;
this.startTime = 0;
this.overallTime = 0;
}
_getTimeElapsedSinceLastStart () {
if (!this.startTime) {
return 0;
}
return Date.now() - this.startTime;
}
start () {
if (this.isRunning) {
return console.error('Timer is already running');
}
this.isRunning = true;
this.startTime = Date.now();
}
stop () {
if (!this.isRunning) {
return console.error('Timer is already stopped');
}
this.isRunning = false;
this.overallTime = this.overallTime + this._getTimeElapsedSinceLastStart();
}
reset () {
this.overallTime = 0;
if (this.isRunning) {
this.startTime = Date.now();
return;
}
this.startTime = 0;
}
getTime () {
if (!this.startTime) {
return 0;
}
if (this.isRunning) {
return this.overallTime + this._getTimeElapsedSinceLastStart();
}
return this.overallTime;
}
}
const timer = new Timer();
timer.start();
setInterval(() => {
const timeInSeconds = Math.round(timer.getTime() / 1000);
document.getElementById('time').innerText = timeInSeconds;
}, 100)
<p>Elapsed time: <span id="time">0</span>s</p>
The snippet also includes a solution for your problem. So instead of incrementing seconds variable every 1000ms interval, we just start the timer and then every 100ms* we just read elapsed time from the timer and update the view accordingly.
* - makes it more accurate than 1000ms
To make your timer more accurate, you would have to round
Most of the timers in the answers here will linger behind the expected time because they set the "expected" value to the ideal and only account for the delay that the browser introduced before that point. This is fine if you just need accurate intervals, but if you are timing relative to other events then you will (nearly) always have this delay.
To correct it, you can keep track of the drift history and use it to predict future drift. By adding a secondary adjustment with this preemptive correction, the variance in the drift centers around the target time. For example, if you're always getting a drift of 20 to 40ms, this adjustment would shift it to -10 to +10ms around the target time.
Building on Bergi's answer, I've used a rolling median for my prediction algorithm. Taking just 10 samples with this method makes a reasonable difference.
var interval = 200; // ms
var expected = Date.now() + interval;
var drift_history = [];
var drift_history_samples = 10;
var drift_correction = 0;
function calc_drift(arr){
// Calculate drift correction.
/*
In this example I've used a simple median.
You can use other methods, but it's important not to use an average.
If the user switches tabs and back, an average would put far too much
weight on the outlier.
*/
var values = arr.concat(); // copy array so it isn't mutated
values.sort(function(a,b){
return a-b;
});
if(values.length ===0) return 0;
var half = Math.floor(values.length / 2);
if (values.length % 2) return values[half];
var median = (values[half - 1] + values[half]) / 2.0;
return median;
}
setTimeout(step, interval);
function step() {
var dt = Date.now() - expected; // the drift (positive for overshooting)
if (dt > interval) {
// something really bad happened. Maybe the browser (tab) was inactive?
// possibly special handling to avoid futile "catch up" run
}
// do what is to be done
// don't update the history for exceptionally large values
if (dt <= interval) {
// sample drift amount to history after removing current correction
// (add to remove because the correction is applied by subtraction)
drift_history.push(dt + drift_correction);
// predict new drift correction
drift_correction = calc_drift(drift_history);
// cap and refresh samples
if (drift_history.length >= drift_history_samples) {
drift_history.shift();
}
}
expected += interval;
// take into account drift with prediction
setTimeout(step, Math.max(0, interval - dt - drift_correction));
}
I agree with Bergi on using Date, but his solution was a bit of overkill for my use. I simply wanted my animated clock (digital and analog SVGs) to update on the second and not overrun or under run creating obvious jumps in the clock updates. Here is the snippet of code I put in my clock update functions:
var milliseconds = now.getMilliseconds();
var newTimeout = 1000 - milliseconds;
this.timeoutVariable = setTimeout((function(thisObj) { return function() { thisObj.update(); } })(this), newTimeout);
It simply calculates the delta time to the next even second, and sets the timeout to that delta. This syncs all of my clock objects to the second. Hope this is helpful.
Here's a solution that pauses when the window is hidden, and can be cancelled with an abort controller.
function animationInterval(ms, signal, callback) {
const start = document.timeline.currentTime;
function frame(time) {
if (signal.aborted) return;
callback(time);
scheduleFrame(time);
}
function scheduleFrame(time) {
const elapsed = time - start;
const roundedElapsed = Math.round(elapsed / ms) * ms;
const targetNext = start + roundedElapsed + ms;
const delay = targetNext - performance.now();
setTimeout(() => requestAnimationFrame(frame), delay);
}
scheduleFrame(start);
}
Usage:
const controller = new AbortController();
// Create an animation callback every second:
animationInterval(1000, controller.signal, time => {
console.log('tick!', time);
});
// And stop it sometime later:
controller.abort();
Modern, Fully Programmable Timer
This timer takes a frequency in Hertz, and a callback that can take up to four arguments, the current frame index, the current time, the time that the current frame would have ideally occurred at, and a reference to the timer instance (so the caller and callback can both access its methods).
