Javascript Array toSentence(connector)

Array.prototype.toSentence = function(connector) {
  connector = connector || 'and';
  
  var sentence = "";
  if (this.length <= 1) { 
    sentence = this[0];// w w w .  j a  v a 2s.  com
  } else {
    //we'll join all but the last error with commas
    var firstErrors = this.slice(0, this.length - 1);
    
    //add the last error, with the connector string
    sentence = String.format("{0} {1} {2}", firstErrors.join(", "), connector, this[this.length - 1]);
  }
  return sentence;
};