function LevelRoller(canvas, options){

  // ...

  this.mousedown = function(e){
    var cell = this.getCursorPosition(e); // <----- why is "this" <HTMLCanvasElement> ? .. how can I get the levelroller instance?
    console.log(cell);
    dragging = true;
    this.canvas.onmousemove = this.dragging;
  };

}

$(function() {
  var can = $('<canvas></canvas>');=
  var canvas = can.get(0);
  $('body').append(can);

  levelroller = new LevelRoller(canvas);

  // ...

  canvas.onmousedown = levelroller.mousedown;
  canvas.onmouseup = levelroller.mouseup;

});