countNegative.js

const countNegative = ( M, n, m ) => {
    var count = 0;
    // start from top right
    var i = 0;
    var j = m - 1; 
 
    while ( j >= 0 && i < n ) {
        if ( M[i][j] < 0 ) {
            
            // last negative number is at index j. Hence, there j + 1 negative numebrs at index i
            count += (j + 1);
            // Move to the next row
            i += 1;
        } else {
            // Move to the next column
            j -= 1;    
        }
    }
    
    return count;
};

var M = [ 
  [-3, -2, -1,  1],
  [-2,  -12,  3,  4],
  [4,   5,  7,  8]
];

console.log(countNegative(M, 3, 4)); // 5