diff --git a/README.md b/README.md
index 8765485..cf8715c 100644
--- a/README.md
+++ b/README.md
@@ -453,14 +453,50 @@ pull requests med kodegjennomgang fra et annet gruppemedlem.
 
 ## Testing
 
-Automatiske funksjons- og komponenttester settes opp i en egen issue.
+Automatiske komponent- og hook-tester kjøres med Vitest.
 Kommandoen `npm run lint` er statisk kodekontroll, ikke en funksjonstest.
 
+### Testoppsett
+
+Kjør testene én gang:
+
+```bash
+npm run test
+```
+
+Kjør testene i vaktmodus under utvikling, der de kjører på nytt ved endringer:
+
+```bash
+npm run test:watch
+```
+
+- **Miljø:** Testene kjører i jsdom, satt opp i `test`-feltet i
+  `vite.config.ts`. `defineConfig` importeres fra `vitest/config` i stedet
+  for `vite`, slik at feltet får riktige typer.
+- **Testoppsett:** `src/test/setup.ts` kjører før alle tester. Den utvider
+  `expect` med jest-dom-matchere som `toBeInTheDocument`, og nullstiller
+  DOM-et (`cleanup`), `localStorage`, `sessionStorage` og mocks/stubbede
+  globaler (for eksempel `fetch`) etter hver test. Ingen test kan dermed
+  påvirke en annen.
+- **Mocking av API-kall:** Tester som trenger `fetch`, stubber den med
+  `vi.stubGlobal('fetch', ...)` og et eget svar. Automatiske tester
+  kontakter derfor aldri det ekte API-et.
+- **QueryClient per test:** `createTestQueryClient` i `src/test/queryClient.ts`
+  gir en fersk `QueryClient` til hver test som bruker TanStack Query, slik at
+  ingen cache deles mellom tester. Klienten har `retry: false` og
+  `retryDelay: 0` som standard, slik at en feiltest ikke venter på ekte
+  forsinkelse mellom forsøk, selv om en hook som `useCountries` setter sin
+  egen `retry`.
+- **Brukerinteraksjon:** Komponenttester bruker `@testing-library/react` og
+  `@testing-library/user-event`, se `src/components/FavoriteButton.test.tsx`.
+- Se `src/hooks/useCountries.test.tsx` for et eksempel som kombinerer mocket
+  `fetch` og en isolert `QueryClient`.
+
 ### Kontroller av API-integrasjonen
 
-Vitest er ikke satt opp ennå. Kontrollene under ble kjørt manuelt eller
-med engangsskript da integrasjonen ble laget. Skriptene er ikke lagt til
-i repoet.
+Vitest var ikke satt opp da denne integrasjonen ble laget. Kontrollene
+under ble derfor kjørt manuelt eller med engangsskript. Skriptene er ikke
+lagt til i repoet.
 
 - `npm run lint`, `npm run format:check` og `npm run build` besto.
 - **Manuell API-verifisering:** Dokumentasjonen ble lest, og faktiske
@@ -638,7 +674,7 @@ Testet i nettleserens enhetsemulering:
 - Lastetilstand, feilmelding og tomt favorittutvalg passer inn i layouten.
 - Ikke testet på ekte mobil eller i Safari ennå.
