return digit mod 10 based on a number

utils.js

/**
 * Author: Weslley Alves <[email protected]>
 * Author URI: http://wezo.com.br
 * Version: 0.0.1
 * License: GNU General Public License
 * License URI: http://www.wezo.com.br/blog/license/gnu-general-public-license/
 *
 * Examples:
 * <script type="text/javascript">
 *
 *  var number = "5432";
 *  var digit = "";
 *  
 *  digit = number.mod10();
 *  // digit return "1"
 *
 *  digit = number.mod10(true);
 *  // digit return "54321"
 *
 *  digit = number.mod10(true , "-");
 *  // digit return "5432-1"
 *  
 *  digit = number.mod10(false , "-");
 *  // digit return "1"
 *
 */

String.prototype.mod10 = function(concatenate , separator){
	var sum = 0;
	var totalDigits = this.length;
	for ( var i = 0; i < totalDigits; i++) {
		sum += this[i] * (totalDigits + 1 - [i]);
	}
	sum = sum % 11;
	sum = 11 - parseInt(sum);
	sum = sum >= 10 ? 0 : sum;
	if(concatenate == false || concatenate == undefined){
		return sum;
	}else{
		
		if(separator == undefined || separator == false){
			separator = "";
		}
		return this + separator + sum;
	}
};