$(document).ready(function () {
  var sketch = createSketchArea(50, 50);
  $('#sketch').append(sketch);
  $('.cell').hover(function() {
    $(this).css({'background-color': 'black'});
  });
});

function createCellDivs(howMany) {
  var divs = document.createDocumentFragment();
  for(var i = 0; i < howMany; i++) {
    var div = document.createElement("div");
    div.className = "cell";
    divs.appendChild(div);
  }
  return divs;
}

function createSketchArea(rows, columns) {
  var area = document.createDocumentFragment();
  for(var i = 0; i < rows; i++) {
    var row = document.createElement("div");
    row.className = "row";
    area.appendChild(row);
    area.appendChild(createCellDivs(columns));
  }
  return area;
}