diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
new file mode 100644
index 0000000..ceaa7da
--- /dev/null
+++ b/.github/workflows/tests.yml
@@ -0,0 +1,30 @@
+name: Tests
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+permissions: {}
+
+jobs:
+  test:
+    name: Run Tests
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    steps:
+      - name: Checkout Code
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          persist-credentials: false
+      - name: Setup Node.js
+        uses: actions/setup-node@53b83947a5a98c8d113130e565377fae1a50d02f # v6.3.0
+        with:
+          node-version: 24
+          cache: npm
+      - name: Install dependencies
+        run: npm ci
+      - name: Run tests
+        run: npm test
diff --git a/package-lock.json b/package-lock.json
index 8d9bbd2..08cb1ba 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -33,7 +33,8 @@
       "devDependencies": {
         "esbuild": "^0.25.0",
         "npm-run-all": "^4.1.5",
-        "postcss-cli": "^11.0.0"
+        "postcss-cli": "^11.0.0",
+        "vitest": "^4.1.2"
       },
       "engines": {
         "node": ">=20.0.0"
@@ -227,6 +228,37 @@
         "url": "https://github.com/sponsors/sindresorhus"
       }
     },
+    "node_modules/@emnapi/core": {
+      "version": "1.9.2",
+      "resolved": "https://registry.npmjs.org/@emnapi/core/-/core-1.9.2.tgz",
+      "integrity": "sha512-UC+ZhH3XtczQYfOlu3lNEkdW/p4dsJ1r/bP7H8+rhao3TTTMO1ATq/4DdIi23XuGoFY+Cz0JmCbdVl0hz9jZcA==",
+      "license": "MIT",
+      "optional": true,
+      "dependencies": {
+        "@emnapi/wasi-threads": "1.2.1",
+        "tslib": "^2.4.0"
+      }
+    },
+    "node_modules/@emnapi/runtime": {
+      "version": "1.9.2",
+      "resolved": "https://registry.npmjs.org/@emnapi/runtime/-/runtime-1.9.2.tgz",
+      "integrity": "sha512-3U4+MIWHImeyu1wnmVygh5WlgfYDtyf0k8AbLhMFxOipihf6nrWC4syIm/SwEeec0mNSafiiNnMJwbza/Is6Lw==",
+      "license": "MIT",
+      "optional": true,
+      "dependencies": {
+        "tslib": "^2.4.0"
+      }
+    },
+    "node_modules/@emnapi/wasi-threads": {
+      "version": "1.2.1",
+      "resolved": "https://registry.npmjs.org/@emnapi/wasi-threads/-/wasi-threads-1.2.1.tgz",
+      "integrity": "sha512-uTII7OYF+/Mes/MrcIOYp5yOtSMLBWSIoLPpcgwipoiKbli6k322tcoFsxoIIxPDqW01SQGAgko4EzZi2BNv2w==",
+      "license": "MIT",
+      "optional": true,
+      "dependencies": {
+        "tslib": "^2.4.0"
+      }
+    },
     "node_modules/@esbuild/aix-ppc64": {
       "version": "0.25.12",
       "resolved": "https://registry.npmjs.org/@esbuild/aix-ppc64/-/aix-ppc64-0.25.12.tgz",
@@ -714,6 +746,314 @@
         "@jridgewell/sourcemap-codec": "^1.4.14"
       }
     },
+    "node_modules/@napi-rs/wasm-runtime": {
+      "version": "1.1.2",
+      "resolved": "https://registry.npmjs.org/@napi-rs/wasm-runtime/-/wasm-runtime-1.1.2.tgz",
+      "integrity": "sha512-sNXv5oLJ7ob93xkZ1XnxisYhGYXfaG9f65/ZgYuAu3qt7b3NadcOEhLvx28hv31PgX8SZJRYrAIPQilQmFpLVw==",
+      "license": "MIT",
+      "optional": true,
+      "dependencies": {
+        "@tybys/wasm-util": "^0.10.1"
+      },
+      "funding": {
+        "type": "github",
+        "url": "https://github.com/sponsors/Brooooooklyn"
+      },
+      "peerDependencies": {
+        "@emnapi/core": "^1.7.1",
+        "@emnapi/runtime": "^1.7.1"
+      }
+    },
+    "node_modules/@oxc-project/types": {
+      "version": "0.122.0",
+      "resolved": "https://registry.npmjs.org/@oxc-project/types/-/types-0.122.0.tgz",
+      "integrity": "sha512-oLAl5kBpV4w69UtFZ9xqcmTi+GENWOcPF7FCrczTiBbmC0ibXxCwyvZGbO39rCVEuLGAZM84DH0pUIyyv/YJzA==",
+      "dev": true,
+      "license": "MIT",
+      "funding": {
