‘TypeError: Cannot set properties of undefined (setting ‘0’)’—-Array assignment

When I was solving a problem on Leetcode, I’ve defined an empty array. I tried push some numbers then I got this Errow. I don’t know why. My code here.

        // r and c are already defined numbers，arr is already defined array.
        let n = [[]]
        let index = 0
        for (let i = 0; i < r; i++) {
            for (let j = 0; j < c; j++) {
                    n[i][j] = arr[index]
                    index++;
            }
        }
        return n; 

Leetcode told me n[i][j] = arr[index] had errow；

Anyone knows why? thanks.