diff --git a/.github/dependabot.yml b/.github/dependabot.yml
new file mode 100644
index 0000000..ca79ca5
--- /dev/null
+++ b/.github/dependabot.yml
@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly
diff --git a/.github/workflows/commentReleasedPRs.yml b/.github/workflows/commentReleasedPRs.yml
index e67df31..63687b0 100644
--- a/.github/workflows/commentReleasedPRs.yml
+++ b/.github/workflows/commentReleasedPRs.yml
@@ -8,7 +8,10 @@ on:
 jobs:
   comment:
     name: Comment released PRs
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-24.04
+    permissions:
+      contents: read           # read the release and its commits
+      pull-requests: write     # comment on and label the released PRs
     steps:
       - name: Comment released PRs
         uses: rdlf0/comment-released-prs-action@v3
diff --git a/.github/workflows/compilabilityCheck.yml b/.github/workflows/compilabilityCheck.yml
deleted file mode 100644
index 6831ff6..0000000
--- a/.github/workflows/compilabilityCheck.yml
+++ /dev/null
@@ -1,17 +0,0 @@
-name: Compilability check
-
-on:
-  push:
-    branches-ignore:
-      - 'master'
-
-jobs:
-  compile:
-    name: Compile
-    runs-on: ubuntu-22.04
-    steps:
-    - name: Check out the code
-      uses: actions/checkout@v3
-      
-    - name: Compile
-      run: tsc --noEmit
diff --git a/.github/workflows/pr-validation.yml b/.github/workflows/pr-validation.yml
new file mode 100644
index 0000000..4ebe861
--- /dev/null
+++ b/.github/workflows/pr-validation.yml
@@ -0,0 +1,26 @@
+name: PR Validation
+
+on:
+  pull_request:
+    types: [opened, reopened, synchronize]
+
+jobs:
+  pr-validation:
+    name: PR Validation
+    runs-on: ubuntu-24.04
+    permissions:
+      contents: read
+    steps:
+    - name: Check out the code
+      uses: actions/checkout@v6
+
+    - name: Set up Node.js
+      uses: actions/setup-node@v6
+      with:
+        node-version: 22
+
+    - name: Install TypeScript
+      run: npm install -g typescript@6.0.3   # keep in sync with the release workflow
+
+    - name: Compile and test
+      run: tsc && node --test   # tsc emits to dist/ and type-checks (fails on errors); tests import the build
diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index 93e5d19..c0ab3de 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -8,48 +8,74 @@ on:
 env:
   ARTIFACT_NAME: minesweeper-compiled
 
+concurrency:
+  group: release
+  cancel-in-progress: false
+
 jobs:
   build:
     name: Build
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-24.04
+    permissions:
+      contents: write        # needed to upload the release asset via the gh CLI
     env:
       ARTIFACT_PATH: artifact
       ASSET_NAME: minesweeper.zip
     steps:
       - name: Check out the code
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           ref: ${{ github.sha }}
 
+      - name: Set up Node.js
+        uses: actions/setup-node@v6
+        with:
+          node-version: 22
+
+      - name: Install TypeScript
+        run: npm install -g typescript@6.0.3   # keep in sync with the PR validation workflow
+
       - name: Compile the source
         run: tsc --outDir "${ARTIFACT_PATH}/dist"
 
       - name: Prepare artifact
-        run: mv index.html require.js main.js styles.css favicon.svg robots.txt sitemap.xml manifest.webmanifest sw.js LICENSE assets $ARTIFACT_PATH
+        run: |
+          rsync -a \
+            --exclude="/.git" \
+            --exclude="/.github" \
+            --exclude="/.vscode" \
+            --exclude="/src" \
+            --exclude="/test" \
+            --exclude="/node_modules" \
+            --exclude="/tsconfig.json" \
+            --exclude="/.gitignore" \
+            --exclude="*.md" \
+            --exclude="/${ARTIFACT_PATH}" \
+            ./ "${ARTIFACT_PATH}/"
 
       - name: Prepare release asset
-        run: zip -r $ASSET_NAME $ARTIFACT_PATH
+        run: zip -r "$ASSET_NAME" "$ARTIFACT_PATH"
 
       - name: Upload artifact
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v7
         with:
           name: ${{ env.ARTIFACT_NAME }}
           path: ./${{ env.ARTIFACT_PATH }}
 
       - name: Upload release asset
-        uses: AButler/upload-release-assets@v3.0
-        with:
-          files: "./${{ env.ASSET_NAME }}"
-          repo-token: ${{ secrets.GITHUB_TOKEN }}
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: gh release upload "${{ github.event.release.tag_name }}" "$ASSET_NAME" --repo "$GITHUB_REPOSITORY"
 
   deploy:
     name: Deploy
     needs: build
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-24.04
     env:
       AWS_S3_BUCKET: theminesweeper.com
       AWS_REGION: us-east-2
       AWS_ROLE: arn:aws:iam::067429979131:role/github_actions_minesweeper
+      CLOUDFRONT_DISTRIBUTION_ID: E32QGZLPPEMLDF
     environment:
       name: production
       url: "https://${{ env.AWS_S3_BUCKET }}"
@@ -58,63 +84,53 @@ jobs:
       contents: read
     steps:
       - name: Download artifact
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v8
         with:
           name: ${{ env.ARTIFACT_NAME }}
 
