How to implement a Stack using Queues in JavaScript

my favorite stack : pancake stack

Hello! Today, I worked on how to make a stack with queues. Queue has “First in First out”, but Stack has “Last in First out” rules. In order to make a Stack, I needed to make two queues. First, in Queue 1 array, I push data(elements) in order using .push() method. And then, while there are more than two elements in the Queue 1 array, I took out the first element in the Queue 1 and then push the element into the Queue2. When there is only one element left, which is the last element, I just return it. After return the last element, I took the first element in the Queue 2 array, and then push it to the Queue 1 to repeat the same process.

class StackFromQueues{
 constructor(){
  this.queue1 = [];
  this.queue2 = [];
 }
 
 push(value){
  this.queue1.push(value); 
 }
 pop(){
  //O(n) time
   while(this.queue1.length > 1){
   this.queue2.push(this.queue1.shift());
  }
  const value = this.queue1.shift();
  //O(n) time
  while(this.queue2.length){
   this.queue1.push(this.queue2.shift());
  }
  return value;
  
  //Time Complexity : O(n) + O(n) = O(2n) ~= O(n) time
  //Space Complexity : O(n), but is required for the problem (2 queues)
 }
}
const stack = new StackFromQueues();
stack.push(1);
stack.push(2);
stack.push(3);
stack.push(4);
console.log('pop 4 off from the stack', stack.pop());
console.log('pop 3 off from the stack', stack.pop());

What is happening in the above code