Note: All times are based on performance.now, and are relative to the moment that the page loaded.
Timer instances have three API methods:
stop: Takes no args. Kills the timer immediately (and permanently).
Returns the frame index for the next frame (the cancelled frame).
adapt: Takes a frequency in Hertz and adapts the timer to it, beginning
from the next frame. Returns the implied interval in milliseconds.
redefine: Takes a new callback function. Swaps it with the current
callback. Effects the next frame. Returns undefined.
Note: The tick method passes this around explicitly (as self) to work around the problem of this referencing window when the tick method is invoked via setTimeout.
class ProgrammableTimer {
constructor(hertz, callback) {
this.target = performance.now(); // target time for the next frame
this.interval = 1 / hertz * 1000; // the milliseconds between ticks
this.callback = callback;
this.stopped = false;
this.frame = 0;
this.tick(this);
}
tick(self) {
if (self.stopped) return;
const currentTime = performance.now();
const currentTarget = self.target;
const currentInterval = (self.target += self.interval) - currentTime;
setTimeout(self.tick, currentInterval, self);
self.callback(self.frame++, currentTime, currentTarget, self);
}
stop() { this.stopped = true; return this.frame }
adapt(hertz) { return this.interval = 1 / hertz * 1000 }
redefine(replacement) { this.callback = replacement }
}
Doesn't get much more accurate than this.
var seconds = new Date().getTime(), last = seconds,
intrvl = setInterval(function() {
var now = new Date().getTime();
if(now - last > 5){
if(confirm("Delay registered, terminate?")){
clearInterval(intrvl);
return;
}
}
last = now;
timer.innerHTML = now - seconds;
}, 333);
As to why it is not accurate, I would guess that the machine is busy doing other things, slowing down a little on each iteration adds up, as you see.
This is an old question but figured I'd share some code I use sometimes:
function Timer(func, delay, repeat, runAtStart)
{
this.func = func;
this.delay = delay;
this.repeat = repeat || 0;
this.runAtStart = runAtStart;
this.count = 0;
this.startTime = performance.now();
if (this.runAtStart)
this.tick();
else
{
var _this = this;
this.timeout = window.setTimeout( function(){ _this.tick(); }, this.delay);
}
}
Timer.prototype.tick = function()
{
this.func();
this.count++;
if (this.repeat === -1 || (this.repeat > 0 && this.count < this.repeat) )
{
var adjustedDelay = Math.max( 1, this.startTime + ( (this.count+(this.runAtStart ? 2 : 1)) * this.delay ) - performance.now() );
var _this = this;
this.timeout = window.setTimeout( function(){ _this.tick(); }, adjustedDelay);
}
}
Timer.prototype.stop = function()
{
window.clearTimeout(this.timeout);
}
Example:
time = 0;
this.gameTimer = new Timer( function() { time++; }, 1000, -1);
Self-corrects the setTimeout, can run it X number of times (-1 for infinite), can start running instantaneously, and has a counter if you ever need to see how many times the func() has been run. Comes in handy.
Edit: Note, this doesn't do any input checking (like if delay and repeat are the correct type. And you'd probably want to add some kind of get/set function if you wanted to get the count or change the repeat value.
One of my simplest implementations is down below. It can even survive page reloads. :-
Code pen: https://codepen.io/shivabhusal/pen/abvmgaV
$(function() {
var TTimer = {
startedTime: new Date(),
restoredFromSession: false,
started: false,
minutes: 0,
seconds: 0,
tick: function tick() {
// Since setInterval is not reliable in inactive windows/tabs we are using date diff.
