Learning JavaScript Data Structures and Algorithms - Third Edition

Verifying whether the queue is empty and its size

The next method is the isEmpty method, which returns true if the queue is empty, and false otherwise:

isEmpty() {  return this.count - this.lowestCount === 0;} 

To calculate how many elements there are in the queue, we simply calculate the difference between the key count and the lowestCount. 

Suppose the count property has value 2 and the lowestCount has value 0. This means we have two elements in the queue. Next, we remove one element from the queue. The property lowestCount will be updated to value 1 and count will continue with value 2. Now the queue has only one element, and so on.

So to implement the size method, we simply return this difference:

size() { return this.count - this.lowestCount; ...