JavaScript uses so-called "event handlers" as a trigger for certain actions or functions to occur. For instance, a web developer may embed a JavaScript function in a web page that automatically checks the content of a form for completeness or accuracy. An event handler associated with a "submit" button can be used to trigger the function before the form is actually submitted to the server for processing. The advantage for the government agency is that it saves government resources by not requiring the government's server to do the initial checking. The advantage for the computer user is that feedback about errors is almost instantaneous because the user is told about the error before the information is even submitted over the Internet.

Web developers must exercise some caution when deciding which event handlers to use in their web pages, because different screen readers provide different degrees of support for different event handlers. The following table includes recommendations for using many of the more popular event handlers:

• onClick – The onClick event handler is triggered when the user clicks once on a particular item. It is commonly used on links and button elements and, used in connection with these elements, it works well with screen readers. If clicking on the element associated with the onClick event handler triggers a function or performs some other action, developers should ensure that the context makes that fact clear to all users. Do not use the onClick event handlers for form elements that include several options (e.g. select lists, radio buttons, checkboxes) unless absolutely necessary.
• onDblClick – The onDblClick event handler is set off when the user clicks twice rapidly on the same element. In addition to the accessibility problems it creates, it is very confusing to users and should be avoided.
• onMouseDown and onMouseUp – The onMouseDown and onMouseUp event handlers each handle the two halves of clicking a mouse while over an element – the process of (a) clicking down on the mouse button and (b) then releasing the mouse button. Like onDblClick, this tag should be used sparingly, if at all, by web developers because it is quite confusing. In most cases, developers should opt for the onClick event handler instead of onMouseDown.
• onMouseOver and onMouseOut – These two event handlers are very popular on many web sites. For instance, so-called rollover gif's, which swap images on a web page when the mouse passes over an image, typically use both of these event handlers. These event handlers neither can be accessed by the mouse nor interfere with accessibility – a screen reader simply bypasses them entirely. Accordingly, web designers who use these event handlers should be careful to duplicate the information (if any) provided by these event handlers through other means.
• onLoad and onUnload – Both of these event handlers are used frequently to perform certain functions when a web page has either completed loading or when it unloads. Because neither event handler is triggered by any user interaction with an element on the page, they do not present accessibility problems.
• onChange – This event handler is very commonly used for triggering JavaScript functions based on a selection from within a <select> tag. Surprisingly, it presents tremendous accessibility problems for many commonly used screen readers and should be avoided. Instead, web developers should use the onClick event handler (associated with a link or button that is adjacent to a <select> tag) to accomplish the same functions.
• onBlur and onFocus – These event handlers are not commonly used in web pages. While they don't necessarily present accessibility problems, their behavior is confusing enough to a web page visitor that they should be avoided.

Sample Code

<SCRIPT LANGUAGE = "JavaScript">

<!--

var isIMG = document.images;
var bon;
var boff;

    if (isIMG) {
      bon = new Image(); 
      boff = new Image();   
      bon.src = 'Images/on.gif';
      boff.src = 'Images/off.gif';
    }

    function b(bname) {
        if (isIMG) {
          document[bname].src = (document[bname].src.indexOf('Images/on.gif') != -1) ? 'Images/off.gif' : 'Images/on.gif';
        }
    }
//-->
</SCRIPT>
<NOSCRIPT>
This is a rollover script that makes a red arrow appear to the left of
a primary link item when the cursor is placed over the link.  As the cursor leaves an option,
the red arrow is replaced with a blank graphic.
</NOSCRIPT>

Working Example

Rule A Rule B Rule C