A Javascript Timer that you can bind to any DOM object

View the Project on GitHub pchiwan/ko-Timer


Need to display a timer on your page? I'd say it's a pretty common necessity. And it's certainly easy to implement your own timer; plus there are many ways to accomplish this! There's only one constant: you will have to refresh it every second, which means changing/updating the DOM. So I thought why not do it with KnockoutJS: an observable will take care of updating the DOM for me.

Here it is then, a really simple Timer implemented with KnockoutJS (which means you WILL NEED to include KnockoutJS to your project in order for the timer to work).

NOTE: Further versions will also provide a chrono mode -whereas, as of now, the timer only provides a countdown mode-.

How does it work?

It's extremely easy! You instantiate a timer object like this.

var timer = new koTimer(300); 

Just like that, and your timer is already running! The first parameter is the time limit in seconds (so, in this example, 300 seconds = 5 minutes). If you don't provide it the default time limit is 60 seconds.


You can further configure the timer by providing it with an options dictionary upon instantiation. The available options are as follows.


The timer will start running upon instantiation unless you tell it otherwise. If you send wait: true the timer will not start running until you call its start method. False by default.


This should be a Knockout observable, a Knockout computed observable or a delegate function that will be used to check whether the timer must keep going or stop after each tick.


Optional callback function that will be executed when the timer's countdown reaches zero.


Optional array of integer time marks (in seconds) that must trigger a timeMarkHit event from the timer when hit.


As of now, the timer only has two methods.


Starts the timer. If the timer had been stopped, the countdown will pick up from where it left off.


Stops the timer.


Starts the timer if it's stopped, or stops it if it's running.


Resets the timer. You can optionally provide a parameter with the new time limit; otherwise the time limit provided on instatiation will be used. The timer will stop running when reset, if you want it to automatically start running again pass true as the second parameter. I.e.:

var newTimeLimit = 240;

//timer is reset with new time limit and starts running right away
timer.reset(newTimeLimit, true); 


The koTimer notifies some events that you can bind to.


This event is triggered when the timer's countdown reaches zero.


This event is triggered when the timer is stopped because the keepGoing condition wasn't met. If you stop the timer by calling the stop method this event is not triggered.


Whenever a time mark provided in the notifyTimeMarks array is hit, this event is triggered. The event data includes a property called timeElapsed, the name of which speaks for itself. What could this be useful for? Keep reading and find out.


The koTimer has the following properties exposed for you.


A computed observable which tells you how much time has already gone by, in seconds.


A computed observable which returns a nicely formatted string representing the time that's already gone by.


A computed observable which returns a nicely formatted string representing the time that's left.


A computed observable which returns true if the timer is running, and false otherwise.


A computed observable which returns true if the timer's countdown has reached zero, and false otherwise.


You don't need to memorize the names of the events you want to bind to, you can access the timer's events dictionary.

But really, how do I use it?

Yeah, I guess by now you're asking yourself just that. The idea is that you bind your DOM to the timer's exposed properties, and the magic of KnockoutJS will deal with updating the DOM as the countdown goes on. Have a look.

<div class="main">    
    <div id="timer">
        <span data-bind="text: TimeLeftStr"></span>

And here's the code that accompanies that html.

var continueCondition = ko.observable(true); //the timer will stop when this is false

var timer = new koTimer(300, {
    wait: true,
    keepGoing: continueCondition, 
    callback: function () {
        console.log('Timer reached zero.');
    notifyTimeMarks: [150] //notify event when second 150 is reached

$(document).on(timer.Events.TimeMarkHit, function (ev) {
    if (ev.timeElapsed === 150) {
        console.log('Halfway there');

//apply ko binding
ko.applyBindings(timer, $('#timer')[0]);

//start timer

Get it? Piece of cake, right?


Behold! A collection of running timers!

And these are some examples I created on jsfiddle which you can play with.

Example 1

Example 2