Develop Reference

SetInterval not completly accurate [duplicate]

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.

Is there a way to reset performance.now?

Hope you all having a good day!
I was requested to make a timer in javascript and I thought using performance.now would be a good idea, but when I try to reset the timer the timer stays where it stops instead of becoming zero.
let time = performance.now();
function initInterval(){
let taken = performance.now() - time;
let current = (time += taken);
setInterval(function () {
// time calculation to make it easy to read, display
// and eventListener to stop, end of the function
}
}
I want to reset the timer with a function call but it keeps giving me the same numbers before stopping the interval.
function resetInterval(){
closebutton.addEventListener('click', () => {
staticTimer.innerHTML = "00:00:00"
initInterval()
console.log('beep');
})
}

There's no way to reset performance.now(). What you can do is to save current performance.now() when the timer starts and just substract it afterwards. Looking at your code, simply set ´time = performance.now()´ again and you are done.

Nope, performance.now() cannot be reset. The value which is returned is the elapsed time since time origin
Here is a simple implementation using js classes. I've saved the time since page load as a constant which I subtract from both this.startTime and this.endTime before returning the difference between them.
class Timer {
constructor() {
this.startTime = undefined;
this.endTime = undefined;
this.constantTimeSincePageLoad = Math.trunc(performance.now());
}
endTimer() {
if (!this.startTime) return 'please start the timer';
this.endTime = Math.trunc(performance.now());
this.constantTimeSincePageLoad = Math.trunc(performance.now());
this.startTime -+ this.constantTimeSincePageLoad;
this.endTime -+ this.constantTimeSincePageLoad;
const result = this.endTime - this.startTime;
this.startTime = undefined;
this.endTime = undefined;
return result;
}
startTimer() {
this.startTime = Math.trunc(performance.now());
}
}
const timer = new Timer();
// activate the timer
timer.startTimer();
// stop the timer - meant to show that the time is reset on each invocation
setTimeout(() => {
console.log(timer.endTimer());
}, 100)

I encountered the problem that the execution result of javascript is not as expected

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)

JS Interval accurate to 1ms

I found a function to create a self correcting interval in JS since setInterval cannot be trusted to be accurate. When logging out the nextTick, when the interval is 1 second, 1 100th of a second, or 1 10th of a second, the nextTick, which is the drift fixer upper (to be technical) prints as expected.
Instead of the next tick being 1000 miliseconds, it's usually 999, sometimes 998. Following the aforementioned intervals, you see values such as 99, 98 for 100ths, and usually 9 for 10ths of a second.
Now, when it comes to milliseconds - that is creating an interval that is supposed to be called every millisecond, the nextTick is printing out NaN or negative numbers. I supposed this is just because the interval is so small. I am wondering if its even possible to get accuracy down to 1 ms
class Interval {
constructor(interval, onTick){
this.interval = interval;
this.onTick = onTick || function(){};
this.timer = false;
this.ticks = 0;
this.startTime = 0;
this.currentTime = 0;
this.elapsedTime = 0;
return this;
}
run(){
this.currentTime = Date.now();
if(!this.startTime){
this.startTime = this.currentTime;
}
this.onTick();
let nextTick = this.interval - (this.currentTime - (this.startTime + (this.ticks * this.interval)));
//console.log(nextTick);
this.ticks++;
let self = this;
this.timer = setTimeout(function(){
self.run();
}, nextTick);
return this;
}
start(){
this.run();
return this;
}
stop(){
clearTimeout(this.timer);
return this;
}
}
Original
Fiddle
In the fiddle you could uncomment one by one the interval constructors to see what I mean.

How to stop asynchronous function in JavaScript?

I have some asynchronous problems. I'm working on an ECMAScript 6 object. It's a timer and I want to be able to restart during its countdown.
Here is my work:
export class Timer {
constructor(sec){
this.sec = sec;
this.count = sec;
this.running = false;
}
start() {
this.running = true;
this._run();
}
_run(){
if(this.running){
setTimeout(()=>{
this.count --;
console.log(this.count);
if(this.count<0){
this.running = false;
}
this._run();
}, 1000);
}
}
restart(){
this.running = false;
/*
Wait until _run() is done then :
*/
this.count = this.sec;
this.start();
}
}
In the restart() function, how can I know when _run() has stopped running?

