Changing the interval of SetInterval while it's running

I have written a javascript function that uses setInterval to manipulate a string every tenth of a second for a certain number of iterations.

function timer() {
    var section = document.getElementById('txt').value;
    var len = section.length;
    var rands = new Array();

    for (i=0; i<len; i++) {
        rands.push(Math.floor(Math.random()*len));
    };

    var counter = 0
    var interval = setInterval(function() {
        var letters = section.split('');
        for (j=0; j < len; j++) {
            if (counter < rands[j]) {
                letters[j] = Math.floor(Math.random()*9);
            };
        };
        document.getElementById('txt').value = letters.join('');
        counter++
        
        if (counter > rands.max()) {
            clearInterval(interval);
        }
    }, 100);
};

Instead of having the interval set at a specific number, I would like to update it every time it runs, based on a counter. So instead of:

var interval = setInterval(function() { ... }, 100);

It would be something like:

var interval = setInterval(function() { ... }, 10*counter);

Unfortunately, that did not work. It seemed like "10*counter" equals 0.

So, how can I adjust the interval every time the anonymous function runs?

You could use an anonymous function:

var counter = 10;
var myFunction = function(){
    clearInterval(interval);
    counter *= 10;
    interval = setInterval(myFunction, counter);
}
var interval = setInterval(myFunction, counter);

UPDATE: As suggested by A. Wolff, use setTimeout to avoid the need for clearInterval .

var counter = 10;
var myFunction = function() {
    counter *= 10;
    setTimeout(myFunction, counter);
}
setTimeout(myFunction, counter);

Use setTimeout() instead. The callback would then be responsible for firing the next timeout, at which point you can increase or otherwise manipulate the timing.

EDIT

Here's a generic function you can use to apply a "decelerating" timeout for ANY function call.

function setDeceleratingTimeout(callback, factor, times)
{
    var internalCallback = function(tick, counter) {
        return function() {
            if (--tick >= 0) {
                window.setTimeout(internalCallback, ++counter * factor);
                callback();
            }
        }
    }(times, 0);
  
    window.setTimeout(internalCallback, factor);
};

// console.log() requires firebug    
setDeceleratingTimeout(function(){ console.log('hi'); }, 10, 10);
setDeceleratingTimeout(function(){ console.log('bye'); }, 100, 10);

I like this question - inspired a little timer object in me:

window.setVariableInterval = function(callbackFunc, timing) {
  var variableInterval = {
    interval: timing,
    callback: callbackFunc,
    stopped: false,
    runLoop: function() {
      if (variableInterval.stopped) return;
      var result = variableInterval.callback.call(variableInterval);
      if (typeof result == 'number')
      {
        if (result === 0) return;
        variableInterval.interval = result;
      }
      variableInterval.loop();
    },
    stop: function() {
      this.stopped = true;
      window.clearTimeout(this.timeout);
    },
    start: function() {
      this.stopped = false;
      return this.loop();
    },
    loop: function() {
      this.timeout = window.setTimeout(this.runLoop, this.interval);
      return this;
    }
  };

  return variableInterval.start();
};

Example use

var vi = setVariableInterval(function() {
  // this is the variableInterval - so we can change/get the interval here:
  var interval = this.interval;

  // print it for the hell of it
  console.log(interval);

  // we can stop ourselves.
  if (interval>4000) this.stop();

  // we could return a new interval after doing something
  return interval + 100;
}, 100);  

// we can change the interval down here too
setTimeout(function() {
  vi.interval = 3500;
}, 1000);

// or tell it to start back up in a minute
setTimeout(function() {
  vi.interval = 100;
  vi.start();
}, 60000);

I had the same question as the original poster, did this as a solution. Not sure how efficient this is ....

let interval = 5000; // initial condition
let run = setInterval(request, interval); // start setInterval as "run"

function request() {

    console.log(interval); // firebug or chrome log
    clearInterval(run); // stop the setInterval()