 
-Automatiske tester som gjenstår når Vitest er satt opp:
+Automatiske tester som gjenstår, planlagt i en egen issue:
 
 - `fetchCountries`: vellykket henting, HTTP-feil, nettverksfeil, ugyldig
   JSON, uventet format, tom respons, manglende felter og avbrutt
diff --git a/package-lock.json b/package-lock.json
index 3d2acda..b79bb78 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -14,6 +14,9 @@
       },
       "devDependencies": {
         "@eslint/js": "^10.0.1",
+        "@testing-library/jest-dom": "^7.0.1",
+        "@testing-library/react": "^16.3.3",
+        "@testing-library/user-event": "^14.6.7",
         "@types/node": "^24.13.3",
         "@types/react": "^19.2.18",
         "@types/react-dom": "^19.2.7",
@@ -23,16 +26,76 @@
         "eslint-plugin-react-hooks": "^7.1.1",
         "eslint-plugin-react-refresh": "^0.5.6",
         "globals": "^17.12.0",
+        "jsdom": "^29.1.1",
         "prettier": "^3.9.6",
         "typescript": "~6.0.2",
         "typescript-eslint": "^8.69.0",
-        "vite": "^8.3.0"
+        "vite": "^8.3.0",
+        "vitest": "^5.0.1"
       },
       "engines": {
         "node": ">=24.6.0",
         "npm": ">=11.0.0"
       }
     },
+    "node_modules/@adobe/css-tools": {
+      "version": "4.5.0",
+      "resolved": "https://registry.npmjs.org/@adobe/css-tools/-/css-tools-4.5.0.tgz",
+      "integrity": "sha512-6OzddxPio9UiWTCemp4N8cYLV2ZN1ncRnV1cVGtve7dhPOtRkleRyx32GQCYSwDYgaHU3USMm84tNsvKzRCa1Q==",
+      "dev": true,
+      "license": "MIT"
+    },
+    "node_modules/@asamuzakjp/css-color": {
+      "version": "5.1.11",
+      "resolved": "https://registry.npmjs.org/@asamuzakjp/css-color/-/css-color-5.1.11.tgz",
+      "integrity": "sha512-KVw6qIiCTUQhByfTd78h2yD1/00waTmm9uy/R7Ck/ctUyAPj+AEDLkQIdJW0T8+qGgj3j5bpNKK7Q3G+LedJWg==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@asamuzakjp/generational-cache": "^1.0.1",
+        "@csstools/css-calc": "^3.2.0",
+        "@csstools/css-color-parser": "^4.1.0",
+        "@csstools/css-parser-algorithms": "^4.0.0",
+        "@csstools/css-tokenizer": "^4.0.0"
+      },
+      "engines": {
+        "node": "^20.19.0 || ^22.12.0 || >=24.0.0"
+      }
+    },
+    "node_modules/@asamuzakjp/dom-selector": {
+      "version": "7.1.1",
+      "resolved": "https://registry.npmjs.org/@asamuzakjp/dom-selector/-/dom-selector-7.1.1.tgz",
+      "integrity": "sha512-67RZDnYRc8H/8MLDgQCDE//zoqVFwajkepHZgmXrbwybzXOEwOWGPYGmALYl9J2DOLfFPPs6kKCqmbzV895hTQ==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@asamuzakjp/generational-cache": "^1.0.1",
+        "@asamuzakjp/nwsapi": "^2.3.9",
+        "bidi-js": "^1.0.3",
+        "css-tree": "^3.2.1",
+        "is-potential-custom-element-name": "^1.0.1"
+      },
+      "engines": {
+        "node": "^20.19.0 || ^22.12.0 || >=24.0.0"
+      }
+    },
+    "node_modules/@asamuzakjp/generational-cache": {
+      "version": "1.0.1",
+      "resolved": "https://registry.npmjs.org/@asamuzakjp/generational-cache/-/generational-cache-1.0.1.tgz",
+      "integrity": "sha512-wajfB8KqzMCN2KGNFdLkReeHncd0AslUSrvHVvvYWuU8ghncRJoA50kT3zP9MVL0+9g4/67H+cdvBskj9THPzg==",
+      "dev": true,
+      "license": "MIT",
+      "engines": {
+        "node": "^20.19.0 || ^22.12.0 || >=24.0.0"
+      }
+    },
+    "node_modules/@asamuzakjp/nwsapi": {
+      "version": "2.3.9",
+      "resolved": "https://registry.npmjs.org/@asamuzakjp/nwsapi/-/nwsapi-2.3.9.tgz",
+      "integrity": "sha512-n8GuYSrI9bF7FFZ/SjhwevlHc8xaVlb/7HmHelnc/PZXBD2ZR49NnN9sMMuDdEGPeeRQ5d0hqlSlEpgCX3Wl0Q==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/@babel/code-frame": {
