Lets say you have a web application that needs to track how long an AJAX request took. If you were writing it in Ruby, PHP or Python you’d have a timer object that you can start() and stop() and a seconds() method to get the number of seconds that passed between calling the first two methods.

To build this in JavaScript we first create our constructor:

Timer = function() {
  var seconds_passed = 0;
}

Here we are creating a Timer object with a private seconds_passed variable. To create this object we we simply create a new timer:

var elapsed_timer = new Timer();

This timer object now needs some public methods. We can attach a public method by using this.method_name:

Timer = function() {
  var seconds_passed = 0;

  this.start = function() {
    // implementation here
  }
}

Our Timer class now has a start() function. Additional public methods can be added in the same way.

To add a private method, don’t define it with the this keyword but instead create a regular function inside the class:

Timer = function() {
  function increment() {
    // implementation of private method here
  }
}

Now we have a Timer object with a public start method and a private increment method. The rest of the class is fairly simple, so lets jump to the finished solution.

Complete Example

Timer = function() {
  var timeout_id = null;
  var seconds_passed = 0;

  this.start = function() {
    increment();
  }

  this.stop = function() {
    clearTimeout(timeout_id);
    timeout_id = null;
  }

  this.seconds = function() {
    return seconds_passed;
  }

  function increment() {
    seconds_passed++;

    timeout_id = window.setTimeout(
      function() {
        increment();
      }
      , 1000
    );
  }
}

var elapsed_timer = new Timer();
elapsed_timer.start();

// do some stuff

elapsed_timer.stop();
alert(elapsed_timer.seconds());

There you have it. A simple JavaScript timer to keep track of time.