Professional JavaScript For Web Developers

Chapter 9
Can you still assign an event handler to document.body.onload? Yes, you can. The problem is that
document.body doesn’t exist until the page has loaded the tag. This means that if you try to
assign the event handler in the element, where it should be done, you get an error. Try it for
yourself:
Onload Example
document.body.onload = function () {
alert(“loaded”);
}
If you run this code, you get an error saying that document.body isn’t defined. So, it is always best to
assign the onload event handler to the window object.
The unload event can be handled the same way, either by assigning the event handler to the window
object or by assigning it in the element. The unload event fires when you navigate from one
page to another (by clicking a link or using the Back/Forward buttons) or when you close the browser
window:
OnUnload Example
You have very short amount of time in which to execute the event handler code
before the window is closed or the next page takes control, so it’s usually best to
avoid using an onunload event handler. The best reason to use onunload is to dereference
objects that were used on the page; any functionality more complicated than
this should be avoided.
The resize event
At times your Web page may change depending on the size of the browser window. For this case, you
can use the resize event to determine when to change these dynamic elements.
Similar to the load and unload events, the event handler for the resize event must be assigned either to
the window object using JavaScript code or to the element in HTML:
OnResize Example
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