Sum All Numbers Over Main Diagonal (Exercise)

let matrix = [
    [1, 2, 3, 4, 5],
    [2, 1, 4, 5, 6],
    [3, 4, 1, 6, 7],
    [4, 5, 6, 1, 8],
    [5, 6, 7, 8, 1],
];

let sum = 0;
let len = matrix.length - 1;

for (let i = len; i >= 0; i -= 1) {
    for (let j = len; j > i; j -= 1) {
        sum += matrix[i][j];
    }
}

console.log(sum);