Welcome to the Treehouse Community

The Treehouse Community is a meeting place for developers, designers, and programmers of all backgrounds and skill levels to get support. Collaborate here on code errors or bugs that you need feedback on, or asking for an extra set of eyes on your latest project. Join thousands of Treehouse students and alumni in the community today. (Note: Only Treehouse students can comment or ask questions, but non-students are welcome to browse our conversations.)

Looking to learn something new?

Treehouse offers a seven day free trial for new students. Get access to thousands of hours of content and a supportive community. Start your free trial today.


Nancy Melucci
Nancy Melucci
Front End Web Development Techdegree Student 34,543 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() {