Trouble with JavaScript 2D array and nested loop

Question

Will you please explain what I'm missing? And possibly what I can do to fix it. I'm receiving the error: TypeError: Cannot set properties of undefined (setting '0') while trying to add data to this 2D array.

        wixData.query("paidLineItems").eq("paidOrderId", orderNumber).find().then((result)=>{

        let nbrRecs = result.totalCount
        let newOrder = [];

        matrix = new Array(nbrRecs).fill(0).map(() => new Array(8).fill(0));
        for (var i = 0; i < nbrRecs; i++) {
            let lineItem = result.items[i];
            if (result.items[i].prodQty > 1) {
                let qty = result.items[i].prodQty
                while (multipleLineCtr < qty) {
                        matrix[multipleLineCtr][0] = lineItem.email
                        matrix[multipleLineCtr][1] = lineItem.prodName
                        matrix[multipleLineCtr][2] = lineItem.prodQty
                        matrix[multipleLineCtr][3] = multipleLineCtr
                        matrix[multipleLineCtr][4] = "Jesus"
                        matrix[multipleLineCtr][5] = lineItem.paidOrderId
                        matrix[multipleLineCtr][6] = lineItem.purchaseDt
                        matrix[multipleLineCtr][7] = lineItem.dtSupportEnds
                        multipleLineCtr++;
                }
            }else {
                matrix[cnt][0] = lineItem.email
                matrix[cnt][1] = lineItem.prodName
                matrix[cnt][2] = lineItem.prodQty
                matrix[cnt][3] = 1
                matrix[cnt][4] = "Jesus"
                matrix[cnt][5] = lineItem.paidOrderId
                matrix[cnt][6] = lineItem.purchaseDt
                matrix[cnt][7] = lineItem.dtSupportEnds
                cnt++;
            }
        }
    })
    .catch((error) => {
        console.log(error);
    }); 
});

Answer 1

You should check if the matrix[index] is an array or not, and if it is, you should create an array at matrix[index].

matrix[multipleLineCtr] ||= [];

or the traditional way:

if (!Array.isArray(matrix[index])) {
    matrix[index] = [];
}