       - name: Configure AWS credentials
-        uses: aws-actions/configure-aws-credentials@v4
+        uses: aws-actions/configure-aws-credentials@v6
         with:
           role-to-assume: ${{ env.AWS_ROLE }}
           aws-region: ${{ env.AWS_REGION }}
 
       - name: Upload to S3
         run: |
-          declare -a files=(
-          [0]="index.html"
-          [1]="*.js"
-          [2]="*.css"
-          [3]="*.svg"
-          [4]="*.png"
-          [5]="*.webmanifest"
-          [6]="*.txt"
-          [7]="*.xml"
-          [8]="LICENSE"
-          )
-
-          declare -a caches=(
-          [0]="no-caches"
-          [1]="max-age=31536000 immutable"
-          [2]="max-age=31536000 immutable"
-          [3]="max-age=31536000 immutable"
-          [4]="max-age=31536000 immutable"
-          [5]="max-age=31536000 immutable"
-          [6]="max-age=31536000 immutable"
-          [7]="max-age=31536000 immutable"
-          [8]=""
-          )
+          set -euo pipefail
 
-          declare -a contentTypes=(
-          [0]="text/html; charset=utf-8"
-          [1]="application/javascript; charset=utf-8"
-          [2]="text/css; charset=utf-8"
-          [3]="image/svg+xml; charset=utf-8"
-          [4]="image/png; charset=utf-8"
-          [5]="application/manifest+json; charset=utf-8"
-          [6]="text/plain; charset=utf-8"
-          [7]="application/xml; charset=utf-8"
-          [8]=""
+          # Each rule: <include-glob>|<content-type>|<cache-control>
+          rules=(
+            "index.html|text/html; charset=utf-8|no-cache"
+            "*.js|application/javascript; charset=utf-8|public, max-age=3600"
+            "*.css|text/css; charset=utf-8|public, max-age=3600"
+            "*.svg|image/svg+xml; charset=utf-8|public, max-age=3600"
+            "*.png|image/png; charset=utf-8|public, max-age=3600"
+            "*.webmanifest|application/manifest+json; charset=utf-8|public, max-age=3600"
+            "*.txt|text/plain; charset=utf-8|public, max-age=3600"
+            "*.xml|application/xml; charset=utf-8|public, max-age=3600"
+            "LICENSE|text/plain; charset=utf-8|public, max-age=3600"
           )
 
-          for i in "${!files[@]}"
-          do
-              echo "Uploading ${files[$i]}"
-
-              aws s3 sync . s3://$AWS_S3_BUCKET \
-                    --delete \
-                    --acl public-read \
-                    --exclude "*" \
-                    --include "${files[$i]}" \
-                    --cache-control "${caches[$i]}" \
-                    --content-type "${contentTypes[$i]}"
+          for rule in "${rules[@]}"; do
+            IFS="|" read -r glob ctype cache <<< "$rule"
+            echo "Uploading $glob"
+            aws s3 sync . "s3://$AWS_S3_BUCKET" \
+              --delete \
+              --exclude "*" \
+              --include "$glob" \
+              --content-type "$ctype" \
+              --cache-control "$cache"
           done
+
+          # sw.js is uploaded by the *.js rule above; the service worker must always
+          # revalidate, so force its Cache-Control afterwards.
+          aws s3 cp "s3://$AWS_S3_BUCKET/sw.js" "s3://$AWS_S3_BUCKET/sw.js" \
+            --metadata-directive REPLACE \
+            --content-type "application/javascript; charset=utf-8" \
+            --cache-control "no-cache"
+
+      - name: Invalidate CloudFront
+        run: |
+          aws cloudfront create-invalidation \
+            --distribution-id "$CLOUDFRONT_DISTRIBUTION_ID" \
+            --paths "/*"
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index d57cc06..1162f9e 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -32,5 +32,12 @@ $ git commit -am "Something descriptive but concise about the changes I made"
 $ git push
 ```
 6. Repeat 3-5 as needed
-7. Test your changes locally
+7. Test your changes locally - serve the project root over HTTP and open the printed URL in your browser (the game loads as ES modules, so opening `index.html` from the filesystem won't work):
+```sh
+$ npx serve
+```
+   And run the unit tests (Node's built-in runner - no dependencies; build first so the compiled `dist/` exists):
+```sh
+$ tsc && node --test
+```
 8. Create a new pull request - assign it to yourself, add some appropriate tags and link it to the corresponding issue
diff --git a/README.md b/README.md
index 33bff0f..180adec 100644
--- a/README.md
+++ b/README.md
@@ -1,22 +1,28 @@
-![](https://github.com/rdlf0/minesweeper/workflows/CI/CD/badge.svg)
+[![Release](https://github.com/rdlf0/minesweeper/actions/workflows/release.yml/badge.svg)](https://github.com/rdlf0/minesweeper/actions/workflows/release.yml)
 
 # Minesweeper
 
 ## Requirements
-- typescript v3 or later (if you choose the compile option below)
 - a non-ancient browser
+- _to play locally:_ a static file server — the game loads as ES modules, which browsers refuse to load over `file://`, so opening `index.html` straight from the filesystem won't work. Any server will do, e.g. `npx serve` (requires [Node.js](https://nodejs.org)).