var diffInSeconds = Math.floor((new Date() - this.startedTime) / 1000);
this.minutes = Math.floor(diffInSeconds / 60);
this.seconds = diffInSeconds - this.minutes * 60;
this.render();
this.updateSession();
},
utilities: {
pad: function pad(number) {
return number < 10 ? '0' + number : number;
}
},
container: function container() {
return $(document);
},
render: function render() {
this.container().find('#timer-minutes').text(this.utilities.pad(this.minutes));
this.container().find('#timer-seconds').text(this.utilities.pad(this.seconds));
},
updateSession: function updateSession() {
sessionStorage.setItem('timerStartedTime', this.startedTime);
},
clearSession: function clearSession() {
sessionStorage.removeItem('timerStartedTime');
},
restoreFromSession: function restoreFromSession() {
// Using sessionsStorage to make the timer persistent
if (typeof Storage == "undefined") {
console.log('No sessionStorage Support');
return;
}
if (sessionStorage.getItem('timerStartedTime') !== null) {
this.restoredFromSession = true;
this.startedTime = new Date(sessionStorage.getItem('timerStartedTime'));
}
},
start: function start() {
this.restoreFromSession();
this.stop();
this.started = true;
this.tick();
this.timerId = setInterval(this.tick.bind(this), 1000);
},
stop: function stop() {
this.started = false;
clearInterval(this.timerId);
this.render();
}
};
TTimer.start();
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
<h1>
<span id="timer-minutes">00</span> :
<span id="timer-seconds">00</span>
</h1>
Inspired by Bergi's answer I created the following complete non drifting timer. What I wanted was a way to set a timer, stop it, and do this simply.
var perfectTimer = { // Set of functions designed to create nearly perfect timers that do not drift
timers: {}, // An object of timers by ID
nextID: 0, // Next available timer reference ID
set: (callback, interval) => { // Set a timer
var expected = Date.now() + interval; // Expected currect time when timeout fires
var ID = perfectTimer.nextID++; // Create reference to timer
function step() { // Adjusts the timeout to account for any drift since last timeout
callback(); // Call the callback
var dt = Date.now() - expected; // The drift (ms) (positive for overshooting) comparing the expected time to the current time
expected += interval; // Set the next expected currect time when timeout fires
perfectTimer.timers[ID] = setTimeout(step, Math.max(0, interval - dt)); // Take into account drift
}
perfectTimer.timers[ID] = setTimeout(step, interval); // Return reference to timer
return ID;
},
clear: (ID) => { // Clear & delete a timer by ID reference
if (perfectTimer.timers[ID] != undefined) { // Preventing errors when trying to clear a timer that no longer exists
console.log('clear timer:', ID);
console.log('timers before:', perfectTimer.timers);
clearTimeout(perfectTimer.timers[ID]); // Clear timer
delete perfectTimer.timers[ID]; // Delete timer reference
console.log('timers after:', perfectTimer.timers);
}
}
}
// Below are some tests
var timerOne = perfectTimer.set(() => {
console.log(new Date().toString(), Date.now(), 'timerOne', timerOne);
}, 1000);
console.log(timerOne);
setTimeout(() => {
perfectTimer.clear(timerOne);
}, 5000)
var timerTwo = perfectTimer.set(() => {
console.log(new Date().toString(), Date.now(), 'timerTwo', timerTwo);
}, 1000);
console.log(timerTwo);
setTimeout(() => {
perfectTimer.clear(timerTwo);
}, 8000)
driftless is a drop-in replacement for setInterval that mitigates drift. Makes life easy, import the npm package, then use it like setInterval / setTimeout:
setDriftlessInterval(() => {
this.counter--;
}, 1000);
setDriftlessInterval(() => {
this.refreshBounds();
}, 20000);
you can use a function called setTimeout that we can use to set the countdown.
Firstly, create a javascript snippet and add it to your page as follows;
var remainingTime = 30;
var elem = document.getElementById('countdown_div');
var timer = setInterval(countdown, 1000); //set the countdown to every second
function countdown() {
if (remainingTime == -1) {
clearTimeout(timer);
doSomething();
} else {
elem.innerHTML = remainingTime + ' left';
remainingTime--; //we subtract the second each iteration
}
}
Source + more details -> https://www.growthsnippets.com/30-second-countdown-timer-javascript/
Many of these answers here are great, but they typically their code examples are pages and pages of code (the good ones even have instructions on the best way to copy/paste it all). I just wanted to understand this problem with a very simple example.
Working Demo
var lastpause = 0;
var totaltime = 0;
function goFunction(e) {
if(this.innerText == 'Off - Timer Not Going') {
this.innerText = 'On - Timer Going';
} else {
totaltime += Date.now() - lastpause;
this.innerText = 'Off - Timer Not Going';
}
lastpause = Date.now();
document.getElementById('count').innerText = totaltime + ' milliseconds.';
}
document.getElementById('button').addEventListener('click', goFunction);
<button id="button">Off - Timer Not Going</button> <br>
Seconds: <span id="count">0 milliseconds.</span>
Explanation of Demo
totaltime — This is the total time calculated.
lastpause — This is the only real temporary variable we have. Whenever someone hits pause, we set lastpause to Date.now(). When someone unpauses, and re-pauses again, we calculate the time diff of Date.now() subtracted from the last pause.
We only need those two variables: Our total and the last time we stopped the timer. The other answers seem to use this approach, but I wanted a compact explanation.
I gave up building my own and ended up using this neat library.