priority queue question. priority is undefined in while loop. how to enqueue element to queue?

//min-heap
class PriorityQueue {
  constructor(){
    this.values = [];
  }
  enqueue(value, priority){
    let newNode = new Node(value, priority);
    this.values.push(newNode);
    this.bubbleUp();
  }
  bubbleUp(){
    let childIndex = this.values.length - 1;
    let parentIndex = Math.floor((childIndex - 1) / 2);
    let childElement = this.values[childIndex].priority;
    let parentElement = this.values[parentIndex].priority;

    while(childElement < parentElement){
      let temp = this.values[childIndex];
      this.values[childIndex] = this.values[parentIndex];
      this.values[parentIndex] = temp;

      childIndex = parentIndex;
      parentIndex = Math.floor((childIndex - 1) / 2);
    }
  }
}