+        "url": "https://github.com/sponsors/Boshen"
+      }
+    },
+    "node_modules/@rolldown/binding-android-arm64": {
+      "version": "1.0.0-rc.12",
+      "resolved": "https://registry.npmjs.org/@rolldown/binding-android-arm64/-/binding-android-arm64-1.0.0-rc.12.tgz",
+      "integrity": "sha512-pv1y2Fv0JybcykuiiD3qBOBdz6RteYojRFY1d+b95WVuzx211CRh+ytI/+9iVyWQ6koTh5dawe4S/yRfOFjgaA==",
+      "cpu": [
+        "arm64"
+      ],
+      "dev": true,
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "android"
+      ],
+      "engines": {
+        "node": "^20.19.0 || >=22.12.0"
+      }
+    },
+    "node_modules/@rolldown/binding-darwin-arm64": {
+      "version": "1.0.0-rc.12",
+      "resolved": "https://registry.npmjs.org/@rolldown/binding-darwin-arm64/-/binding-darwin-arm64-1.0.0-rc.12.tgz",
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+      "cpu": [
+        "arm64"
+      ],
+      "dev": true,
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "darwin"
+      ],
+      "engines": {
+        "node": "^20.19.0 || >=22.12.0"
+      }
+    },
+    "node_modules/@rolldown/binding-darwin-x64": {
+      "version": "1.0.0-rc.12",
+      "resolved": "https://registry.npmjs.org/@rolldown/binding-darwin-x64/-/binding-darwin-x64-1.0.0-rc.12.tgz",
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+      "cpu": [
+        "x64"
+      ],
+      "dev": true,
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "darwin"
+      ],
+      "engines": {
+        "node": "^20.19.0 || >=22.12.0"
+      }
+    },
+    "node_modules/@rolldown/binding-freebsd-x64": {
+      "version": "1.0.0-rc.12",
+      "resolved": "https://registry.npmjs.org/@rolldown/binding-freebsd-x64/-/binding-freebsd-x64-1.0.0-rc.12.tgz",
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+      "cpu": [
+        "x64"
+      ],
+      "dev": true,
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "freebsd"
+      ],
+      "engines": {
+        "node": "^20.19.0 || >=22.12.0"
+      }
+    },
+    "node_modules/@rolldown/binding-linux-arm-gnueabihf": {
+      "version": "1.0.0-rc.12",
+      "resolved": "https://registry.npmjs.org/@rolldown/binding-linux-arm-gnueabihf/-/binding-linux-arm-gnueabihf-1.0.0-rc.12.tgz",
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+      "cpu": [
+        "arm"
+      ],
+      "dev": true,
+      "license": "MIT",
+      "optional": true,
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+        "linux"
+      ],
+      "engines": {
+        "node": "^20.19.0 || >=22.12.0"
+      }
+    },
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+      "cpu": [
+        "arm64"
+      ],
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+      "libc": [
+        "glibc"
+      ],
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "linux"
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+      "engines": {
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+      }
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+      }
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+      "resolved": "https://registry.npmjs.org/@rolldown/binding-linux-s390x-gnu/-/binding-linux-s390x-gnu-1.0.0-rc.12.tgz",
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+        "s390x"
+      ],
+      "dev": true,
+      "libc": [
+        "glibc"
+      ],
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "linux"
+      ],
+      "engines": {
+        "node": "^20.19.0 || >=22.12.0"
+      }
+    },
+    "node_modules/@rolldown/binding-linux-x64-gnu": {
+      "version": "1.0.0-rc.12",
+      "resolved": "https://registry.npmjs.org/@rolldown/binding-linux-x64-gnu/-/binding-linux-x64-gnu-1.0.0-rc.12.tgz",
