Sum All Numbers Under 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;

for (let i = 0; i < matrix.length; i += 1) {
    for (let j = 0; j < i; j += 1) {
        sum += matrix[i][j];
    }
}

console.log(sum);