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Yehuda Shiran September 11, 2000
Detecting a Modifier Key
Tips: September 2000

Yehuda Shiran, Ph.D.
Doc JavaScript

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 Suppose you want to prevent the user from using the Shift key on your site. The Shift, Alt, and CTRL keys are called modifiers. In Internet Explorer, there is a boolean property for each modifier:

  • event.shiftKey
  • event.altKey
  • event.ctrlKey

You can detect if a modifier was pressed by simply checking if the appropriate property is true. Unlike Internet Explorer 4.0x and above, Navigator 4.0x's modifier keys are reflected in one property: modifiers. This property must be tested against a constant property of the Event object with the bitwise AND operator in order to find out which modifier keys were pressed when the event occurred. For example, the expression:

e.modifiers & Event.ALT_MASK

evaluates to a Boolean value indicating the state of the Alt key when the event occurred. The four masks are:

  • SHIFT_MASK
  • ALT_MASK
  • CONTROL_MASK
  • META_MASK

The following code is included in the current page:


<BODY ... onKeyPress="handlePress(event)">
<SCRIPT LANGUAGE="JavaScript">
<!--
function handlePress(e) {
  var shiftPressed = (window.Event) ? e.modifiers & Event.SHIFT_MASK : e.shiftKey;
  if (shiftPressed) {
    alert("The Shift Key is not supported on this site");
	return false;
  }
  else return true;
}
// -->
</SCRIPT>

Notice how we detect the browser: Netscape Navigator supports window.Event while Internet Explorer does not. Try now to type upper-case and lower-case character. Lower-case letter go unnoticed. Upper case characters trigger a proper alert box.

Learn more about the event model in Column 11, The Cross-Browser Event Model.

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