    // dynamically change the run interval
    if (interval > 200) {
        interval = interval * .8;
    } else {
        interval = interval * 1.2;
    }

    run = setInterval(request, interval); // start the setInterval()
}

This is my way of doing this, i use setTimeout:

var timer = {
	running: false,
	iv: 5000,
	timeout: false,
	cb : function(){},
	start : function(cb,iv){
		var elm = this;
		clearInterval(this.timeout);
		this.running = true;
		if(cb) this.cb = cb;
		if(iv) this.iv = iv;
		this.timeout = setTimeout(function(){elm.execute(elm)}, this.iv);
	},
	execute : function(e){
		if(!e.running) return false;
		e.cb();
		e.start();
	},
	stop : function(){
		this.running = false;
	},
	set_interval : function(iv){
		clearInterval(this.timeout);
		this.start(false, iv);
	}
};

Usage:

timer.start(function(){
    console.debug('go');
}, 2000);

timer.set_interval(500);

timer.stop();

A much simpler way would be to have an if statement in the refreshed function and a control to execute your command at regular time intervals . In the following example, I run an alert every 2 seconds and the interval (intrv) can be changed dynamically...

var i=1;
var intrv=2; // << control this variable

var refreshId = setInterval(function() {
  if(!(i%intrv)) {
    alert('run!');
  }
  i++;
}, 1000);

This can be initiated however you want. timeout is the method i used to keep it on the top of the hour.

I had the need for every hour to begin a code block on the hour. So this would start at server startup and run the interval hourly. Basicaly the initial run is to begin the interval within the same minute. So in a second from init, run immediately then on every 5 seconds.

var interval = 1000;
var timing =function(){
    var timer = setInterval(function(){
        console.log(interval);
        if(interval == 1000){ /*interval you dont want anymore or increment/decrement */
            interval = 3600000; /* Increment you do want for timer */
            clearInterval(timer);
            timing();
        }
    },interval);
}
timing();

Alternately if you wanted to just have something happen at start and then forever at a specific interval you could just call it at the same time as the setInterval. For example:

var this = function(){
 //do
}
setInterval(function(){
  this()
},3600000)
this()

Here we have this run the first time and then every hour.

Simple answer is you can't update an interval of already created timer. (There is only two functions setInterval/setTimer and clearInterval/clearTimer, so having a timerId you can only deactivate it.) But you can made some workarounds. Take a look at this github repo.

I couldn't synchronize and change the speed my setIntervals too and I was about to post a question. But I think I've found a way. It should certainly be improved because I'm a beginner. So, I'd gladly read your comments/remarks about this.

<body onload="foo()">
<div id="count1">0</div>
<div id="count2">2nd counter is stopped</div>
<button onclick="speed0()">pause</button>
<button onclick="speedx(1)">normal speed</button>
<button onclick="speedx(2)">speed x2</button>
<button onclick="speedx(4)">speed x4</button>
<button onclick="startTimer2()">Start second timer</button>
</body>
<script>
var count1 = 0,
	count2 = 0,
	greenlight = new Boolean(0), //blocks 2nd counter
	speed = 1000,	//1second
	countingSpeed;
function foo(){
	countingSpeed = setInterval(function(){
		counter1();
		counter2();
	},speed);
}
function counter1(){
	count1++;
	document.getElementById("count1").innerHTML=count1;
}
function counter2(){
	if (greenlight != false) {
		count2++;
		document.getElementById("count2").innerHTML=count2;
	}
}
function startTimer2(){
	//while the button hasn't been clicked, greenlight boolean is false
	//thus, the 2nd timer is blocked
	greenlight = true;
	counter2();
	//counter2() is greenlighted
}

//these functions modify the speed of the counters
function speed0(){
	clearInterval(countingSpeed);
}
function speedx(a){
	clearInterval(countingSpeed);
	speed=1000/a;
	foo();
}
</script>

If you want the counters to begin to increase once the page is loaded, put counter1() and counter2() in foo() before countingSpeed is called. Otherwise, it takes speed milliseconds before execution. EDIT : Shorter answer.

