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<Understanding Keyboard Event Data ⁄ >




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If you examine the Events module of the DOM Level 2 recommendation, you may notice that keyboard events are nowhere to be found. As the W3C working group discovered, keyboard events in a Unicode world are tricky things to mold into an acceptable standard. But keyboard events in one form or another have been implemented in browsers since Netscape 4 and IE 4. Even though the Version 6 browsers carry forward the concepts from earlier days, the drafts of future W3C DOM Level 3 Events module indicate that keyboard event processing may take on new syntax in the future. In the meantime, we have "old-fashioned" keyboard events, with processing that is not always straightforward due to differences in event object details across browser versions.

The most important data related to a keyboard event is the identity of either the physical key being activated or the character generated by that key. These are not the same things. Every key has a numeric code associated with it. For example, a U.S. English keyboard assigns the number 65 to the key labeled A. That same key, however, can produce at least two different characters (A and a) on every U.S. computer, and even more characters on operating systems like the Macintosh (where the Option and Option-Shift modifier keys let that A key generate even more characters).

It so happens that the code "65" is also the ASCII and Unicode value of the uppercase A letter (this isn't a coincidence as much as it reflects the English-centric basis of early computing). The character codes for the A and a characters are 65 and 97, respectively. For some scripting tasks, the character code is important--such as whether the character is a numeral, regardless of whether the user pressed a top row keyboard key or a numeric keypad key; for other tasks, the key pressed is important--such as whether the user pressed the PageDown key (which doesn't have a character associated with it).

Complicating the issue is that the IE event object has only one property value that conveys a code for a keyboard event (event.keyCode). IE 5/Mac and Netscape 6 have a second property (charCode) that conveys additional event information. To expose both the key and character codes, even with only one property, the browsers let the different keyboard events deliver different information. To read the key code, use only the onkeydown or onkeyup events; use the onkeypress event to read the character code.



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