text: function Queue() {
    this.queue = [];
  }
  
  Queue.prototype.enqueue = function(value) {
    this.queue.push(value);
  };
  
  Queue.prototype.dequeue = function() {
    return this.queue.shift();
  };
  
  Queue.prototype.peek = function() {
    return this.queue[0];
  };
  
  Queue.prototype.length = function() {
    return this.queue.length;
  };
  
  Queue.prototype.print = function() {
    console.log(this.queue.join(' '));
  };
  
  var queue = new Queue();
  queue.enqueue(1);
  queue.enqueue(2);
  queue.enqueue(3);
  queue.print(); // => 1 2 3
  console.log('length is 3:', queue.length()); // => 3
  console.log('peek is 1:', queue.peek()); // => 3
  console.log('dequeue is 1:', queue.dequeue()); // => 1
  queue.print(); // => 2 3
  console.log('dequeue is 2:', queue.dequeue());  // => 2
  console.log('length is 1:', queue.length()); // => 1
  console.log('dequeue is 3:', queue.dequeue()); // => 3
  queue.print(); // => ''
  console.log('peek is undefined:', queue.peek()); // => undefined
  console.log('dequeue is undefined:', queue.dequeue()); // => undefined
  