+- _to compile from source:_ typescript v4 or later
 
 ## Options to play
 ### Online  
 Enjoy the published version of the game at [theminesweeper.com](https://theminesweeper.com). You can also install it as a progressive web app!
 ### Locally - download precompiled
-Simply download the asset from the [latest release](https://github.com/rdlf0/minesweeper/releases/latest), unzip, and then open `index.html`.
+Download the asset from the [latest release](https://github.com/rdlf0/minesweeper/releases/latest), unzip, then serve the folder over HTTP and open the printed URL in your browser:
+```
+$ npx serve
+```
+(Opening `index.html` directly from the filesystem won't work — the game loads as ES modules, which browsers block over `file://`.)
 ### Locally - compile from source
-Clone, go to the root project directory and run:  
+Clone, go to the root project directory, compile, then serve:  
 ```
 $ tsc
+$ npx serve
 ```
-After that open the `index.html`.
+After that open the printed URL (e.g. http://localhost:3000) in your browser.
 
 ## Settings
 | Setting | Options | Default |
diff --git a/index.html b/index.html
index a22460a..b639a5c 100644
--- a/index.html
+++ b/index.html
@@ -9,7 +9,7 @@
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <meta name="theme-color" content="#1e1e1e" />
     <title>Minesweeper</title>
-    <script data-main="dist/main" src="require.js"></script>
+    <script type="module" src="dist/main.js"></script>
     <link rel="icon" href="favicon.svg" />
     <link rel="apple-touch-icon" href="/assets/icons/favicon_192_apple.png" />
     <link rel="stylesheet" href="styles.css" />
@@ -43,7 +43,7 @@
         <div id="board"></div>
         <div id="settings"></div>
     </main>
-    <script type="text/javascript" src="main.js"></script>
+    <script type="text/javascript" src="pwa-resize.js"></script>
 </body>
 
 </html>
\ No newline at end of file
diff --git a/main.js b/pwa-resize.js
similarity index 100%
rename from main.js
rename to pwa-resize.js
diff --git a/require.js b/require.js
deleted file mode 100644
index a4203f0..0000000
--- a/require.js
+++ /dev/null
@@ -1,5 +0,0 @@
-/** vim: et:ts=4:sw=4:sts=4
- * @license RequireJS 2.3.6 Copyright jQuery Foundation and other contributors.
- * Released under MIT license, https://github.com/requirejs/requirejs/blob/master/LICENSE
- */
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diff --git a/src/board.ts b/src/board.ts
index a48eb4a..bb61d04 100644
--- a/src/board.ts
+++ b/src/board.ts
@@ -1,14 +1,14 @@
-import { Cell } from "./cell";
-import { Mode, FIRST_CLICK } from "./config";
-import { State } from "./state";
+import { Cell } from "./cell.js";
+import { Mode, FIRST_CLICK } from "./config.js";
+import { State } from "./state.js";
 import {
     EVENT_CELL_CLICKED,
     EVENT_CELL_REVEALED,
     EVENT_GAME_OVER,
     EVENT_SAFE_AREA_CREATED,
     PubSub,
-} from "./util/pub-sub";
-import { Session } from "./util/session";
+} from "./util/pub-sub.js";
+import { Session } from "./util/session.js";
 
 interface EventSubscriber {
     event: string;
@@ -31,7 +31,7 @@ export class Board {
 
     constructor(
         private mode: Mode,
-        private state: State,
+        private state: State | null,
         private el: HTMLElement,
     ) {
         this.initGrid();
@@ -92,8 +92,13 @@ export class Board {
     }
 
     private plantMinesFromState(): void {
+        const state = this.state;
+        if (state == null) {
+            return;
+        }
+
         for (let i = 0; i < this.mode.rows * this.mode.cols; i++) {
-            if (this.state.isHighBit(i)) {
+            if (state.isHighBit(i)) {
                 const row = Math.floor(i / this.mode.cols);
                 const col = i % this.mode.cols;
                 this.grid[row][col].setMine();
diff --git a/src/cell.ts b/src/cell.ts
index 4e6acbe..5b6a3c3 100644
--- a/src/cell.ts
+++ b/src/cell.ts
@@ -5,8 +5,8 @@ import {
     EVENT_CELL_UNFLAGGED,
     EVENT_GAME_OVER,
     PubSub,
-} from "./util/pub-sub";
-import { Session } from "./util/session";
+} from "./util/pub-sub.js";
+import { Session } from "./util/session.js";
 
 enum CellState {
     Default = "default",
@@ -14,7 +14,7 @@ enum CellState {
     Questioned = "questioned",
     Revealed = "revealed",
     RevealedMine = "revealedMine",
-    Exploаded = "exploaded",
+    Exploded = "exploded",
     WronglyFlagged = "wronglyFlagged",
 }
 
@@ -106,7 +106,7 @@ export class Cell {
     }
 
     private explode(): void {
-        this.setState(CellState.Exploаded);
