Min Heap JavaScript implementation

MinHeap.js

// Based on AlgoExpert's version

class MinHeap {
  heap = null;
  
  constructor(array) {
    this.heap = array;
    this.buildHeap();
  }

  buildHeap() {
    const firstParentIndex = Math.floor((this.heap.length - 2) / 2);
    for (let currentIndex = firstParentIndex; currentIndex >= 0; currentIndex--) {
      this.siftDown(currentIndex, this.heap.length);
    }
  }

  siftDown(currentIndex, endIndex) {
    let childOneIndex = currentIndex * 2 + 1;
    while (childOneIndex <= endIndex) {
      let childTwoIndex = -1;
      if (currentIndex * 2 + 2 <= endIndex) {
        childTwoIndex = currentIndex * 2 + 2;
      }
      let indexToSwap = childOneIndex;
      if (childTwoIndex > -1 && this.heap[childTwoIndex] < this.heap[childOneIndex]) {
        indexToSwap = childTwoIndex;
      }
      
      if (this.heap[indexToSwap] < this.heap[currentIndex]) {
        this.swap(currentIndex, indexToSwap);
        currentIndex = indexToSwap;
        childOneIndex = currentIndex * 2 + 1;
      } else {
        return;
      }
    }
  }

  siftUp() {
    let currentIndex = this.heap.length - 1;
    let parentIndex = Math.floor((currentIndex - 1) / 2);
    while (currentIndex >= 0 && this.heap[currentIndex] < this.heap[parentIndex]) {
      this.swap(currentIndex, parentIndex);
      currentIndex = parentIndex;
      parentIndex = Math.floor((currentIndex - 1) / 2); 
    }
  }

  remove() {
    this.swap(0, this.heap.length - 1);
    const peek = this.heap.pop();
    this.siftDown(0, this.heap.length - 1);
    return peek;
  }

  peek() {
    if (this.heap.length == 0) {
      return -1;
    }
    return this.heap[0];
  }

  insert(value) {
    this.heap.push(value);
    this.siftUp();
  }

  swap(i,j) {
    [this.heap[i], this.heap[j]] = [this.heap[j], this.heap[i]]
  }

  isEmpty() {
    return this.heap.length === 0;
  }
}

const heap = new MinHeap([]);
heap.insert(9);
heap.insert(4);
heap.insert(20);
heap.insert(2);
heap.insert(3);
console.log(heap.peek());
heap.insert(1);
console.log(heap.peek());
console.log(heap.remove());
console.log(heap.peek());
console.log(heap.isEmpty());