       "version": "7.29.7",
       "resolved": "https://registry.npmjs.org/@babel/code-frame/-/code-frame-7.29.7.tgz",
@@ -225,6 +288,16 @@
         "node": ">=6.0.0"
       }
     },
+    "node_modules/@babel/runtime": {
+      "version": "7.29.7",
+      "resolved": "https://registry.npmjs.org/@babel/runtime/-/runtime-7.29.7.tgz",
+      "integrity": "sha512-Nq8OhGWiZIZGV6hLHoyAKLLcJihP/xFeBMGJoUrxTX2psI8dCifzLhZISFb+VWS3wFMRDmCGw5R+dOySCqPLhw==",
+      "dev": true,
+      "license": "MIT",
+      "engines": {
+        "node": ">=6.9.0"
+      }
+    },
     "node_modules/@babel/template": {
       "version": "7.29.7",
       "resolved": "https://registry.npmjs.org/@babel/template/-/template-7.29.7.tgz",
@@ -273,6 +346,19 @@
         "node": ">=6.9.0"
       }
     },
+    "node_modules/@bramus/specificity": {
+      "version": "2.4.2",
+      "resolved": "https://registry.npmjs.org/@bramus/specificity/-/specificity-2.4.2.tgz",
+      "integrity": "sha512-ctxtJ/eA+t+6q2++vj5j7FYX3nRu311q1wfYH3xjlLOsczhlhxAg2FWNUXhpGvAw3BWo1xBcvOV6/YLc2r5FJw==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "css-tree": "^3.0.0"
+      },
+      "bin": {
+        "specificity": "bin/cli.js"
+      }
+    },
     "node_modules/@cacheable/memory": {
       "version": "2.2.0",
       "resolved": "https://registry.npmjs.org/@cacheable/memory/-/memory-2.2.0.tgz",
@@ -297,6 +383,146 @@
         "keyv": "^5.6.0"
       }
     },
+    "node_modules/@csstools/color-helpers": {
+      "version": "6.1.1",
+      "resolved": "https://registry.npmjs.org/@csstools/color-helpers/-/color-helpers-6.1.1.tgz",
+      "integrity": "sha512-gLNsunvwf3mCi5u5o46/Z/JcJMnhbHSaZ69rkgPzNM3J4s8hWwpPUQB6/tt0EDFyCiWzxANlx+2LJwpYj4zS1w==",
+      "dev": true,
+      "funding": [
+        {
+          "type": "github",
+          "url": "https://github.com/sponsors/csstools"
+        },
+        {
+          "type": "opencollective",
+          "url": "https://opencollective.com/csstools"
+        }
+      ],
+      "license": "MIT-0",
+      "engines": {
+        "node": ">=20.19.0"
+      }
+    },
+    "node_modules/@csstools/css-calc": {
+      "version": "3.4.0",
+      "resolved": "https://registry.npmjs.org/@csstools/css-calc/-/css-calc-3.4.0.tgz",
+      "integrity": "sha512-XQKj5B7QiZcHiegCOCAzcAOJdhGgWOHbbu62h5e5mkHnn8lWcfiJhllkqWmxu5zWR9jucPHuo1iTB56P033hcg==",
+      "dev": true,
+      "funding": [
+        {
+          "type": "github",
+          "url": "https://github.com/sponsors/csstools"
+        },
+        {
+          "type": "opencollective",
+          "url": "https://opencollective.com/csstools"
+        }
+      ],
+      "license": "MIT",
+      "engines": {
+        "node": ">=20.19.0"
+      },
+      "peerDependencies": {
+        "@csstools/css-parser-algorithms": "^4.0.0",
+        "@csstools/css-tokenizer": "^4.0.0"
+      }
+    },
+    "node_modules/@csstools/css-color-parser": {
+      "version": "4.2.3",
+      "resolved": "https://registry.npmjs.org/@csstools/css-color-parser/-/css-color-parser-4.2.3.tgz",
+      "integrity": "sha512-y4LpL+lmpuyKDiEFq2PnZUVFdAjsoB/qQJod79yLNokXyW7jewi+/WJ69EfItj8A2unWtxXnGjw6LYXgXu5ZjA==",
+      "dev": true,
+      "funding": [
+        {
+          "type": "github",
+          "url": "https://github.com/sponsors/csstools"
+        },
+        {
+          "type": "opencollective",
+          "url": "https://opencollective.com/csstools"
+        }
+      ],
+      "license": "MIT",
+      "dependencies": {