+        this.setState(CellState.Exploded);
         PubSub.publish(EVENT_GAME_OVER);
     }
 
@@ -114,8 +114,8 @@ export class Cell {
         // Leave flags
         if (this.state === CellState.Flagged) return;
 
-        // Reveal not exploaded mines
-        if (this.state !== CellState.Exploаded) {
+        // Reveal not exploded mines
+        if (this.state !== CellState.Exploded) {
             this.setState(CellState.RevealedMine)
         }
     }
diff --git a/src/config.ts b/src/config.ts
index fcc1363..451f68d 100644
--- a/src/config.ts
+++ b/src/config.ts
@@ -1,5 +1,5 @@
-import { Encoder } from "./encoder/encoder"
-import { Pairer } from "./pairer/pairer"
+import { Encoder } from "./encoder/encoder.js"
+import { Pairer } from "./pairer/pairer.js"
 
 
 export interface Config {
diff --git a/src/encoder/binaryToBase64UrlEncoder.ts b/src/encoder/binaryToBase64UrlEncoder.ts
index c55220a..16887c0 100644
--- a/src/encoder/binaryToBase64UrlEncoder.ts
+++ b/src/encoder/binaryToBase64UrlEncoder.ts
@@ -1,4 +1,4 @@
-import { Encoder } from "./encoder";
+import { Encoder } from "./encoder.js";
 
 enum Side {
     BEGINING,
@@ -12,7 +12,7 @@ export class BinaryToBase64UrlEncoder implements Encoder {
 
     public encode(binary: string): string {
         const padded = BinaryToBase64UrlEncoder.padString(binary, Side.END, 8, "0");
-        const bytes = padded.match(/.{8}/g);
+        const bytes = padded.match(/.{8}/g) ?? [];
         const chars = bytes
             .map(b => String.fromCharCode(parseInt(b, 2)))
             .join("");
@@ -36,7 +36,7 @@ export class BinaryToBase64UrlEncoder implements Encoder {
         try {
             chars = atob(b64);
         } catch (e) {
-            throw "Invalid Base64Url string!";
+            throw new Error("Invalid Base64Url string!");
         }
 
         const bytes = chars.split("")
diff --git a/src/encoder/binaryToBase64UrlEncoderV2.ts b/src/encoder/binaryToBase64UrlEncoderV2.ts
index 4f12bfc..d60f228 100644
--- a/src/encoder/binaryToBase64UrlEncoderV2.ts
+++ b/src/encoder/binaryToBase64UrlEncoderV2.ts
@@ -1,4 +1,4 @@
-import { Encoder } from "./encoder";
+import { Encoder } from "./encoder.js";
 
 enum Side {
     BEGINING,
@@ -16,7 +16,7 @@ export class BinaryToBase64UrlEncoderV2 implements Encoder {
 
     public encode(binary: string): string {
         const padded = BinaryToBase64UrlEncoderV2.padString(binary, Side.END, 6, "0");
-        const b64u = padded.match(/.{6}/g)
+        const b64u = (padded.match(/.{6}/g) ?? [])
             .map(sextet => ALPHABET[parseInt(sextet, 2)])
             .join("");
 
@@ -28,7 +28,7 @@ export class BinaryToBase64UrlEncoderV2 implements Encoder {
             .map(ch => {
                 const index = ALPHABET.indexOf(ch);
                 if (index == -1) {
-                    throw "Invalid Base64Url string!";
+                    throw new Error("Invalid Base64Url string!");
                 }
 
                 return index;
diff --git a/src/game.ts b/src/game.ts
index da07012..046a50a 100644
--- a/src/game.ts
+++ b/src/game.ts
@@ -1,9 +1,9 @@
-import { Board } from "./board";
-import { Timer } from "./timer";
-import { Counter } from "./counter";
-import { Config, Mode, BOARD_CONFIG, MODE_NAME } from "./config";
-import { State } from "./state";
-import { UrlTool } from "./urlTool";
+import { Board } from "./board.js";
+import { Timer } from "./timer.js";
+import { Counter } from "./counter.js";
+import { Config, Mode, BOARD_CONFIG, MODE_NAME } from "./config.js";
+import { State } from "./state.js";
+import { UrlTool } from "./urlTool.js";
 import {
     EVENT_CELL_REVEALED,
     EVENT_CELL_FLAGGED,
@@ -12,9 +12,9 @@ import {
     EVENT_SAFE_AREA_CREATED,
     EVENT_SETTINGS_CHANGED,
     PubSub,
-} from "./util/pub-sub";
-import { Session } from "./util/session";
-import { Settings } from "./settings";
+} from "./util/pub-sub.js";
+import { Session } from "./util/session.js";
+import { Settings } from "./settings.js";
 
 export class Game {
 
@@ -39,20 +39,20 @@ export class Game {
     constructor(private config: Config) {
         document.body.classList.toggle("dark", this.config.darkModeOn);
 
-        this.counter = new Counter(document.getElementById("mines-counter"));
-        this.timer = new Timer(document.getElementById("timer"));
+        this.counter = new Counter(document.getElementById("mines-counter")!);
+        this.timer = new Timer(document.getElementById("timer")!);
 
-        this.resetBtn = document.getElementById("reset");
