Event Handlers

Note that event handlers, which are created by global on... event attributes, are not dispatched during the capture phase. They are only dispatched during the at target phase and bubbling phase.

onerror Event Handlers

When an error event is propagated to a node with an onerror event handler, the JavaScript code in the onerror attribute is invoked. In the onerror event handler code, an arguments array provides access to values from the event constructor and the event variable provides access to other details of the event and a boolean return value:

Parameters:

arguments[0] - the message being reported

arguments[1] - absolute URL of the resource (document) in which the error occurred

arguments[2] - in that resource, the line number where the error occurred

Return value:

event.returnValue = false if the error has been handled or event.returnValue = true if not

onmouseover Event Handlers

When a mouseover event is propagated to a node with an onmouseover event handler, the JavaScript code in the onmouseover attribute is invoked. In the onmouseover event handler code, the event variable provides access to details of the event and a boolean return value:

Parameters:

event - the event

Return value:

event.returnValue = true if the event is to be cancelled or event.returnValue = false if not

Note that the return value for mouseover event handlers is the opposite from the return value for other event handlers (below).

onbeforeunload Event Handlers

When a beforeUnload event is propagated to a node with an onbeforeunload event handler, the JavaScript code in the onbeforeunload attribute is invoked. In the onbeforeunload event handler code, the event variable provides access to details of the event and a return value that contains a dialog prompt string:

Parameters:

event - the event

Return value:

event.returnValue = "Prompt text" to display a prompt for the user to confirm leaving the page or event.returnValue = "" if not

Other Event Handlers

When any other event is propagated to a node with an event handler, the JavaScript code in the event handler attribute for the event type is invoked. In the event handler code, the event variable provides access to details of the event and a boolean return value:

Parameters:

event - the event

Return value:

event.returnValue = false if the event is to be cancelled or event.returnValue = true if not

onclick Event Handler Examples

event.returnValue=false cancels almost any event:

Clicking on this link does not load the href page because the event is cancelled with onclick="event.returnValue=false".

<p>Clicking on <a href="http://www.ExampleOnly.com/" title="Just a tool tip"
   onclick="alert('clicked'); event.returnValue=false">this link</a> does <em>not</em> load the
   href page because the event is cancelled with <code>onclick="event.returnValue=false"</code>.
</p>

event.preventDefault() is another way to cancel an event:

Clicking on this link does not load the href page because the event is cancelled with onclick="event.preventDefault()".

<p>Clicking on <a href="http://www.ExampleOnly.com/" title="Just a tool tip"
   onclick="alert('clicked'); event.preventDefault()">this link</a> does <em>not</em> load the
   href page because the event is cancelled with <code>onclick="event.preventDefault()"</code>.
</p>

Using event.preventDefault() is the recommended method because it is more browser independent. There are some HTML 5 web browsers, such as Firefox 3, where the event.returnValue=false method does not cancel the event and the browser loads the page referenced by the <a href> attribute after the alert box is dismissed.

onmouseover / onmouseout Event Handler Examples

Without stopPropagation(), events on the inner element propagate to the outer element, changing both to a lighter color at the same time:

Try it:

<div style="width: 90px; height: 90px; margin: auto; border: 1px solid black; padding: 0; background-color: blue"
   onmouseover="this.style.backgroundColor = 'lightblue'"
   onmouseout="this.style.backgroundColor = 'blue'"
>
<p style="width: 40px; height: 40px; margin: 0; border: 1px solid black; padding: 0; background-color: green"
   onmouseover="this.style.backgroundColor = 'lightgreen'"
   onmouseout="this.style.backgroundColor = 'green'"
> </p>
</div>

With stopPropagation(), events on the inner element do not affect the outer element so only one changes to a lighter color at a time:

Try it:

<div style="width: 90px; height: 90px; margin: auto; border: 1px solid black; padding: 0; background-color: blue"
   onmouseover="this.style.backgroundColor = 'lightblue'; event.stopPropagation()"
   onmouseout="this.style.backgroundColor = 'blue'; event.stopPropagation()"
>
<p style="width: 40px; height: 40px; margin: 0; border: 1px solid black; padding: 0; background-color: green"
   onmouseover="this.style.backgroundColor = 'lightgreen'; event.stopPropagation()"
   onmouseout="this.style.backgroundColor = 'green'; event.stopPropagation()"
> </p>
</div>