[Alexandroppolus]

function maxdiags(m) {
  var len = m.length;
  var res = new Array(len * 2 - 1);
  for (var i = 0; i < len; ++i) {
    for (var j = 0; j < len; ++j) {
      res[i + j] = Math.max(res[i + j] || -Infinity, m[i][j]);
    }
  }
  return res;
}

var matrix = [
[1, 2, 3, 4],
[5, 6, 7, 8],
[9, 8, 7, 6],
[5, 4, 3, 2]
];

console.log(matrix);
console.log(maxdiags(matrix));