docs: function substring(str, start, end) {
    start = start || 0;
    end   = (end === undefined) ? Infinity : (end || 0);
  
    if (start < 0) {
      start = 0;
    }
    if (end < 0) {
      end = 0;
    }
  
    const length = Math.abs(end - start);
    start = (start < end) ? start : end;
    return substr(str, start, length);
  }