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+      "cpu": [
+        "x64"
+      ],
+      "dev": true,
+      "libc": [
+        "glibc"
+      ],
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "linux"
+      ],
+      "engines": {
+        "node": "^20.19.0 || >=22.12.0"
+      }
+    },
+    "node_modules/@rolldown/binding-linux-x64-musl": {
+      "version": "1.0.0-rc.12",
+      "resolved": "https://registry.npmjs.org/@rolldown/binding-linux-x64-musl/-/binding-linux-x64-musl-1.0.0-rc.12.tgz",
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+      "optional": true,
+      "os": [
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+      ],
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+      }
+    },
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+      "version": "1.0.0-rc.12",
+      "resolved": "https://registry.npmjs.org/@rolldown/binding-openharmony-arm64/-/binding-openharmony-arm64-1.0.0-rc.12.tgz",
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+      "cpu": [
+        "arm64"
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+      "dev": true,
+      "license": "MIT",
+      "optional": true,
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+    },
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+      "resolved": "https://registry.npmjs.org/@rolldown/binding-wasm32-wasi/-/binding-wasm32-wasi-1.0.0-rc.12.tgz",
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+      "cpu": [
+        "wasm32"
+      ],
+      "dev": true,
+      "license": "MIT",
+      "optional": true,
+      "dependencies": {
+        "@napi-rs/wasm-runtime": "^1.1.1"
+      },
+      "engines": {
+        "node": ">=14.0.0"
+      }
+    },
+    "node_modules/@rolldown/binding-win32-arm64-msvc": {
+      "version": "1.0.0-rc.12",
+      "resolved": "https://registry.npmjs.org/@rolldown/binding-win32-arm64-msvc/-/binding-win32-arm64-msvc-1.0.0-rc.12.tgz",
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+        "arm64"
+      ],
+      "dev": true,
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "win32"
+      ],
+      "engines": {
+        "node": "^20.19.0 || >=22.12.0"
+      }
+    },
+    "node_modules/@rolldown/binding-win32-x64-msvc": {
+      "version": "1.0.0-rc.12",
+      "resolved": "https://registry.npmjs.org/@rolldown/binding-win32-x64-msvc/-/binding-win32-x64-msvc-1.0.0-rc.12.tgz",
+      "integrity": "sha512-PyqoipaswDLAZtot351MLhrlrh6lcZPo2LSYE+VDxbVk24LVKAGOuE4hb8xZQmrPAuEtTZW8E6D2zc5EUZX4Lw==",
+      "cpu": [
+        "x64"
+      ],
+      "dev": true,
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "win32"
+      ],
+      "engines": {
+        "node": "^20.19.0 || >=22.12.0"
+      }
+    },
+    "node_modules/@rolldown/pluginutils": {
+      "version": "1.0.0-rc.12",
+      "resolved": "https://registry.npmjs.org/@rolldown/pluginutils/-/pluginutils-1.0.0-rc.12.tgz",
+      "integrity": "sha512-HHMwmarRKvoFsJorqYlFeFRzXZqCt2ETQlEDOb9aqssrnVBB1/+xgTGtuTrIk5vzLNX1MjMtTf7W9z3tsSbrxw==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/@sindresorhus/slugify": {
       "version": "2.2.1",
       "resolved": "https://registry.npmjs.org/@sindresorhus/slugify/-/slugify-2.2.1.tgz",
@@ -745,6 +1085,13 @@
         "url": "https://github.com/sponsors/sindresorhus"
       }
     },
+    "node_modules/@standard-schema/spec": {
+      "version": "1.1.0",
+      "resolved": "https://registry.npmjs.org/@standard-schema/spec/-/spec-1.1.0.tgz",
+      "integrity": "sha512-l2aFy5jALhniG5HgqrD6jXLi/rUWrKvqN/qJx6yoJsgKhblVd+iqqU4RCXavm/jPityDo5TCvKMnpjKnOriy0w==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/@tailwindcss/node": {
       "version": "4.2.2",
