queue.js

var Queue = function() {
  var someInstance = {};
  var size = 0;
  // Use an object with numeric keys to store values
  var storage = {};

  // Implement the methods below

  someInstance.enqueue = function(value) {
    storage[size] = value;
    size++;
  };

  someInstance.dequeue = function() {
    var dequeue = storage[0];
    if (size) {
      size--;
    }
    for (var key in storage) {
      var oldIndex = key;
      var newIndex = key - 1;
      storage[oldIndex] = storage[newIndex];
    }
    return dequeue;
  };

  someInstance.size = function() {
    return size;
  };

  return someInstance;
};

// test to be passed;

queue.enqueue('a');
queue.enqueue('b');
queue.dequeue();
queue.enqueue('c');
expect(queue.dequeue()).to.equal('b');