docs: function randomString(size) {
    if (size === 0) {
      throw new Error('Zero-length randomString is useless.');
    }
    const chars = ('ABCDEFGHIJKLMNOPQRSTUVWXYZ' +
                 'abcdefghijklmnopqrstuvwxyz' +
                 '0123456789');
    let objectId = '';
    const bytes = crypto.randomBytes(size);
    for (let i = 0; i < bytes.length; ++i) {
      objectId += chars[bytes.readUInt8(i) % chars.length];
    }
    return objectId;
  }