       "resolved": "https://registry.npmjs.org/@tailwindcss/node/-/node-4.2.2.tgz",
@@ -1013,6 +1360,127 @@
         "tailwindcss": "4.2.2"
       }
     },
+    "node_modules/@tybys/wasm-util": {
+      "version": "0.10.1",
+      "resolved": "https://registry.npmjs.org/@tybys/wasm-util/-/wasm-util-0.10.1.tgz",
+      "integrity": "sha512-9tTaPJLSiejZKx+Bmog4uSubteqTvFrVrURwkmHixBo0G4seD0zUxp98E1DzUBJxLQ3NPwXrGKDiVjwx/DpPsg==",
+      "license": "MIT",
+      "optional": true,
+      "dependencies": {
+        "tslib": "^2.4.0"
+      }
+    },
+    "node_modules/@types/chai": {
+      "version": "5.2.3",
+      "resolved": "https://registry.npmjs.org/@types/chai/-/chai-5.2.3.tgz",
+      "integrity": "sha512-Mw558oeA9fFbv65/y4mHtXDs9bPnFMZAL/jxdPFUpOHHIXX91mcgEHbS5Lahr+pwZFR8A7GQleRWeI6cGFC2UA==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@types/deep-eql": "*",
+        "assertion-error": "^2.0.1"
+      }
+    },
+    "node_modules/@types/deep-eql": {
+      "version": "4.0.2",
+      "resolved": "https://registry.npmjs.org/@types/deep-eql/-/deep-eql-4.0.2.tgz",
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+      "license": "MIT"
+    },
+    "node_modules/@types/estree": {
+      "version": "1.0.8",
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+          "optional": true
+        },
+        "sugarss": {
+          "optional": true
+        },
+        "terser": {
+          "optional": true
+        },
+        "tsx": {
+          "optional": true
+        },
+        "yaml": {
+          "optional": true
+        }
+      }
+    },
     "node_modules/which": {
       "version": "1.3.1",
       "resolved": "https://registry.npmjs.org/which/-/which-1.3.1.tgz",
@@ -5054,6 +5870,23 @@
         "url": "https://github.com/sponsors/ljharb"
       }
     },
+    "node_modules/why-is-node-running": {
+      "version": "2.3.0",
+      "resolved": "https://registry.npmjs.org/why-is-node-running/-/why-is-node-running-2.3.0.tgz",
+      "integrity": "sha512-hUrmaWBdVDcxvYqnyh09zunKzROWjbZTiNy8dBEjkS7ehEDQibXJ7XvlmtbwuTclUiIyN+CyXQD4Vmko8fNm8w==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "siginfo": "^2.0.0",
+        "stackback": "0.0.2"
+      },
+      "bin": {
+        "why-is-node-running": "cli.js"
+      },
+      "engines": {
+        "node": ">=8"
+      }
+    },
     "node_modules/wrap-ansi": {
       "version": "7.0.0",
       "resolved": "https://registry.npmjs.org/wrap-ansi/-/wrap-ansi-7.0.0.tgz",
diff --git a/package.json b/package.json
index 79156f8..115a92a 100644
--- a/package.json
+++ b/package.json
@@ -28,7 +28,8 @@
   "devDependencies": {
     "esbuild": "^0.25.0",
     "npm-run-all": "^4.1.5",
-    "postcss-cli": "^11.0.0"
+    "postcss-cli": "^11.0.0",
+    "vitest": "^4.1.2"
   },
   "scripts": {
     "css:build": "postcss ./src/assets/css/tailwind-input.css -o ./src/assets/css/tailwind.css",
@@ -39,7 +40,9 @@
     "eleventy:serve": "eleventy --serve",
     "dev:watch": "npm-run-all --parallel css:watch js:watch eleventy:serve",
     "start": "npm-run-all --sequential js:build css:build dev:watch",
-    "build": "npm-run-all --sequential js:build css:build eleventy:build"
+    "build": "npm-run-all --sequential js:build css:build eleventy:build",
+    "test": "vitest run",
+    "test:watch": "vitest"
   },
   "repository": {
     "type": "git",
diff --git a/src/assets/js/games/config.js b/src/assets/js/games/config.js
index 61fe8d5..2340348 100644
--- a/src/assets/js/games/config.js
+++ b/src/assets/js/games/config.js
@@ -118,3 +118,16 @@ function buildGameConfig(overrides) {
     overrides || {},
   );
 }
+
+// Export for testing (Node.js / Vitest)
+if (typeof module !== "undefined" && module.exports) {
+  module.exports = {
