function substr (str, start, len) {
    str += '';
    var end = str.length;
    if (start < 0) {        start += end;}
    end = typeof len === 'undefined' ? end : (len < 0 ? len + end : len + start);
    return start >= str.length || start < 0 || start > end ? !1 : str.slice(start, end);
}