miscoded

Why User JavaScript events?

Any script can ask Opera to be notified when a specific event occurs, for instance a mouse click on a button. By using the special User JavaScript events your user script can ask Opera to be notified in advance of the page script. The user script can then either cancel the event, change the event or allow the page script to receive it unchanged.

Opera also can notify the user script about special events that normal scripts can not detect, such as when an external JavaScript library will be loaded.


How to use User JavaScript event handling

A user JavaScript can add event listeners to the window.opera object with the command window.opera.addEventListener(). The following new events are available to allow the user script to enhance and control the environment:


BeforeExternalScript
This event takes place before Opera loads an external JavaScript file.

BeforeScript / AfterScript
These events happen before and after any SCRIPT tag is ran.

BeforeEvent / AfterEvent
BeforeEventListener / AfterEventListener
These are 'meta-events' you can use to either be notified in advance or after about every event taking place in the document, or every event that the scripts in the page will use.

To the 'meta-events' you can add .TYPE to detect specific events only, for instance BeforeEventListener.load . This is the preferred way of using these events because a generic handler will be called a lot - for instance every time you move the mouse.

Cancelling events with event.preventDefault() in the event handler stops the default action. If you cancel 'Before' script notification events, scripts will not be loaded or ran. If you cancel 'meta-events' the page script will not receive the real events. (If this explanation is too confusing see the examples! )

Writing User JavaScript event handlers

The event handler function receives an object with information about the event. The function can check various properties of this object to get information about the event. The element property tells you what element the event is happening to, and the event property is the event object that will be passed to the page's script.

Examples
1
To prevent the script on a page from running its onload handler you could add the following to a User JavaScript:
opera.addEventListener('BeforeEventListener.load', function(ev){ ev.preventDefault(); }, false);

2
Say you have a form with an onsubmit handler that prevents Opera from sending the form:
<form action=''http://example.com'' onsubmit=''return false;''>

The event.preventDefault() in the following example will cancel the return false from the page's script because the page's onsubmit event handler will not be called:
opera.addEventListener('BeforeEvent.submit', function(ev){ ev.preventDefault(); }, false);

3
The following user script will fix a buggy DHTML script on <http://www.thegreenbutton.com/downloads.aspx> by rewriting the script source code with a regular expression:

if(window.location.href.indexOf('thegreenbutton.com')>-1){

window.opera.addEventListener( 'BeforeScript', function (ev){ev.element.text = ev.element.text.replace(/!=
null/g,''); },false);

}