+        this.resetBtn = document.getElementById("reset")!;
         this.resetBtn.addEventListener("click", this.reset.bind(this));
 
-        this.replayBtn = document.getElementById("replay");
+        this.replayBtn = document.getElementById("replay")!;
         this.replayBtn.addEventListener("click", this.replay.bind(this));
 
-        this.toggleSettingsBtn = document.getElementById("toggle-settings");
+        this.toggleSettingsBtn = document.getElementById("toggle-settings")!;
         this.toggleSettingsBtn.addEventListener("click", this.toggleSettings.bind(this));
 
-        this.boardEl = document.getElementById("board");
-        this.settingsEl = document.getElementById("settings");
+        this.boardEl = document.getElementById("board")!;
+        this.settingsEl = document.getElementById("settings")!;
         window.addEventListener("hashchange", this.handleHashChange.bind(this));
 
         this.urlTool = new UrlTool(
@@ -131,7 +131,7 @@ export class Game {
 
     private generateBoard(): void {
         let mode: Mode;
-        let state: State;
+        let state: State | null;
 
         if (this.isReset) {
             mode = this.board.getMode();
@@ -150,7 +150,7 @@ export class Game {
                 console.warn("Could not extract mode or state from hash. Falling back to defaults.");
             }
         } else {
-            mode = BOARD_CONFIG[this.config.mode];
+            mode = BOARD_CONFIG[this.config.mode]!;
             state = null;
             Session.set("applyFirstClickRule", true);
         }
@@ -161,8 +161,7 @@ export class Game {
     }
 
     private getModeNameFromMode(mode: Mode): MODE_NAME {
-        for (const modeKey in MODE_NAME) {
-            const modeValue = MODE_NAME[modeKey];
+        for (const modeValue of Object.values(MODE_NAME)) {
             const m = BOARD_CONFIG[modeValue];
             if (m == null) {
                 continue;
diff --git a/src/main.ts b/src/main.ts
index e92edd2..06340ff 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -1,7 +1,7 @@
-import { Game } from "./game";
-import { FIRST_CLICK, MODE_NAME, Config } from "./config";
-import { CantorPairer } from "./pairer/cantorPairer";
-import { BinaryToBase64UrlEncoderV2 } from "./encoder/binaryToBase64UrlEncoderV2";
+import { Game } from "./game.js";
+import { FIRST_CLICK, MODE_NAME, Config } from "./config.js";
+import { CantorPairer } from "./pairer/cantorPairer.js";
+import { BinaryToBase64UrlEncoderV2 } from "./encoder/binaryToBase64UrlEncoderV2.js";
 
 const config: Config = {
     mode: MODE_NAME.Expert,
diff --git a/src/pairer/cantorPairer.ts b/src/pairer/cantorPairer.ts
index f5a6b3b..f1e6397 100644
--- a/src/pairer/cantorPairer.ts
+++ b/src/pairer/cantorPairer.ts
@@ -1,4 +1,4 @@
-import { Pairer, Tuple } from "./pairer";
+import { Pairer, Tuple } from "./pairer.js";
 
 /**
  * Implemetns Cantor pairing and inverted pairing functions
diff --git a/src/pairer/mortonPairer.ts b/src/pairer/mortonPairer.ts
index 045a514..9c5f151 100644
--- a/src/pairer/mortonPairer.ts
+++ b/src/pairer/mortonPairer.ts
@@ -1,4 +1,4 @@
-import { Pairer, Tuple } from "./pairer";
+import { Pairer, Tuple } from "./pairer.js";
 
 /**
  * Implements Morton order function (Z-Order curve)
diff --git a/src/pairer/peterPairer.ts b/src/pairer/peterPairer.ts
index 7874215..cd2b036 100644
--- a/src/pairer/peterPairer.ts
+++ b/src/pairer/peterPairer.ts
@@ -1,4 +1,4 @@
-import { Pairer, Tuple } from "./pairer";
+import { Pairer, Tuple } from "./pairer.js";
 
 export class PeterPairer implements Pairer {
 
diff --git a/src/pairer/szudzikPairer.ts b/src/pairer/szudzikPairer.ts
index ecad411..626d8df 100644
--- a/src/pairer/szudzikPairer.ts
+++ b/src/pairer/szudzikPairer.ts
@@ -1,4 +1,4 @@
-import { Pairer, Tuple } from "./pairer";
+import { Pairer, Tuple } from "./pairer.js";
 
 export class SzudzikPairer implements Pairer {
 
diff --git a/src/settings.ts b/src/settings.ts
index 46445ac..011a19a 100644
--- a/src/settings.ts
+++ b/src/settings.ts
@@ -1,5 +1,5 @@
-import { BOARD_CONFIG, Config, FIRST_CLICK, MODE_NAME } from "./config";
-import { EVENT_SETTINGS_CHANGED, PubSub } from "./util/pub-sub";
+import { BOARD_CONFIG, Config, FIRST_CLICK, Mode, MODE_NAME } from "./config.js";
+import { EVENT_SETTINGS_CHANGED, PubSub } from "./util/pub-sub.js";
 
 enum AVAILABLE_SETTINGS {
     mode = "Mode",
@@ -42,8 +42,7 @@ export class Settings {
         modeSwitchWrapper.id = "mode_switch_wrapper";
         fieldset.append(modeSwitchWrapper);
 
-        Object.keys(MODE_NAME).forEach(modeKey => {