+        "@csstools/color-helpers": "^6.1.1",
+        "@csstools/css-calc": "^3.4.0"
+      },
+      "engines": {
+        "node": ">=20.19.0"
+      },
+      "peerDependencies": {
+        "@csstools/css-parser-algorithms": "^4.0.0",
+        "@csstools/css-tokenizer": "^4.0.0"
+      }
+    },
+    "node_modules/@csstools/css-parser-algorithms": {
+      "version": "4.0.0",
+      "resolved": "https://registry.npmjs.org/@csstools/css-parser-algorithms/-/css-parser-algorithms-4.0.0.tgz",
+      "integrity": "sha512-+B87qS7fIG3L5h3qwJ/IFbjoVoOe/bpOdh9hAjXbvx0o8ImEmUsGXN0inFOnk2ChCFgqkkGFQ+TpM5rbhkKe4w==",
+      "dev": true,
+      "funding": [
+        {
+          "type": "github",
+          "url": "https://github.com/sponsors/csstools"
+        },
+        {
+          "type": "opencollective",
+          "url": "https://opencollective.com/csstools"
+        }
+      ],
+      "license": "MIT",
+      "engines": {
+        "node": ">=20.19.0"
+      },
+      "peerDependencies": {
+        "@csstools/css-tokenizer": "^4.0.0"
+      }
+    },
+    "node_modules/@csstools/css-syntax-patches-for-csstree": {
+      "version": "1.1.14",
+      "resolved": "https://registry.npmjs.org/@csstools/css-syntax-patches-for-csstree/-/css-syntax-patches-for-csstree-1.1.14.tgz",
+      "integrity": "sha512-HpbVXyrofRXpHpgkNIjU/3EWR4WJvOkO3emNK/L6X/mTJU7bGUI3AkkpoTNXznQLp0KRjLHELTGeKI5dIkI9JQ==",
+      "dev": true,
+      "funding": [
+        {
+          "type": "github",
+          "url": "https://github.com/sponsors/csstools"
+        },
+        {
+          "type": "opencollective",
+          "url": "https://opencollective.com/csstools"
+        }
+      ],
+      "license": "MIT-0",
+      "peerDependencies": {
+        "css-tree": "^3.2.1"
+      },
+      "peerDependenciesMeta": {
+        "css-tree": {
+          "optional": true
+        }
+      }
+    },
+    "node_modules/@csstools/css-tokenizer": {
+      "version": "4.0.0",
+      "resolved": "https://registry.npmjs.org/@csstools/css-tokenizer/-/css-tokenizer-4.0.0.tgz",
+      "integrity": "sha512-QxULHAm7cNu72w97JUNCBFODFaXpbDg+dP8b/oWFAZ2MTRppA3U00Y2L1HqaS4J6yBqxwa/Y3nMBaxVKbB/NsA==",
+      "dev": true,
+      "funding": [
+        {
+          "type": "github",
+          "url": "https://github.com/sponsors/csstools"
+        },
+        {
+          "type": "opencollective",
+          "url": "https://opencollective.com/csstools"
+        }
+      ],
+      "license": "MIT",
+      "engines": {
+        "node": ">=20.19.0"
+      }
+    },
     "node_modules/@eslint-community/eslint-utils": {
       "version": "4.10.1",
       "resolved": "https://registry.npmjs.org/@eslint-community/eslint-utils/-/eslint-utils-4.10.1.tgz",
@@ -425,6 +651,24 @@
         "node": "^20.19.0 || ^22.13.0 || >=24"
       }
     },
+    "node_modules/@exodus/bytes": {
+      "version": "1.15.1",
+      "resolved": "https://registry.npmjs.org/@exodus/bytes/-/bytes-1.15.1.tgz",
+      "integrity": "sha512-S6mL0yNB/Abt9Ei4tq8gDhcczc4S3+vQ4ra7vxnAf+YHC02srtqxKKZghx2Dq6p0e66THKwR6r8N6P95wEty7Q==",
+      "dev": true,
+      "license": "MIT",
+      "engines": {
+        "node": "^20.19.0 || ^22.12.0 || >=24.0.0"
+      },
+      "peerDependencies": {
+        "@noble/hashes": "^1.8.0 || ^2.0.0"
+      },
+      "peerDependenciesMeta": {
+        "@noble/hashes": {
+          "optional": true
+        }
+      }
+    },
     "node_modules/@humanfs/core": {
       "version": "0.19.2",
       "resolved": "https://registry.npmjs.org/@humanfs/core/-/core-0.19.2.tgz",
@@ -685,9 +929,6 @@
         "arm64"
