gistfile1.txt

/**
   * Find next available server number
   *
   * @param servers
   *
   * @return {int}
   */

  function next_server_number( servers ) {
    'use strict';

    if ( ! servers || ! servers.length ) {
      return 1;
    }

    var availCount = 1;
    var available;

    servers = servers.sort( function  ( a, b ) {
      return a - b;
    });

    servers.some( function  ( el ) {

      if ( el != availCount ) {
        return true;
      }

      availCount++;
    });

    return available ? available : availCount;
  }

  console.log( next_server_number( [5, 3, 1] ) );
  console.log( next_server_number( [5, 4, 1, 2] ) );
  console.log( next_server_number( [3, 2, 1] ) );
  console.log( next_server_number( [2, 3] ) );
  console.log( next_server_number( [] ) );