-            const modeValue = MODE_NAME[modeKey];
+        Object.entries(MODE_NAME).forEach(([modeKey, modeValue]) => {
             if (BOARD_CONFIG[modeValue] == null) {
                 return;
             }
@@ -54,7 +53,7 @@ export class Settings {
         const modeDetailsWrapper = document.createElement("div");
         modeDetailsWrapper.id = "mode_details_wrapper";
         fieldset.append(modeDetailsWrapper);
-        Object.keys(BOARD_CONFIG[MODE_NAME.Beginner]).forEach(modeProperty => {
+        Object.keys(BOARD_CONFIG[MODE_NAME.Beginner]!).forEach(modeProperty => {
             this.drawModeDetails(modeDetailsWrapper, modeProperty);
         });
     }
@@ -65,12 +64,13 @@ export class Settings {
                 return;
             }
 
+            const value = FIRST_CLICK[firstClickKey as keyof typeof FIRST_CLICK];
             this.drawRadioButton(
                 fieldset,
                 firstClickKey,
                 "firstClick",
-                FIRST_CLICK[firstClickKey],
-                this.config.firstClick == FIRST_CLICK[firstClickKey]);
+                value.toString(),
+                this.config.firstClick == value);
         })
     }
 
@@ -123,9 +123,10 @@ export class Settings {
     }
 
     private drawModeSwitch(parent: HTMLElement, modeKey: string, modeValue: MODE_NAME, fieldset: HTMLElement) {
-        const rows = BOARD_CONFIG[modeValue].rows;
-        const cols = BOARD_CONFIG[modeValue].cols;
-        const mines = BOARD_CONFIG[modeValue].mines;
+        const modeConfig = BOARD_CONFIG[modeValue]!;
+        const rows = modeConfig.rows;
+        const cols = modeConfig.cols;
+        const mines = modeConfig.mines;
         const current = rows == BOARD_CONFIG[this.config.mode]?.rows &&
             cols == BOARD_CONFIG[this.config.mode]?.cols &&
             mines == BOARD_CONFIG[this.config.mode]?.mines;
@@ -153,7 +154,8 @@ export class Settings {
         const input = document.createElement("input");
         input.setAttribute("type", "number");
         input.setAttribute("id", `${modeProperty}Input`);
-        input.setAttribute("value", BOARD_CONFIG[this.config.mode] == null ? "" : BOARD_CONFIG[this.config.mode][modeProperty].toString());
+        const currentModeConfig = BOARD_CONFIG[this.config.mode];
+        input.setAttribute("value", currentModeConfig == null ? "" : currentModeConfig[modeProperty as keyof Mode].toString());
         input.disabled = true;
         wrapper.appendChild(input);
         parent.appendChild(wrapper);
@@ -207,14 +209,19 @@ ${navigator.userAgent}`;
 
     private updateConfig(fieldset: HTMLElement, e: MouseEvent) {
         const target = e.currentTarget as HTMLElement;
-        this.config[target.getAttribute("data-configKey")] = target.getAttribute("data-configValue");
-        this.drawFieldset(target.getAttribute("data-configKey") as keyof typeof AVAILABLE_SETTINGS, fieldset);
+        const configKey = target.getAttribute("data-configKey");
+        if (configKey == null) {
+            return;
+        }
+        // Dynamic write of a config field (mode / firstClick) from a DOM attribute string
+        (this.config as any)[configKey] = target.getAttribute("data-configValue");
+        this.drawFieldset(configKey as keyof typeof AVAILABLE_SETTINGS, fieldset);
         PubSub.publish(EVENT_SETTINGS_CHANGED)
     }
 
     private toggleDarkMode() {
         const state = document.body.classList.toggle("dark");
-        document.getElementById("darkModeCheckbox").setAttribute("checked", String(state));
+        document.getElementById("darkModeCheckbox")!.setAttribute("checked", String(state));
     }
 
 }
diff --git a/src/state.ts b/src/state.ts
index bc3e5bd..d5817ab 100644
--- a/src/state.ts
+++ b/src/state.ts
@@ -16,7 +16,7 @@ export class State {
 
     public isHighBit(index: number): boolean {
         if (index >= this.size) {
-            throw "Index out of bounds!";
+            throw new Error("Index out of bounds!");
         }
 
         return this.data[index] == 1;
@@ -24,7 +24,7 @@ export class State {
 
     public setBit(index: number): void {
         if (index >= this.size) {
-            throw "Index out of bounds!";
+            throw new Error("Index out of bounds!");
         }
 
         this.data[index] = 1;
@@ -32,7 +32,7 @@ export class State {
 
     public unsetBit(index: number): void {
         if (index >= this.size) {
-            throw "Index out of bounds!";
+            throw new Error("Index out of bounds!");
         }
 
