A Cross-Browser Solution for Handling JavaScript Events

It all starts with attaching event listeners to elements. Say you have a button that increments a counter each time you click it. You can add an inline onclick attribute and that will work across all browsers but will be violating the separation of concerns and the progressive enhancement. So you should strive for attaching the listener in JavaScript, outside of any markup.

Say you have the following markup:

<button id="clickme">Click me: 0</button>

You can assign a function to the onclick property of the node, but you can do this only once:

var b = document.getElementById('clickme'), count = 0;
b.onclick = function()
   count += 1;
   b.innerHTML = "Click me: " + count;

If you want to have several functions executed on click, you cannot do it with this pattern while maintaining loose coupling. Technically you can check if onclick already contains a function and if so, add the existing one to your own function and replace the onclick value with your new function. But a much cleaner solution is to use the addEventListener() method. This method doesn't exist in IE up to and including version 8, so you need attachEvent() for those browsers.

When we looked at the init-time branching pattern (Chapter 4) you saw an example of implementing a good solution for defining a cross-browser event listener utility. Without going into all the details right now, let's just attach a listener to our button:

var b = document.getElementById('clickme');
   b.addEventListener('click', myHandler, false);
else if(document.attachEvent)
   b.attachEvent('onclick', myHandler);
   b.onclick = myHandler;

Now when this button is clicked, the function myHandler() will be executed. Let's have the function increment the number in the "Click me: 0" button label. To make it a little more interesting, let's assume we have several buttons and a single myHandler() for all of them. Keeping a reference to each button node and a counter for the number will be inefficient, given that we can get that information from the event object that is created on every click.

Let's see the solution first and comment on it after:

function myHandler(e)
   var src, parts;

   e = e || window.event;
   src = || e.srcElement;

   parts = src.innerHTML.split(": ");
   parts[1] = parseInt(parts[1], 10) + 1;
   src.innerHTML = parts[0] + ": " + parts[1];

   if(typeof e.stopPropagation === "function")
   if(typeof e.cancelBubble !== "undefined")
      e.cancelBubble =  true;

   if(typeof e.preventDefault === "function")
   if(typeof e.returnValue !== "undefined")
      e.returnValue = false;

There are four parts in the event handler function:

First, we gain access to the event object, which contains information about the event and the page element that triggered that event. This event object is passed to the callback event handler, but not when using the onclick property where it's accessible through the global property window.event instead.

The second part is doing the actual work of updating the label.

Next is canceling the propagation of the event. This is not required in this particular example, but in general if you don't do it, than the event bubbles up all the way to the document root or even the window object. Again we need to do it two ways: the W3C standard way (stopPropagation()) and then differently form IE (using cancelBubble).

Finally, prevent the default action, if this is required. Some events (clicking a link, submitting a form) have default actions, but you can prevent them by using preventDefault() (or for IE, by setting returnValue to false).

As you can see there's quite a bit of duplicate work involved, so it makes sense to create your event utility with facade methods as discussed in Chapter 7.

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