add tile merging

js/grid.js

@@ -62,10 +62,23 @@ Grid.prototype.cellOccupied = function (cell) {
 };
 
 Grid.prototype.cellContent = function (cell) {
-  return this.cells[cell.x][cell.y];
+  if (this.withinBounds(cell)) {
+    return this.cells[cell.x][cell.y];
+  } else {
+    return null;
+  }
 };
 
 // Inserts a tile at its position
 Grid.prototype.insertTile = function (tile) {
   this.cells[tile.x][tile.y] = tile;
 };
+
+Grid.prototype.removeTile = function (tile) {
+  this.cells[tile.x][tile.y] = null;
+};
+
+Grid.prototype.withinBounds = function (position) {
+  return position.x >= 0 && position.x < this.size &&
+         position.y >= 0 && position.y < this.size;
+};