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Nancy Melucci
Nancy Melucci
Courses Plus Student 35,157 Points

Creating a recursive timer in the DOM with JavaScript

I am trying to make a JS function that will run a timer (like a clock) from 1 to 12 and then reset it using setInterval. I can't figure out how to make it run. I can toggle from AM to PM using setInterval, and change an image. But not this. Does anyone have a hint? My code is posted below. Just to be clear, the document text needs to change from sequentially to 12, and then reset to 1 and start again. This is why I figure it needs to be recursive...

<!doctype html>
<meta charset="utf-8">
<title>Untitled Document</title>

    <script type="text/javascript">

    function updateTime () {
        var t = document.getElementbyId("output").value;
        t = 1;
        if t < 12;
        setInterval (t++, 200);
        else if (t === 12) {
            t = 1;


    <textarea id="output">1</textarea>

2 Answers

29,445 Points

Solution based on your code:

function updateTime() {

    var time = 1;

    setInterval(function() {
        document.querySelector("#output").value = time;
        if (time > 12) {
            time = 1;
    }, 200);


solution I would go for so you can easily start, stop and customize the timer:

var timer = {
    currentTime: 1,
    startTime: 1,
    maxTime: 12,
    interval: 200,
    target: "#output",
    updateTime: function() {
        document.querySelector(this.target).value = this.currentTime;
        if (this.currentTime > this.maxTime) {
            this.currentTime = this.startTime;
    start: function() {
        this.currentInterval = setInterval(this.updateTime.bind(this), this.interval);
    stop: function() {