code: function longestCommonPathPrefix(stra, strb) {
    const minlen = min(stra.length, strb.length);
    let common = 0;
    let lastSep = -1;
    for (; common < minlen; ++common) {
      const chr = stra[common];
      if (chr !== strb[common]) {
        break;
      }
      if (chr === sep) {
        lastSep = common;
      }
    }
    if (common === minlen &&
        (stra.length === minlen ?
          strb.length === minlen || strb[minlen] === sep :
          stra[minlen] === sep)) {
      return minlen + 1;
    }
    return lastSep + 1;
  }
docs: Longest common prefix that ends on a path element boundary. Each input is assumed to end with an implicit path separator.