+    PHASER_CDN_URL,
+    RARITY_WEIGHTS,
+    RARITY_COLORS,
+    GENERIC_SKILLS,
+    skillRarity,
+    getGameTheme,
+    buildGameConfig,
+  };
+}
diff --git a/tests/build-data.test.mjs b/tests/build-data.test.mjs
new file mode 100644
index 0000000..6ae8fb2
--- /dev/null
+++ b/tests/build-data.test.mjs
@@ -0,0 +1,45 @@
+import { vi } from "vitest";
+import { createRequire } from "node:module";
+
+const require = createRequire(import.meta.url);
+
+describe("build.js data file", () => {
+  it("should export a function", () => {
+    const buildData = require("../src/_data/build.js");
+    expect(typeof buildData).toBe("function");
+  });
+
+  it("should return an object with timestamp, hash, and repoUrl", () => {
+    const buildData = require("../src/_data/build.js");
+    const result = buildData();
+
+    expect(result).toHaveProperty("timestamp");
+    expect(result).toHaveProperty("hash");
+    expect(result).toHaveProperty("repoUrl");
+  });
+
+  it("should return the correct repository URL", () => {
+    const buildData = require("../src/_data/build.js");
+    const result = buildData();
+
+    expect(result.repoUrl).toBe(
+      "https://github.com/NextCommunity/NextCommunity.github.io",
+    );
+  });
+
+  it("should return a non-empty timestamp string", () => {
+    const buildData = require("../src/_data/build.js");
+    const result = buildData();
+
+    expect(typeof result.timestamp).toBe("string");
+    expect(result.timestamp.length).toBeGreaterThan(0);
+  });
+
+  it("should return a non-empty hash string", () => {
+    const buildData = require("../src/_data/build.js");
+    const result = buildData();
+
+    expect(typeof result.hash).toBe("string");
+    expect(result.hash.length).toBeGreaterThan(0);
+  });
+});
diff --git a/tests/config.test.mjs b/tests/config.test.mjs
new file mode 100644
index 0000000..4aaa35c
--- /dev/null
+++ b/tests/config.test.mjs
@@ -0,0 +1,134 @@
+import { createRequire } from "node:module";
+
+const require = createRequire(import.meta.url);
+const {
+  PHASER_CDN_URL,
+  RARITY_WEIGHTS,
+  RARITY_COLORS,
+  GENERIC_SKILLS,
+  skillRarity,
+} = require("../src/assets/js/games/config");
+
+describe("config.js", () => {
+  describe("PHASER_CDN_URL", () => {
+    it("should be a valid CDN URL for Phaser 3.90.0", () => {
+      expect(PHASER_CDN_URL).toBe(
+        "https://cdnjs.cloudflare.com/ajax/libs/phaser/3.90.0/phaser.min.js",
+      );
+    });
+  });
+
+  describe("RARITY_WEIGHTS", () => {
+    it("should contain all seven rarity tiers", () => {
+      const tiers = [
+        "common",
+        "uncommon",
+        "rare",
+        "epic",
+        "legendary",
+        "mythic",
+        "absolute",
+      ];
+      for (const tier of tiers) {
+        expect(RARITY_WEIGHTS).toHaveProperty(tier);
+      }
+    });
+
+    it("should have ascending weights", () => {
+      const ordered = [
+        RARITY_WEIGHTS.common,
+        RARITY_WEIGHTS.uncommon,
+        RARITY_WEIGHTS.rare,
+        RARITY_WEIGHTS.epic,
+        RARITY_WEIGHTS.legendary,
+        RARITY_WEIGHTS.mythic,
+        RARITY_WEIGHTS.absolute,
+      ];
+      for (let i = 1; i < ordered.length; i++) {
+        expect(ordered[i]).toBeGreaterThan(ordered[i - 1]);
+      }
+    });
+
+    it("should have correct specific values", () => {
+      expect(RARITY_WEIGHTS.common).toBe(1);
+      expect(RARITY_WEIGHTS.uncommon).toBe(2);
+      expect(RARITY_WEIGHTS.rare).toBe(3);
+      expect(RARITY_WEIGHTS.epic).toBe(5);
+      expect(RARITY_WEIGHTS.legendary).toBe(8);
+      expect(RARITY_WEIGHTS.mythic).toBe(13);
+      expect(RARITY_WEIGHTS.absolute).toBe(21);
+    });
+  });
+
+  describe("RARITY_COLORS", () => {
