Javascript – TypeError: Cannot create property '375' on number '1'

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.