Untitled

let sum = 0;
let len = mat.length;

for (let i = 0; i < len; i++) {
    if(i != mat[i].length - i - 1) {
        sum += mat[i][i] + mat[i][mat[i].length - i - 1];
    } else{ 
        sum += mat[i][i];
    }
}

console.log(sum);