extract the grid to its own class

js/grid.js

@@ -0,0 +1,57 @@
+function Grid(size) {
+  this.size = size;
+
+  this.cells = [];
+
+  this.build();
+}
+
+// Build a grid of the specified size
+Grid.prototype.build = function () {
+  for (var x = 0; x < this.size; x++) {
+    var row = this.cells[x] = [];
+
+    for (var y = 0; y < this.size; y++) {
+      row.push(null);
+    }
+  }
+};
+
+// Find the first available random position
+Grid.prototype.randomAvailableCell = function () {
+  var cells = this.availableCells();
+
+  if (cells.length) {
+    return cells[Math.floor(Math.random() * cells.length)];
+  }
+};
+
+Grid.prototype.availableCells = function () {
+  var cells = [];
+
+  for (var x = 0; x < this.size; x++) {
+    for (var y = 0; y < this.size; y++) {
+      var cell = { x: x, y: y };
+
+      if (this.cellAvailable(cell)) {
+        cells.push(cell);
+      }
+    }
+  }
+
+  return cells;
+};
+
+// Check if the specified cell is taken
+Grid.prototype.cellAvailable = function (cell) {
+  return !this.cellOccupied(cell);
+};
+
+Grid.prototype.cellOccupied = function (cell) {
+  return !!this.cells[cell.x][cell.y];
+};
+
+// Inserts a tile at its position
+Grid.prototype.insertTile = function (tile) {
+  this.cells[tile.x][tile.y] = tile;
+};