Queue.js is a simple and efficient queue implementation for JavaScript whose dequeue function runs in amortised constant time. As a result, for larger queues it can be significantly faster than using arrays.

/*

Queue.js

A function to represent a queue

Created by Stephen Morley - http://code.stephenmorley.org/ - and released under
the terms of the CC0 1.0 Universal legal code:

http://creativecommons.org/publicdomain/zero/1.0/legalcode

*/

/* Creates a new queue. A queue is a first-in-first-out (FIFO) data structure -
 * items are added to the end of the queue and removed from the front.
 */
function Queue(){

  // initialise the queue and offset
  var queue  = [];
  var offset = 0;

  /* Returns the length of the queue.
   */
  this.getLength = function(){

    // return the length of the queue
    return (queue.length - offset);

  }

  /* Returns true if the queue is empty, and false otherwise.
   */
  this.isEmpty = function(){

    // return whether the queue is empty
    return (queue.length == 0);

  }

  /* Enqueues the specified item. The parameter is:
   *
   * item - the item to enqueue
   */
  this.enqueue = function(item){

    // enqueue the item
    queue.push(item);

  }

  /* Dequeues an item and returns it. If the queue is empty then undefined is
   * returned.
   */
  this.dequeue = function(){

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

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

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

    // return the item at the front of the queue
    return (queue.length > 0 ? queue[offset] : undefined);

  }

}