code: function checkSkip (skip, keys) {
    if (skip && typeof skip === 'string') skip = +skip;
    if (!skip || Number(skip) !== skip || skip % 1 !== 0 ||
        skip < 0)
      skip = 0;
    if (skip > keys.length) skip = keys.length;
    return skip;
  }
docs: Check that the skip parameter is withing range, or set defaults