Javascript Array Multidimensional array Sum

let grades = [[89, 77, 78],[76, 82, 81],[91, 94, 89]]; 
let total = 0; //  ww  w.j  av  a  2s. co  m
let average = 0.0; 
for (let row = 0; row < grades.length; ++row) { 
   for (let col = 0; col < grades[row].length; ++col) { 
      total += grades[row][col]; 
   } 
   average = total / grades[row].length; 
   console.log("Student " + parseInt(row+1) + " average: " + 
         average.toFixed(2)); 
   total = 0; 
   average = 0.0; 
}