code: function subArrayJoin(a, separator, from, to) {
      return a.slice(from, to + 1).join(separator);
  }
docs: Join part of array.
  
  @param {Array} a Array to join.
  @param {String} separator Separator to use in join.
  @param {Number} from Join start index.
  @param {Number} to Join finish index.
  @returns {String} Joined array string.