概要
A possible way to get notified of Events of a particular type (such as click) for a given object is to specify the event handler using:
- An HTML attribute named
on{eventtype}on an element, for example:<button onclick="return handleClick(event);">, - or by setting the corresponding property from JavaScript, for example:
document.getElementById("mybutton").onclick = function(event) { ... }.
If you're working with a modern browser, use addEventListener() instead.
The available events are documented in the events reference.
Details
Definitions
The term event handler may refer to:
- any function or object registered to be notified of events,
- or, more specifically, to the mechanism of registering event listeners via
on...attributes or properties in HTML and other web APIs, such as<button onclick="alert(this)">orwindow.onload = function() { /* ... */ }.
When discussing the various methods of listening to events,
- event listener refers to a function or object registered via
EventTarget.addEventListener(), - whereas event handler refers to a function registered via
on...attributes or properties.
This page is about the specifics of registering event handlers using on... attributes and properties.
Objects that support event handlers
Event handlers on HTML elements can be set using attributes starting with "on" in the HTML markup (<button onclick="...">) or using Element.setAttribute, and also via the Element's properties named "on..." using JavaScript (i.e. element.onclick = function() { ... }).
Event handlers can also be set using properties on many non-element objects that generate events, including window, document, XMLHttpRequest, and others.
For historical reasons, some attributes/properties on the <body> and <frameset> elements actually set event handlers on their parent Window object. (The HTML specification names these: onblur, onerror, onfocus, onload, onscroll.)
Event handler's parameters, this binding, and the return value
TBD
When the event handler is invoked
TBD (non-capturing listener)
Specifications
| Specification | Status | Comment |
|---|---|---|
| WHATWG HTML Living Standard event handlers |
Living Standard | |
| HTML5 event handlers |
Candidate Recommendation |
Browser compatibility
| Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari (WebKit) |
|---|---|---|---|---|---|
| Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |
| Feature | Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
|---|---|---|---|---|---|
| Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |
Event handler changes in Firefox 9
In order to better match the specifications, and improve cross-browser compatibility, the way event handlers were implemented at a fundamental level changed in Gecko 9.0 (Firefox 9.0 / Thunderbird 9.0 / SeaMonkey 2.6).
Specifically, in the past, event handlers were not correctly implemented as standard IDL attributes. In Gecko 9.0, this was changed. Because of this, certain behaviors of event handlers in Gecko have changed. In particular, they now behave in all the ways standard IDL attributes behave. In most cases, this shouldn't affect web or add-on content at all; however, there are a few specific things to watch out for.
Detecting the presence of event handler properties
You can now detect the presence of an event handler property (that is, for example, onload), using the JavaScript in operator. For example:
if ("onsomenewfeature" in window) {
/* do something amazing */
}
Event handlers and prototypes
You can't set or access the values of any IDL-defined attributes on DOM prototype objects; that means you can't, for example, change Window.prototype.onload anymore. In the past, event handlers (onload, etc) weren't really implemented as IDL attributes in Gecko, so you were able to do this for those. Now you can't. This improves compatibility.