function strspn (str1, str2, start, lgth) {
  // http://kevin.vanzonneveld.net
  // +   original by: Valentina De Rosa
  // +   improved by: Brett Zamir (http://brett-zamir.me)
  // *     example 1: strspn('42 is the answer, what is the question ...', '1234567890');
  // *     returns 1: 2
  // *     example 2: strspn('foo', 'o', 1, 2);
  // *     returns 2: 2
  var found;
  var stri;
  var strj;
  var j = 0;
  var i = 0;

  start = start ? (start < 0 ? (str1.length + start) : start) : 0;
  lgth = lgth ? ((lgth < 0) ? (str1.length + lgth - start) : lgth) : str1.length - start;
  str1 = str1.substr(start, lgth);

  for (i = 0; i < str1.length; i++) {
    found = 0;
    stri = str1.substring(i, i + 1);
    for (j = 0; j <= str2.length; j++) {
      strj = str2.substring(j, j + 1);
      if (stri == strj) {
        found = 1;
        break;
      }
    }
    if (found != 1) {
      return i;
    }
  }

  return i;
}

strspn('42 is the answer, what is the question ...', '1234567890');

2

strspn('foo', 'o', 1, 2);

2