const sum = (arr) => {
    return arr.reduce((acc, n) => acc + n, 0);
}

function hourglassSum(arr) {
    const results = [];
    
    for (let i = 0; i < 4; i++) {
        for (let j = 0; j < 4; j++) {
            const roi = [
                [ arr[  i  ][  j  ],     arr[ i ][j+1],        arr[ i ][j+2]],
                [                         arr[i+1][j+1],                      ],
                [ arr[i+2][ j ],       arr[i+2][j+1],   arr[i+2][j+2]],
            ]                                    
            results.push(sum(roi.flat()));
        }
    }    
    return Math.max(...results);
}