(function variableInterval() {
    //whatever needs to be done
    interval *= 2; //deal with your interval
    setTimeout(variableInterval, interval);
    //whatever needs to be done
})();

can't get any shorter

Here is yet another way to create a decelerating/accelerating interval timer. The interval gets multiplied by a factor until a total time is exceeded.

function setChangingInterval(callback, startInterval, factor, totalTime) {
    let remainingTime = totalTime;
    let interval = startInterval;

    const internalTimer = () => {
        remainingTime -= interval ;
        interval *= factor;
        if (remainingTime >= 0) {
            setTimeout(internalTimer, interval);
            callback();
        }
    };
    internalTimer();
}

This piece of code below accelerates (acceleration > 1) or decelerates (acceleration <1) a setInterval function :

function accelerate(yourfunction, timer, refresh, acceleration) {
    var new_timer = timer / acceleration;
    var refresh_init = refresh;//save this user defined value
    if (refresh < new_timer ){//avoid reseting the interval before it has produced anything.
        refresh = new_timer + 1 ;
    };
    var lastInter = setInterval(yourfunction, new_timer);
    console.log("timer:", new_timer);
    function stopLastInter() {
        clearInterval(lastInter);
        accelerate(yourfunction, new_timer, refresh_init, acceleration);
        console.log("refresh:", refresh);
    };
    setTimeout(stopLastInter, refresh);
}

With :

• timer: the setInterval initial value in ms (increasing or decreasing)
• refresh: the time before a new value of timer is calculated. This is the step lenght
• acceleration: the gap between the old and the next timer value. This is the step height

Make new function:

// set Time interval
$("3000,18000").Multitimeout();

jQuery.fn.extend({
    Multitimeout: function () {
        var res = this.selector.split(",");
        $.each(res, function (index, val) { setTimeout(function () { 
            //...Call function
            temp();
        }, val); });
        return true;
    }
});

function temp()
{
    alert();
}

Inspired by the internal callback above, i made a function to fire a callback at fractions of minutes. If timeout is set to intervals like 6 000, 15 000, 30 000, 60 000 it will continuously adapt the intervals in sync to the exact transition to the next minute of your system clock.

//Interval timer to trigger on even minute intervals
function setIntervalSynced(callback, intervalMs) {

    //Calculate time to next modulus timer event
    var betterInterval = function () {
        var d = new Date();
        var millis = (d.getMinutes() * 60 + d.getSeconds()) * 1000 + d.getMilliseconds();
        return intervalMs - millis % intervalMs;
    };

    //Internal callback
    var internalCallback = function () {
        return function () {
            setTimeout(internalCallback, betterInterval());
            callback();
        }
    }();

    //Initial call to start internal callback
    setTimeout(internalCallback, betterInterval());
};

You can use a variable and change the variable instead.

setInterval(() => function, variable)

This is my idea for times when you do not want loops like setInterval to overlap.
You also want to be able to set the loop execution delay and start and stop the loop, instansly on the fly.
I am using a loop_flag variable and a setTimeout function.
I set the main function to async so that you can call other functions in the body by calling await. When the main body of your code is running, the main loop waits and does not repeat itself. (which is not the case with setInterval)

An example of a simple code is:

//@NabiKAZ

document.getElementById("btn_start").addEventListener("click", function() {
  console.log("Starting...");
  loop_flag = true;
  loop_func();
});
document.getElementById("btn_stop").addEventListener("click", function() {
  console.log("Stoping...");
  loop_flag = false;
});

var n = 0;

var loop_flag = false;
var loop_func = async function() {
  if (!loop_flag) {
    console.log("STOP.");
    return;
  }

  //body main function inhere
  n++;
  console.log(n);
  ////


  if (loop_flag) {
    setTimeout(loop_func, document.getElementById("inp_delay").value);
  } else {
    console.log("STOP.");
  }
}

<input id="inp_delay" value="1000">
<button id="btn_start">START</button>
<button id="btn_stop">STOP</button>

For a more complete code with a fetch request inside the loop, see here:

https://jsfiddle.net/NabiKAZ/a5hdw2bo/

var counter = 15;
var interval = function() {
	setTimeout(function(){
        // Write your code here and remove console.log, remember that you need declare yourDynamicValue and give it a value
        console.log((new Date()).getTime())

        window.counter = yourDynamicValue;
        window.interval();
    }, counter);
}

// It needs to run just once as init

interval();