+    it("should have a hex color for every rarity tier", () => {
+      const tiers = Object.keys(RARITY_WEIGHTS);
+      for (const tier of tiers) {
+        expect(RARITY_COLORS[tier]).toMatch(/^#[0-9a-f]{6}$/i);
+      }
+    });
+  });
+
+  describe("GENERIC_SKILLS", () => {
+    it("should contain 20 skills", () => {
+      expect(GENERIC_SKILLS).toHaveLength(20);
+    });
+
+    it("should contain known tech skills", () => {
+      expect(GENERIC_SKILLS).toContain("JavaScript");
+      expect(GENERIC_SKILLS).toContain("Python");
+      expect(GENERIC_SKILLS).toContain("Docker");
+    });
+
+    it("should contain only unique entries", () => {
+      const unique = new Set(GENERIC_SKILLS);
+      expect(unique.size).toBe(GENERIC_SKILLS.length);
+    });
+  });
+
+  describe("skillRarity()", () => {
+    it("should return a valid rarity tier", () => {
+      const validTiers = [
+        "common",
+        "uncommon",
+        "rare",
+        "epic",
+        "legendary",
+        "mythic",
+        "absolute",
+      ];
+      const result = skillRarity("JavaScript");
+      expect(validTiers).toContain(result);
+    });
+
+    it("should be deterministic (same input → same output)", () => {
+      const first = skillRarity("React");
+      const second = skillRarity("React");
+      expect(first).toBe(second);
+    });
+
+    it("should return consistent results for all generic skills", () => {
+      for (const skill of GENERIC_SKILLS) {
+        const a = skillRarity(skill);
+        const b = skillRarity(skill);
+        expect(a).toBe(b);
+      }
+    });
+
+    it("should handle empty string without error", () => {
+      const result = skillRarity("");
+      expect(typeof result).toBe("string");
+    });
+
+    it("should handle single character", () => {
+      const result = skillRarity("A");
+      expect(typeof result).toBe("string");
+    });
+
+    it("should produce different results for different skills", () => {
+      const results = GENERIC_SKILLS.map(skillRarity);
+      const unique = new Set(results);
+      expect(unique.size).toBeGreaterThan(1);
+    });
+  });
+});
diff --git a/tests/eleventy-config.test.mjs b/tests/eleventy-config.test.mjs
new file mode 100644
index 0000000..016528b
--- /dev/null
+++ b/tests/eleventy-config.test.mjs
@@ -0,0 +1,161 @@
+import { vi } from "vitest";
+import { createRequire } from "node:module";
+
+const require = createRequire(import.meta.url);
+
+describe(".eleventy.js configuration", () => {
+  let eleventyConfig;
+  let configFn;
+
+  beforeEach(() => {
+    eleventyConfig = {
+      shortcodes: {},
+      templateFormats: [],
+      extensions: {},
+      passthroughCopies: [],
+      collections: {},
+
+      addShortcode(name, fn) {
+        this.shortcodes[name] = fn;
+      },
+      addTemplateFormats(formats) {
+        this.templateFormats.push(formats);
+      },
+      addExtension(ext, opts) {
+        this.extensions[ext] = opts;
+      },
+      addPassthroughCopy(src) {
+        this.passthroughCopies.push(src);
+      },
+      addCollection(name, fn) {
+        this.collections[name] = fn;
+      },
+    };
+
+    configFn = require("../.eleventy.js");
+  });
+
+  it("should export a function", () => {
+    expect(typeof configFn).toBe("function");
+  });
+
+  it("should return correct directory configuration", () => {
+    const result = configFn(eleventyConfig);
+
+    expect(result).toEqual({
+      dir: {
+        input: "src",
+        output: "_site",
+        includes: "_includes",
+      },
+    });
+  });
+
+  describe("shortcodes", () => {
+    it("should register a currentYear shortcode", () => {
+      configFn(eleventyConfig);
