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Window onload Event

By Stephen Bucaro

The onload event executes immediately after the webpage finishes loading. You should never write code that attempts to access elements on a document before the page completes loading because the element may not be created yet. The onload event allows you to access webpage elements immediately after all the coded elements have been created.

Originally the onload event was called in the body tag and it was usually used to execute some initialization code as shown below.

<head>
<script language="JavaScript">

function init()
{
   ...
}

</script>
</head>
<body onload="init()">

Today programmer's frown on calling the onload event in the body tag because onload is technically a window event not specifically related to the body element.

Now, the onload event is usually called near the end of a Java Script code block in the head section of the webpage as shown below.

<head>
<script language="JavaScript">

function init()
{
   ...
}

window.onload = init;

</script>
</head>

The problem with the above method is that it allows only one function to be called by the onload event. The method shown below uses addEventListener which allows you to add multiple event listeners to the onload event.

if (window.addEventListener)	// Netscape/Firefox/Opera
{  
   window.addEventListener("load", function1, false);
   window.addEventListener("load", function2, false);
}
else if (window.attachEvent) 	 // IE
{
   window.attachEvent("onload", function1);
   window.attachEvent("onload", function2);
}

However Internet Explorer does not recognize addEventListener, it uses attachEvent instead, and the code above shows you how to make your code compatible with both Firefox and Internet Explorer.

In the last argument of addEventListener, true means you want to use event capturing and false means you want to use event bubbling. To make a complex subject simple, when you execute an event on a element that is nested inside another element (and this is always the case because all elements are nested inside the body element) and multiple elements have event handlers for the same event, event capturing causes the outer element to execute its event handler first, bubbling causes the inner element to execute its event handler first. Internet Explorer supports only event bubbling.

More Java Script Code:
• The Document Object Model (DOM)
• The while Loop
• The Navigator Object
• Java Script Strings
• The Java Script window.open Method
• A Brief History of JavaScript
• Java Script confirm Message Box
• The switch / case Structure
• The Location Object
• Generating Random Numbers