validar numeros integrales con javascript

Código Javascript

<script type="text/javascript">
function IsInt( numstr, allowNegatives ) {
	// Return immediately if an invalid value was passed in
	if (numstr+"" == "undefined" || numstr+"" == "null" || numstr+"" == "")	
		return false;
	// Default allowNegatives to true when undefined or null
	if (allowNegatives+"" == "undefined" || allowNegatives+"" == "null")	
		allowNegatives = true;
	var isValid = true;
	// convert to a string for performing string comparisons.
	numstr += "";	
	// Loop through string and test each character. If any
	// character is not a number, return a false result.
 	// Include special case for negative numbers (first char == '-').   
	for (i = 0; i < numstr.length; i++) {
    	if (!((numstr.charAt(i) >= "0") && (numstr.charAt(i) <= "9") || (numstr.charAt(i) == "-"))) {
       	isValid = false;
       	break;
		} else if ((numstr.charAt(i) == "-" && i != 0) || 
				(numstr.charAt(i) == "-" && !allowNegatives)) {
       	isValid = false;
       	break;
      }
         	         	       
   } // END for   
   
   	return isValid;
}  // end IsInt