         this.data[index] = 0;
diff --git a/src/timer.ts b/src/timer.ts
index 5f77634..c8791a2 100644
--- a/src/timer.ts
+++ b/src/timer.ts
@@ -1,5 +1,5 @@
 export class Timer {
-    private intervaID: number;
+    private intervaID: number | undefined;
     private value: number = 0;
 
     constructor(private el: HTMLElement) { }
diff --git a/src/urlTool.ts b/src/urlTool.ts
index 58d0702..cd22bf2 100644
--- a/src/urlTool.ts
+++ b/src/urlTool.ts
@@ -1,7 +1,7 @@
-import { Encoder } from "./encoder/encoder";
-import { Pairer, Tuple } from "./pairer/pairer";
-import { Mode } from "./config";
-import { State } from "./state";
+import { Encoder } from "./encoder/encoder.js";
+import { Pairer, Tuple } from "./pairer/pairer.js";
+import { Mode } from "./config.js";
+import { State } from "./state.js";
 
 const MODE_SIZE = 24;
 const MIN_ROWS = 5;
@@ -21,7 +21,7 @@ export class UrlTool {
         return window.location.hash.length > 1;
     }
 
-    public extractMode(): Mode {
+    public extractMode(): Mode | null {
         try {
             this.decodedHash = this.encoder.decode(window.location.hash.slice(1));
         } catch (e) {
@@ -59,7 +59,7 @@ export class UrlTool {
         }
     }
 
-    public extractState(mode: Mode): State {
+    public extractState(mode: Mode): State | null {
         const stateString = this.decodedHash.slice(MODE_SIZE, mode.rows * mode.cols + MODE_SIZE);
         if (mode.rows * mode.cols != stateString.length) {
             console.error("Invalid hash! Can't extract state!");
@@ -75,7 +75,7 @@ export class UrlTool {
         return state;
     }
 
-    public updateHash(mode: Mode, state: State): void {
+    public updateHash(mode: Mode | null, state: State | null): void {
         let encodedHash = "";
 
         if (mode != null && state != null) {
@@ -84,7 +84,7 @@ export class UrlTool {
             encodedHash = this.encoder.encode(decodedHash);
         }
 
-        history.replaceState(undefined, undefined, `#${encodedHash}`);
+        history.replaceState(undefined, "", `#${encodedHash}`);
     }
 
     private encodeMode(mode: Mode): string {
diff --git a/src/util/pub-sub.ts b/src/util/pub-sub.ts
index fbfeda3..d215f2f 100644
--- a/src/util/pub-sub.ts
+++ b/src/util/pub-sub.ts
@@ -1,4 +1,4 @@
-import { Session } from "./session";
+import { Session } from "./session.js";
 
 interface CallbackFunc {
     (data?: any): any;
diff --git a/src/util/session.ts b/src/util/session.ts
index bfc32f3..8855eb8 100644
--- a/src/util/session.ts
+++ b/src/util/session.ts
@@ -14,7 +14,7 @@ export class Session {
         }
     }
 
-    public static get(key: string, defaultValue?: ValueType): ValueType {
+    public static get(key: string, defaultValue?: ValueType): ValueType | undefined {
         if (Session.data.has(key)) {
             return Session.data.get(key);
         }
diff --git a/styles.css b/styles.css
index 35dd644..abc8bdb 100644
--- a/styles.css
+++ b/styles.css
@@ -164,7 +164,7 @@ main {
 
 .cell.state-revealed,
 .cell.state-revealedMine,
-.cell.state-exploaded,
+.cell.state-exploded,
 .cell.state-wronglyFlagged {
     background-color: #bdbdbd;
     border-style: solid none none solid;
@@ -185,11 +185,11 @@ main {
 }
 
 .cell.state-revealedMine,
-.cell.state-exploaded {
+.cell.state-exploded {
     background-image: url(data:image/svg+xml;base64,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);
 }
 
-.cell.state-exploaded {
+.cell.state-exploded {
     background-color: #ff0000;
 }
 
diff --git a/test/cantorPairer.test.mjs b/test/cantorPairer.test.mjs
new file mode 100644
index 0000000..461284f
--- /dev/null
+++ b/test/cantorPairer.test.mjs
@@ -0,0 +1,43 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+
+import { CantorPairer } from "../dist/pairer/cantorPairer.js";
+
+const pairer = new CantorPairer();
+
+test("pair() produces the known Cantor values for small tuples", () => {
+    assert.equal(pairer.pair({ a: 0, b: 0 }), 0);
+    assert.equal(pairer.pair({ a: 1, b: 0 }), 1);
+    assert.equal(pairer.pair({ a: 0, b: 1 }), 2);
+    assert.equal(pairer.pair({ a: 1, b: 1 }), 4);
+});
+
+test("unpair() inverts pair() for a range of tuples", () => {
+    for (let a = 0; a < 40; a++) {
+        for (let b = 0; b < 40; b++) {
+            const x = pairer.pair({ a, b });
+            assert.deepEqual(pairer.unpair(x), { a, b }, `round-trip failed for (${a}, ${b})`);
+        }
+    }
+});
+
+// This mirrors how UrlTool folds a mode into a single integer:
+// pair(pair(rows, cols), mines) — then unpairs twice to recover it.
