Question

When a page finishes loading, the "load" event fires on the window and the document body objects. This is often used to schedule initialization actions that require the whole document to have been built. Remember that the content of <script> tags is run immediately when the tag is encountered. This is often too soon, such as when the script needs to do something with parts of the document that appear after the <script> tag.

Elements such as images and script tags that load an external file also have a "load" event that indicates the files they reference were loaded. Like the focus-related events, loading events do not propagate.

When a page is closed or navigated away from (for example by following a link), a "beforeunload" event fires. The main use of this event is to prevent the user from accidentally losing work by closing a document. Preventing the page from unloading is not, as you might expect, done with the preventDefault method. Instead, it is done by returning a string from the handler. The string will be used in a dialog that asks the user if they want to stay on the page or leave it. This mechanism ensures that a user is able to leave the page, even if it is running a malicious script that would prefer to keep them there forever in order to force them to look at dodgy weight loss ads.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.