validar caracteres alfannmericos con javascript

Código Javascript

<script type="text/javascript">
function IsAlphaNum( str ) {
	// Return immediately if an invalid value was passed in
	if (str+"" == "undefined" || str+"" == "null" || str+"" == "")	
		return false;
	var isValid = true;
	
	// convert to a string for performing string comparisons.
   	str += "";	
	// Loop through length of string and test for any alpha numeric 
	// characters
   	for (i = 0; i < str.length; i++)
   	{
			// Alphanumeric must be between "0"-"9", "A"-"Z", or "a"-"z"
      	if (!(((str.charAt(i) >= "0") && (str.charAt(i) <= "9")) || 
      			((str.charAt(i) >= "a") && (str.charAt(i) <= "z")) ||
      			((str.charAt(i) >= "A") && (str.charAt(i) <= "Z"))))
			{
				isValid = false;
				break;
			}	
   	} // END for   
   
   	return isValid;
}  // end IsAlphaNum
</SCRIPT>