+test("nested pairing round-trips the built-in game modes", () => {
+    const modes = [
+        { rows: 9, cols: 14, mines: 10 },   // beginner
+        { rows: 16, cols: 16, mines: 40 },  // intermediate
+        { rows: 16, cols: 30, mines: 99 },  // expert
+    ];
+
+    for (const { rows, cols, mines } of modes) {
+        const paired = pairer.pair({ a: pairer.pair({ a: rows, b: cols }), b: mines });
+
+        const outer = pairer.unpair(paired);
+        const inner = pairer.unpair(outer.a);
+
+        assert.equal(inner.a, rows);
+        assert.equal(inner.b, cols);
+        assert.equal(outer.b, mines);
+    }
+});
diff --git a/test/encoder.test.mjs b/test/encoder.test.mjs
new file mode 100644
index 0000000..1e2bdd0
--- /dev/null
+++ b/test/encoder.test.mjs
@@ -0,0 +1,25 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+
+import { BinaryToBase64UrlEncoder } from "../dist/encoder/binaryToBase64UrlEncoder.js";
+import { BinaryToBase64UrlEncoderV2 } from "../dist/encoder/binaryToBase64UrlEncoderV2.js";
+
+// 24 bits — a multiple of both encoders' block sizes (8 and 6), so encode -> decode is exact.
+const binary = "100100011010001010111000";
+
+for (const Encoder of [BinaryToBase64UrlEncoder, BinaryToBase64UrlEncoderV2]) {
+    const encoder = new Encoder();
+
+    test(`${Encoder.name}: decode() inverts encode()`, () => {
+        assert.equal(encoder.decode(encoder.encode(binary)), binary);
+    });
+
+    test(`${Encoder.name}: produces a URL-safe string (no +, /, =)`, () => {
+        const encoded = encoder.encode(binary);
+        assert.doesNotMatch(encoded, /[+/=]/);
+    });
+
+    test(`${Encoder.name}: decode() rejects invalid input`, () => {
+        assert.throws(() => encoder.decode("!!!not base64url!!!"));
+    });
+}
diff --git a/test/state.test.mjs b/test/state.test.mjs
new file mode 100644
index 0000000..cee1629
--- /dev/null
+++ b/test/state.test.mjs
@@ -0,0 +1,28 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+
+import { State } from "../dist/state.js";
+
+test("a fresh State is all zeros", () => {
+    assert.equal(new State(8).toString(), "00000000");
+});
+
+test("setBit / isHighBit / unsetBit flip a single bit", () => {
+    const state = new State(8);
+
+    assert.equal(state.isHighBit(3), false);
+
+    state.setBit(3);
+    assert.equal(state.isHighBit(3), true);
+    assert.equal(state.toString(), "00010000");
+
+    state.unsetBit(3);
+    assert.equal(state.isHighBit(3), false);
+    assert.equal(state.toString(), "00000000");
+});
+
+test("out-of-bounds access throws", () => {
+    const state = new State(4);
+    assert.throws(() => state.setBit(4));
+    assert.throws(() => state.isHighBit(4));
+});
diff --git a/tsconfig.json b/tsconfig.json
index 2cdf007..2019420 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -1,9 +1,11 @@
 {
     "compilerOptions": {
-        "baseUrl": "./",
+        "rootDir": "./src",
         "outDir": "./dist",
-        "module": "amd",
-        "target": "ES2020"
+        "module": "es2020",
+        "target": "ES2020",
+        "strict": true,
+        "strictPropertyInitialization": false
     },
     "include": [
         "src/**/*"