You don't need to iterate over the entire array, you can just get half of it and retrieve values from the 4 "corners", and then add the middle point in case of a odd number of rows.

class Solution {
    diagonalSum(mat) {
        let totalSum = 0;  // Initialize the total sum
        const len = mat.length;
        for (let i = 0; i < Math.floor(len / 2); i += 1) {
            totalSum += ( mat[i][i] + 
							            mat[i][len - 1 - i] + 
							            mat[len - 1 - i][i] + 
							            mat[len - 1 - i][len - 1 - i]
						            );
        }
        if (len % 2 === 1) {
            // add center
            let pt = Math.floor(len / 2);
            totalSum += mat[pt][pt];
        }
        return totalSum;  // Return the calculated total sum
    }
}