diff --git a/minesweeper.js b/minesweeper.js
index a76e568..a3770a8 100644
--- a/minesweeper.js
+++ b/minesweeper.js
@@ -1,4 +1,4 @@
-var board = [];
+const board = [];
 
 const canvas = document.getElementById("minesweeper");
 
@@ -49,6 +49,13 @@ function draw() {
             context.fillRect(x * CELL_SIZE + CELL_SIZE - STROKE_SIZE, y * CELL_SIZE + STROKE_SIZE, STROKE_SIZE, CELL_SIZE - STROKE_SIZE);
             context.fillRect(x * CELL_SIZE + STROKE_SIZE, y * CELL_SIZE + CELL_SIZE - STROKE_SIZE, CELL_SIZE - STROKE_SIZE, STROKE_SIZE);
 
+            if (cell.isFlagged) {
+                context.fillStyle = "red";
+                context.beginPath();
+                context.arc(x * CELL_SIZE + CELL_SIZE / 2, y * CELL_SIZE + CELL_SIZE / 2, CELL_SIZE / 3, 0, 2 * Math.PI);
+                context.fill();
+            }
+
             if (cell.isRevealed) {
                 if (cell.isMine) {
                     context.fillStyle = "black";
@@ -144,6 +151,9 @@ function getCellNumber(x, y) {
 }
 
 function revealCell(x, y) {
+    if (board[x][y].isFlagged) {
+        return;
+    }
     board[x][y].isRevealed = true;
 
     if (getCellNumber(x, y) === 0) {
@@ -156,10 +166,60 @@ function revealCell(x, y) {
     }
 }
 
+function cellMarkedAsMine(cell) {
+    return cell.isFlagged || (cell.isRevealed && cell.isMine);
+}
+
+function revealSafeCell(x, y) {
+    if (!board[x][y].isRevealed || board[x][y].isMine) {
+        return;
+    }
+    const targetValue = getCellNumber(x, y);
+    const neighbours = getCellsAround(x, y);
+    const revealedBombs = neighbours
+        .map(([x, y]) => board[x][y])
+        .map(cellMarkedAsMine)
+        .reduce((acc, isMine) => acc + (isMine ? 1 : 0), 0);
+    if (revealedBombs === targetValue) {
+        neighbours.forEach(([x, y]) => revealCell(x, y))
+    }
+}
+
+function flagCell(x, y) {
+    if (board[x][y].isRevealed) {
+        return;
+    }
+    board[x][y].isFlagged = !board[x][y].isFlagged;
+}
+
+window.addEventListener("contextmenu", event => {
+    event.preventDefault();
+})
+
+const MouseButton = Object.freeze({
+    Left: 0,
+    Middle: 1,
+    Right: 2,
+})
+
 window.addEventListener("mousedown", event => {
     const { x, y } = screenCoordsToBoardCoords(event.pageX + scrollX * 2, event.pageY + scrollY * 2);
 
-    revealCell(x, y);
+    switch (event.button) {
+        case MouseButton.Left:
+            revealCell(x, y);
+            break;
+        case MouseButton.Middle:
+            event.preventDefault();
+            revealSafeCell(x, y);
+            break;
+        case MouseButton.Right:
+            event.preventDefault();
+            flagCell(x, y);
+            break;
+        default:
+            return true;
+    }
 
     draw();
 });