矩阵每一列的总和

function matrix(m, n) {
  var table = document.createDocumentFragment();

  var arr = [];
  var colSums = [];
  for (var j = 0; j < n; j++) {
    colSums[j] = 0;
  }

  for (var i = 0; i < m; i++) {
    var tr = document.createElement('tr');
    arr[i] = [];
    var summM = 0;

    for (var j = 0; j < n; j++) {
      var td = document.createElement('td');
      td.id = "unicId-" + i + "-" + j;
      td.innerHTML = arr[i][j] = getRandom();
      tr.appendChild(td);
      summM += arr[i][j]; //сумма каждой строки
      colSums[j] += arr[i][j];
    }

    tr.setAttribute("data-summ", " → Cуммa = " + summM);
    table.appendChild(tr);
  }

  //console.log(colSums);
  tr = document.createElement('tr');
  for (var j = 0; j < n; j++) {
    td = document.createElement('td');
    td.innerHTML = "<b>" + colSums[j] + "</b>";
    tr.appendChild(td);
  }
  table.appendChild(tr);
  
  document.getElementById('matrix').appendChild(table);

  function getRandom() {
    var min = 1;
    var max = 4;
    return Math.floor(Math.random() * (max - min)) + min;
  }
}

matrix(4, 4);

tr {
  position: relative;
}

tr:after {
  content: attr(data-summ);
}

td {
  border: 1px solid lightgray;
  padding: 5px;
}

<section id="matrix"></section>