function strncmp (str1, str2, lgth) {
  // http://kevin.vanzonneveld.net
  // +      original by: Waldo Malqui Silva
  // +         input by: Steve Hilder
  // +      improved by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
  // +       revised by: gorthaur
  // + reimplemented by: Brett Zamir (http://brett-zamir.me)
  // *     example 1: strncmp('aaa', 'aab', 2);
  // *     returns 1: 0
  // *     example 2: strncmp('aaa', 'aab', 3 );
  // *     returns 2: -1
  var s1 = (str1 + '').substr(0, lgth);
  var s2 = (str2 + '').substr(0, lgth);

  return ((s1 == s2) ? 0 : ((s1 > s2) ? 1 : -1));
}

strncmp('aaa', 'aab', 2);

0

strncmp('aaa', 'aab', 3 );

-1