Найдите среднее значение по индексу в матрице

const matrix = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9]
];
const averages = [];
for (let col = 0; col < matrix[0].length; col++) { // loop to iterate over columns
  let sum = 0;
  for (let row = 0; row < matrix.length; row++) { // loop to iterate over rows
    sum += matrix[row][col]; // calculate accumulated sum
  }
  const average = sum / matrix.length; // get sum divided by matrix length
  averages.push(average);
}

console.info(JSON.stringify(averages));

const 
      input = [
        [1, 2, 3],
        [4, 5, 6],
        [7, 8, 9]
      ],
      
      output = input.reduce(
          (acc,cur,i,a) => acc.map(
              (x,j) => x + cur[j]/a.length
          ), 
          Array(input.length).fill(0)
      );

console.info( output );