deque.js

function Queue(originalQueue) {
  let queue = originalQueue || [];

  function removeElement(q, i) {
    const index = i === 'last' ? q.length - 1 : i;
    if (!q[index + 1]) {
      q[index] = undefined;
      q.length = q.length - 1;
    }
  }

  return {
    enqueue (element) {
      for (let i = 0; i < queue.length + 1; i++) {
        if (!queue[i]) {
          queue[i] = element;
          return;
        }
      }
    },
    dequeue () {
      for (let i = 1; i < queue.length + 1; i++) {
        // Moves every element forward (towards index 0) by 1.
        queue[i - 1] = queue[i];
        removeElement(queue, i);
      }
    },
    buttIn (element) {
      for (let i = queue.length; i >= 0; i--) {
        queue[i] = queue[i - 1];
      }
      queue[0] = element;
    },
    giveUp () {
      removeElement(queue, 'last');
    },
    peek () {
      return queue[0];
    },
    isEmpty() {
      return !queue.length;
    },
    _view () {
      return queue;
    }
  };
}

function test(q, a) {
  let aQueue = new Queue(q);
  console.log('initial array:', aQueue._view());
  aQueue.enqueue('x');
  aQueue.dequeue();
  console.log('dequeued', aQueue._view());
  aQueue.enqueue('z');
  aQueue.enqueue('q');
  aQueue.buttIn('t');
  aQueue.giveUp();
  aQueue.enqueue('a');
  aQueue.buttIn('e');
  console.log('should be:', a);
  console.log('is:', aQueue._view());
  console.log('');
}

let q = ['a','s','d','f','g'];
let a = '[e, t, s, d, f, g, x, z, a]';
test(q, a);
test(null, '[e, t, z, a]');
test(['g'], '[e, t, x, z, a]');

q = new Queue();
q.buttIn('x');
console.log(q.isEmpty());
console.log(q._view());