Queue.js

function Queue(){
  // initialise the queue and offset
  this.queue  = [];
  this.offset = 0;
}
  // Returns the length of the queue.
Queue.prototype.getLength = function(){
    return (this.queue.length - offset);
}

// Returns true if the queue is empty, and false otherwise.
Queue.prototype.isEmpty = function(){
    return (this.queue.length == 0);
  }

  /* Enqueues the specified item. The parameter is:
   *
   * item - the item to enqueue
   */
Queue.prototype.enqueue = function(item){
    this.queue.push(item);
  }

  /* Dequeues an item and returns it. If the queue is empty, the value
   * 'undefined' is returned.
   */
Queue.prototype.dequeue = function(){

    // if the queue is empty, return immediately
    if (this.queue.length == 0) return undefined;

    // store the item at the front of the queue
    var item = this.queue[this.offset];

    // increment the offset and remove the free space if necessary
    if (++ this.offset * 2 >= this.queue.length){
      this.queue  = this.queue.slice(this.offset);
      this.offset = 0;
    }

    // return the dequeued item
    return item;

  }

  /* Returns the item at the front of the queue (without dequeuing it). If the
   * queue is empty then undefined is returned.
   */
Queue.prototype.peek = function(){
    return (this.queue.length > 0 ? this.queue[this.offset] : undefined);
  }