Standard FIFO Queue, implemented with two stacks.

Queue.js

/**
 * Standard FIFO Queue, implemented with two stacks.
 * @typedef {Object} Queue
 * @property {function} enqueue - add an item to the back of the queue
 * @property {function} dequeue - remove an item from the front of the queue
 * @property {function} isEmpty - check whether or not the queue is empty
 */

/**
 * Standard FIFO Queue, implemented with two stacks.
 * @constructor
 */
function Queue() {
   // inbox
   var en = [];
   // outbox
   var de = [];

   /**
    * Add an item to the back of the queue.
    * @param {Object} o - the object to add
    */
   this.enqueue = function (o) {
      en.push(o);
   };

   /**
    * Remove and item from the front of the queue.
    * @returns {Object} the first item in the queue
    */
   this.dequeue = function () {
      // if there's nothing in the outbox, flip the inbox
      if (de.length === 0)
         while (en.length !== 0)
            de.push(en.pop());
      return de.pop();
   };

   /**
    * Check whether or not the queue is empty.
    * @returns {boolean} true if the queue is empty, false otherwise
    */
   this.isEmpty = function () {
      return ((en.length === 0) && (de.length === 0));
   };
};