How do I correctly use setInterval and clearInterval to switch between two different functions?

For practice I am trying to display a number that increments from 0 - 9, then decrements from 9 - 0, and infinitely repeats.

The code that I have so far seems to be close, but upon the second iteration the setInterval calls of my 2 respective functions countUp and countDown seem to be conflicting with each other, as the numbers displayed are not counting in the intended order... and then the browser crashes.

Here is my code:

<!DOCTYPE html>
<html>
    <head>
        <title>Algorithm Test</title>
    </head>

    <body onload = "onloadFunctions();">
        <script type = "text/javascript">
            function onloadFunctions()
            {
                countUp();
                setInterval(countUp, 200);
            }

            var count = 0;
            function countUp()
            {
                document.getElementById("here").innerHTML = count;
                count++;

                if(count == 10)
                {
                    clearInterval(this);
                    countDown();
                    setInterval(countDown, 200);
                }
            }
            function countDown()
            {
                document.getElementById("here").innerHTML = count;
                count--;

                if(count == 0)
                {
                    clearInterval(this);
                    countUp();
                    setInterval(countUp, 200);
                }       
            }
        </script>

        From 0 - 9, up and down:&nbsp;&nbsp;<div id = "here"></div>
    </body>
</html>

Answers:

Answer

You need to capture the return value from setInterval( ... ) into a variable as that is the reference to the timer:

var interval;

function onloadFunctions()
{
    countUp();
    interval = setInterval(countUp, 200);
}

/* ... code ... */

function countDown()
{
    document.getElementById("here").innerHTML = count;
    count--;

    if(count == 0)
    {
        clearInterval(interval);
        countUp();
        interval = setInterval(countUp, 200);
    }
}
Answer

@Claude, you are right, the other solution I proposed was too different from original code. This is another possible solution, using setInterval and switching functions:

function onloadFunctions() {
    var count = 0;
    var refId = null;
    var target = document.getElementById("aux");

    var countUp = function() {
        target.innerHTML = count;
        count ++;
        if(count >= 9) {
            window.clearInterval(refId);
            refId = window.setInterval(countDown, 500);
        }
    }

    var countDown = function() {
        target.innerHTML = count;
        count --;
        if(count <= 0) {
            window.clearInterval(refId);
            refId = window.setInterval(countUp, 500);
        }
    }
    refId = window.setInterval(countUp, 500);
}
Answer

clearInterval(this);. You can't do that. You need to save the return value from setInterval.

var interval;
function onloadFunctions()
{
    countUp();
    interval = setInterval(countUp, 200);
}

var count = 0;
function countUp()
{
    document.getElementById("here").innerHTML = count;
    count++;

    if(count == 10)
    {
        clearInterval(interval);
        countDown();
        interval = setInterval(countDown, 200);
    }
}
function countDown()
{
    document.getElementById("here").innerHTML = count;
    count--;

    if(count == 0)
    {
        clearInterval(interval);
        countUp();
        interval = setInterval(countUp, 200);
    }       
}
Answer

try this:

...
<body onload = "onloadFunctions();">

    <script>
        var cup, cdown; // intervals
        var count = 0,
            here  = document.getElementById("here");

        function onloadFunctions() {
            cup = setInterval(countUp, 200);
        }

        function countUp() {
            here.innerHTML = count;
            count++;

            if(count === 10) {
                clearInterval(cup);
                cdown = setInterval(countDown, 200);
            }
        }
        function countDown() {   
            here.innerHTML = count;
            count--;

            if(count === 0) {
                clearInterval(cdown);
                cup = setInterval(countUp, 200);
            }       
        }
    </script>

    From 0 - 9, up and down:&nbsp;&nbsp;<div id = "here"></div>
</body>

you could also create a single reference to #here element. Use always === instead of ==

Answer

There are many ways to solve this problem, the following is my suggestion:

function onloadFunctions() {
    var count = 0;
    var delta = 1;
    var target = document.getElementById("here");
    var step = function() {
        if(count <= 0) delta =  1;
        if(count >= 9) delta = -1;
        count += delta;
        target.innerHTML = count;
        window.setTimeout(step, 500);
    }
    step ();
}

PS: it's safer to use setTimeout than setInteval.

Answer
    /** Tools */
const log = require('ololog').configure({
  locate: false
})

let count = 0
let interval__UP
let interval__DOWN

function countUp () {
  count++
  log.green('countUp(): ', count)

  if (count == 5) {
    clearInterval(interval__UP)
    interval__DOWN = setInterval(function () {
      countDown()
    }, 1000)
  }

}

function countDown () {

  count--
  log.red('countDown(): ', count)

  if (count == 0) {
    clearInterval(interval__DOWN)
    interval__UP = setInterval(function () {
      countUp()
    }, 3000)
  }
}

function start () {
  countUp()
  log.cyan('start()')
  interval__UP = setInterval(function () {
    countUp()
  }, 2000)
}

start()

Console Log shows it's working

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