I’m trying to create an algorithm visualiser for heap sort. In order to keep track of the swaps that happen during the heap sort execution, I created an animations array that will include the indexes of each swap. The heap sort animation works fine, but once I introduce the animations array into my code, I run into this error. TypeError: Cannot create property '375' on number '1'

Sort.js

heapSortVisualiser() {
    const animations = heapSort(this.state.array);
    console.log(animations);
}

HeapSort.js

// i is an index in arr, N is size of heap
function heapify(arr, N, i, animations) {
    let largest = i; // initialise largest as root
    let l = 2 * i + 1; // left = 2i + 1
    let r = 2 * i + 2; // right = 2i + 2

    // If left child is larger than root
    if (l < N && arr[l] > arr[largest]) { largest = l; }

    // If right child is larger than current largest
    if (r < N && arr[r] > arr[largest]) { largest = r; }

    // If largest is no longer the root
    if (largest !== i) {
        animations.push({
            type: 'heapify',
            index1: i,
            index2: largest
        })
        [arr[i], arr[largest]] = [arr[largest], arr[i]]; // swap i and largest

        // Recursively heapify the affected subtree
        heapify(arr, N, largest);
    }
}

export function heapSort(arr, animations = []) {
    let N = arr.length;

    // Build heap (rearrange array)
    for (let i = Math.floor(N / 2) - 1; i >= 0; i--) { heapify(arr, N, i, animations) };

    // Extract top element of heap one by one until heap is empty
    for (let i = N - 1; i > 0; i--) {
        animations.push({
            type: 'sort',
            index1: i,
            index2: 0
        })
        [arr[i], arr[0]] = [arr[0], arr[i]] // move current root to end

        // call max heapify on the reduced heap
        heapify(arr, i, 0, animations);
    }
    return animations;
}

Initially I thought the problem was pushing a JavaScript object to the array was the issue, so I used JSON.stringify() to convert the object to a string but the same error persists.