An easier way to know if the timer is "running" is to perhaps use setInterval instead.
var interval = setInterval(() => updateTimer(), 10); // update every 10ms
It's running if interval is set
if (interval) // timer is running
Stop the timer
window.clearInterval(interval);
interval = null;
// timer is no longer "running"
Additional notes
Beware of creating timers that increment with a fixed value
In your code, you have
setTimeout(() => this.count--, 1000);
The intention is for you to decrement your count property once every second, but this is not the behavior you will be guaranteed.
Check out this little script
var state = {now: Date.now()};
function delta(now) {
let delta = now - state.now;
state.now = now;
return delta;
}
setInterval(() => console.log(delta(Date.now())), 1000);
// Output
1002
1000
1004
1002
1002
1001
1002
1000
We used setInterval(fn, 1000) but the actual interval varies a couple milliseconds each time.
The problem is exaggerated if you do things like switch your browser's focus to a different tab, open a new tab, etc. Look at these more sporadic numbers
1005 // close to 1000 ms
1005 // ...
1004 // a little variance here
1004 // ...
1834 // switched focus to previous browser tab
1231 // let timer tab run in background for a couple seconds
1082 // ...
1330 // ...
1240 // ...
2014 // switched back to timer tab
1044 // switched to previous tab
2461 // rapidly switched to many tabs below
1998 // ...
2000 // look at these numbers...
1992 // not even close to the 1000 ms that we set for the interval
2021 // ...
1989 // switched back to this tab
1040 // ...
1003 // numbers appear to stabilize while this tab is in focus
1004 // ...
1005 // ...
So, this means you can't rely upon your setTimeout (or setInterval) function getting run once per 1000 ms. count will be decremented with much variance depending on a wide variety of factors.
To work around this, you need to use a delta. That means before each "tick" of your timer, you need to take a timestamp using Date.now. On the next tick, take a new timestamp and subtract your previous timestamp from the new one. That is your delta. Using this value, add it to the Timer's total ms to get the precise number of milliseconds the timer has been running for.
Then, all time-sensitive values will be a projection/calculation of the total accumulated ms.
In your case, say you have a count that starts at 10. If you want to count down by -1 each 1000 ms, you could do
function update() {
// update totalMs
this.totalMs += calculateDelta();
// display count based on totalMS
console.log("count %d", Math.ceil(this.count - this.totalMs/1000));
}
Here's a sample ES6 timer that implements a delta function that might help you
class Timer {
constructor(resolution=1000, ms=0) {
this.ms = ms
this.resolution = resolution;
this.interval = null;
}
delta(now) {
let delta = now - this.now;
this.now = now;
return delta;
}
start() {
this.now = Date.now();
this.interval = window.setInterval(() => this.update(), this.resolution);
}
reset() {
this.update();
this.ms = 0;
}
stop() {
this.update();
window.clearInterval(this.interval);
this.interval = null;
}
update() {
this.ms += this.delta(Date.now());
console.log("%d ms - %0.2f sec", this.ms, this.ms/1000);
}
}
Create a new timer with a 50 ms "resolution". All this means is that the timer display is updated every 50 ms. You could set this value to anything and the timer will still keep an accurate value.
var t = new Timer(50);
t.start();
To simulate the reset, we can just create a one-off timeout so you can see the reset working
// in ~5 seconds, reset the timer once
setTimeout(() => t.reset(), 5000);
Here's a demonstration of pausing the timer
// in ~10 seconds, pause the timer
setTimeout(() => t.stop(), 10000);
And you can resume the timer, too
// in ~12 seconds, resume the timer (without reset)
setTimeout(() => t.start(), 12000);
You can start, stop, reset the timer as much as you like
Here's an the ES6 (above) transpiled to ES5 so you can see the code working in a runnable snippet. Open your console and click Run code snippet.
"use strict";
function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }
var Timer = (function () {
function Timer() {
var resolution = arguments.length <= 0 || arguments[0] === undefined ? 1000 : arguments[0];
var ms = arguments.length <= 1 || arguments[1] === undefined ? 0 : arguments[1];
_classCallCheck(this, Timer);
this.ms = ms;
this.resolution = resolution;
this.interval = null;
}
Timer.prototype.delta = function delta(now) {
var delta = now - this.now;
this.now = now;
return delta;
};
Timer.prototype.start = function start() {
var _this = this;
this.now = Date.now();
this.interval = window.setInterval(function () {
return _this.update();
}, this.resolution);
};
Timer.prototype.reset = function reset() {
this.update();
this.ms = 0;
};
Timer.prototype.stop = function stop() {
this.update();
window.clearInterval(this.interval);
this.interval = null;
};
Timer.prototype.update = function update() {
this.ms += this.delta(Date.now());
console.log("%d ms - %0.2f sec", this.ms, this.ms / 1000);
};
return Timer;
})();
var t = new Timer(50);
t.start();
// in ~5 seconds, reset the timer once
setTimeout(function () {
return t.reset();
}, 5000);
// in ~10 seconds, pause the timer
setTimeout(function () {
return t.stop();
}, 10000);
// in ~12 seconds, resume the timer (without reset)
setTimeout(function () {
return t.start();
}, 12000);