Create a queue-like animal shelter in JavaScript

function AnimalShelter() {
  this.size = 0;
  this.first = this.last = null;
}
function Node(data) {
  this.data = data;
  this.next = null;
}
AnimalShelter.prototype.enqueue = function(data) {
  const node = new Node(data);
  if (this.last) this.last.next = node;
  this.last = node;
  if (!this.first) this.first = this.last;
  this.size++; 
  return node; // returning for the sake of testing
};
AnimalShelter.prototype.dequeueAny = function() {
  if (!this.first) return;
  const data = this.first.data;
  this.first = this.first.next;
  if (!this.first) this.last = null;
  this.size = Math.max(0, this.size - 1);
  return data;  
};
AnimalShelter.prototype.dequeueDog = function() {
  let prev = current  = this.first;
  let next = current ? current.next : null;
  let dog;
  while (current) {
    if (current.data === 'dog') {
      // Only one node in list
      if (this.first === current && next === null) {
        this.first = this.last = null;
      } else if (next === null && prev) {
        // Removing the last node
        this.last = prev;
      }
      dog = current.data;
      prev.next = next;
      this.size--;
      break;
    }
    prev = current;
    current = next;
    next = current ? current.next : null;
  }
  
  return dog;
};
AnimalShelter.prototype.dequeueCat = function() {
let prev = current  = this.first;
  let next = current ? current.next : null;
  let cat;
  while (current) {
    if (current.data === 'cat') {
      // Only one node in list
      if (this.first === current && next === null) {
        this.first = this.last = null;
      } else if (next === null && prev) {
        // Removing the last node
        this.last = prev;
      }
      
      cat = current.data;
      prev.next = next;
      this.size--;
      break;
    }
    prev = current;
    current = next;
    next = current ? current.next : null;
  }
  
  return cat;  
};
AnimalShelter.prototype.peek = function() {
  if (!this.first) return;
  return this.first.data;  
};
AnimalShelter.prototype.isEmpty = function() {
  return this.size === 0 && !this.first;  
};
AnimalShelter.prototype.getSize = function() {
  return this.size;
}