Heap implementation in JS. Defaults to Min Heap. Inherit and override compare to implement a Max Heap

heap.js

/*
Heap implementation in JS. 

Description of Heap:
https://en.wikipedia.org/wiki/Heap_(data_structure)

Defaults to MinHeap. Extend class and override
compare(a, b) to implement MaxHeap.
*/

class Heap {
  constructor() {
    this.items = [];
  }

  leftChildIndex(parentIndex) {
    return 2 * parentIndex + 1;
  }
  rightChildIndex(parentIndex) {
    return 2 * parentIndex + 2;
  }
  parentIndex(childIndex) {
    return Math.floor((childIndex - 1) / 2);
  }

  hasLeftChild(index) {
    return this.leftChildIndex(index) < this.items.length;
  }

  hasRightChild(index) {
    return this.rightChildIndex(index) < this.items.length;
  }
  hasParent(index) {
    return this.parentIndex(index) >= 0;
  }

  leftChild(index) {
    return this.items[this.leftChildIndex(index)];
  }
  rightChild(index) {
    return this.items[this.rightChildIndex(index)];
  }

  parent(index) {
    return this.items[this.parentIndex(index)];
  }

  swap(i1, i2) {
    [this.items[i1], this.items[i2]] = [this.items[i2], this.items[i1]];
  }

  peak() {
    if (this.items.length == 0) return null;

    return this.items[0];
  }
  
  poll() {
		if (this.items.length == 0) return null;

		this.swap(0, this.items.length - 1);
		let first = this.items.pop();
		this.heapifyDown();

		return first;
  }
  add(val) {
    this.items.push(val);
    this.heapifyUp();
  }

  heapifyUp() {
    let i = this.items.length - 1;
    while (this.hasParent(i) && this.compare(this.items[i], this.parent(i))) {
      this.swap(this.parentIndex(i), i);
      i = this.parentIndex(i);
    }
  }

  heapifyDown() {
    let i = 0;
    while (this.hasLeftChild(i)) {
      let smallChildIndex = this.leftChildIndex(i);
      if (
        this.hasRightChild(i) &&
        this.compare(this.rightChild(i), this.leftChild(i))
      ) {
        smallChildIndex = this.rightChildIndex(i);
      }
      if (this.compare(this.items[i], this.items[smallChildIndex])) {
        break;
      } else this.swap(i, smallChildIndex);

      i = smallChildIndex;
    }
  }

  print() {
    console.log(this.items);
  }
  //Default to Min Heap
  compare(a, b) {
    return a <= b;
  }
}

class MaxHeap extends Heap {
  compare(a, b) {
    return a >= b;
  }
}