+      expect(eleventyConfig.shortcodes).toHaveProperty("currentYear");
+    });
+
+    it("currentYear should return the current year as a number", () => {
+      configFn(eleventyConfig);
+      const year = eleventyConfig.shortcodes.currentYear();
+      expect(year).toBe(new Date().getFullYear());
+    });
+  });
+
+  describe("template formats", () => {
+    it("should register yaml as a template format", () => {
+      configFn(eleventyConfig);
+      expect(eleventyConfig.templateFormats).toContain("yaml");
+    });
+  });
+
+  describe("YAML extension", () => {
+    it("should register a yaml extension", () => {
+      configFn(eleventyConfig);
+      expect(eleventyConfig.extensions).toHaveProperty("yaml");
+    });
+
+    it("yaml extension should have compile and getData functions", () => {
+      configFn(eleventyConfig);
+      const yamlExt = eleventyConfig.extensions.yaml;
+      expect(typeof yamlExt.compile).toBe("function");
+      expect(typeof yamlExt.getData).toBe("function");
+    });
+
+    it("compile should extract bio from YAML content", async () => {
+      configFn(eleventyConfig);
+      const yamlExt = eleventyConfig.extensions.yaml;
+
+      const yamlContent = "name: Alice\nbio: Hello world\nskills:\n  - JS";
+      const renderFn = await yamlExt.compile(yamlContent);
+      const output = await renderFn();
+
+      expect(output).toBe("Hello world");
+    });
+
+    it("compile should return empty string when bio is missing", async () => {
+      configFn(eleventyConfig);
+      const yamlExt = eleventyConfig.extensions.yaml;
+
+      const yamlContent = "name: Bob\nskills:\n  - Python";
+      const renderFn = await yamlExt.compile(yamlContent);
+      const output = await renderFn();
+
+      expect(output).toBe("");
+    });
+  });
+
+  describe("passthrough copy", () => {
+    it("should copy src/assets to output", () => {
+      configFn(eleventyConfig);
+      expect(eleventyConfig.passthroughCopies).toContain("src/assets");
+    });
+  });
+
+  describe("randomPeople collection", () => {
+    it("should register a randomPeople collection", () => {
+      configFn(eleventyConfig);
+      expect(eleventyConfig.collections).toHaveProperty("randomPeople");
+    });
+
+    it("should return a shuffled copy of all user yaml files", () => {
+      configFn(eleventyConfig);
+      const collectionFn = eleventyConfig.collections.randomPeople;
+
+      const mockPeople = [
+        { data: { name: "Alice" } },
+        { data: { name: "Bob" } },
+        { data: { name: "Charlie" } },
+      ];
+      const collectionApi = {
+        getFilteredByGlob: vi.fn(() => mockPeople),
+      };
+
+      const result = collectionFn(collectionApi);
+
+      expect(collectionApi.getFilteredByGlob).toHaveBeenCalledWith(
+        "src/users/*.yaml",
+      );
+      expect(result).toHaveLength(mockPeople.length);
+
+      for (const person of mockPeople) {
+        expect(result).toContain(person);
+      }
+    });
+
+    it("should not mutate the original collection", () => {
+      configFn(eleventyConfig);
+      const collectionFn = eleventyConfig.collections.randomPeople;
+
+      const original = [{ name: "A" }, { name: "B" }, { name: "C" }];
+      const collectionApi = {
+        getFilteredByGlob: vi.fn(() => original),
+      };
+
+      const result = collectionFn(collectionApi);
+      expect(result).not.toBe(original);
+    });
+  });
+});
diff --git a/vitest.config.js b/vitest.config.js
new file mode 100644
index 0000000..ea000e1
--- /dev/null
+++ b/vitest.config.js
@@ -0,0 +1,9 @@
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+  test: {
+    globals: true,
+    environment: "node",
+    include: ["tests/**/*.test.mjs"],
+  },
+});