Heap with comparator function (Min Heap, Max Heap)

Heap.js

class Heap {
    constructor(arr, comparator) {
        this.values = [];
        this.comparator = comparator;

        if (arr) {
            this.build(arr);
        }
    }
    
    leftIndex(i) {
        return i * 2 + 1;
    }
    
    rightIndex(i) {
        return i * 2 + 2;
    }
    
    parent(i) {
        return Math.floor(i  / 2);
    }
    
    build(arr) {
      arr.forEach(val => this.add(val)); 
    }
    
    heapify(i) {
        let largest  = i, leftChild = this.leftIndex(i), rightChild = this.rightIndex(i);
        
        if (this.values[leftChild] && this.comparator(this.values[largest], this.values[leftChild])) {
            largest = leftChild;
        } 
        
        if(this.values[rightChild] && this.comparator(this.values[largest], this.values[rightChild])) {
            largest = rightChild
        };
        
        
        if (largest !== i) {
            this.swap(i, largest);
            this.heapify(largest);
        }
        
    }

    add(val) {
        this.values.push(val); 
        
        for(let i = Math.floor(this.size() / 2 -1); i >= 0; i--){
            this.heapify(i)   
        }
    }
    
    size() {
        return this.values.length;
    }
    
    swap(i,j) {
        [this.values[i], this.values[j]] = [this.values[j], this.values[i]];
    }

    peek() {
        return this.values[0];
    }

    pull() {
        const max = this.values.shift();

        this.heapify(0);

        return max;
    }
}


// example:

const heap = new Heap([3,9,1,2,4,5], (a,b) => a < b);