       ],
       "dev": true,
-      "libc": [
-        "glibc"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -705,9 +946,6 @@
         "arm64"
       ],
       "dev": true,
-      "libc": [
-        "musl"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -725,9 +963,6 @@
         "ppc64"
       ],
       "dev": true,
-      "libc": [
-        "glibc"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -745,9 +980,6 @@
         "s390x"
       ],
       "dev": true,
-      "libc": [
-        "glibc"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -765,9 +997,6 @@
         "x64"
       ],
       "dev": true,
-      "libc": [
-        "glibc"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -785,9 +1014,6 @@
         "x64"
       ],
       "dev": true,
-      "libc": [
-        "musl"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -881,6 +1107,131 @@
         "react": "^18 || ^19"
       }
     },
+    "node_modules/@testing-library/dom": {
+      "version": "10.4.2",
+      "resolved": "https://registry.npmjs.org/@testing-library/dom/-/dom-10.4.2.tgz",
+      "integrity": "sha512-yzr2S9HyAIdhz2/6qHgbs665Q7PKVcDF05vsOlHPxG1mo36gKVesdYVeDLnXgfjJ03CrKRk08knc6+E/9m8v2Q==",
+      "dev": true,
+      "license": "MIT",
+      "peer": true,
+      "dependencies": {
+        "@babel/code-frame": "^7.10.4",
+        "@babel/runtime": "^7.12.5",
+        "@types/aria-query": "^5.0.1",
+        "aria-query": "5.3.0",
+        "dom-accessibility-api": "^0.5.9",
+        "lz-string": "^1.5.0",
+        "picocolors": "1.1.1",
+        "pretty-format": "^27.0.2"
+      },
+      "engines": {
+        "node": ">=18"
+      }
+    },
+    "node_modules/@testing-library/jest-dom": {
+      "version": "7.0.1",
+      "resolved": "https://registry.npmjs.org/@testing-library/jest-dom/-/jest-dom-7.0.1.tgz",
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+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@adobe/css-tools": "^4.4.0",
+        "aria-query": "^5.0.0",
+        "css.escape": "^1.5.1",
+        "dom-accessibility-api": "^0.6.3",
+        "picocolors": "^1.1.1",
+        "redent": "^3.0.0"
+      },
+      "engines": {
+        "node": ">=22",
+        "npm": ">=6",
+        "yarn": ">=1"
+      },
+      "peerDependencies": {
+        "@testing-library/dom": ">=10 <11",
+        "vitest": ">= 0.32"
+      },
+      "peerDependenciesMeta": {
+        "vitest": {
+          "optional": true
+        }
+      }
+    },
+    "node_modules/@testing-library/jest-dom/node_modules/dom-accessibility-api": {
+      "version": "0.6.3",
+      "resolved": "https://registry.npmjs.org/dom-accessibility-api/-/dom-accessibility-api-0.6.3.tgz",
+      "integrity": "sha512-7ZgogeTnjuHbo+ct10G9Ffp0mif17idi0IyWNVA/wcwcm7NPOD/WEHVP3n7n3MhXqxoIYm8d6MuZohYWIZ4T3w==",
+      "dev": true,
+      "license": "MIT"
+    },
+    "node_modules/@testing-library/react": {
+      "version": "16.3.3",
+      "resolved": "https://registry.npmjs.org/@testing-library/react/-/react-16.3.3.tgz",
+      "integrity": "sha512-Uo193NgQbPMz6lrrhtRQQFcMC6Re/ELLFbbuVL30WDlZxlpZf9/lMHTAVxPRLw1q1iu9OJmR1c2BLiENRstdBg==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@babel/runtime": "^7.12.5"
+      },
+      "engines": {
+        "node": ">=18"
+      },
+      "peerDependencies": {
+        "@testing-library/dom": "^10.0.0",
+        "@types/react": "^18.0.0 || ^19.0.0",
+        "@types/react-dom": "^18.0.0 || ^19.0.0",
+        "react": "^18.0.0 || ^19.0.0",
+        "react-dom": "^18.0.0 || ^19.0.0"
+      },
+      "peerDependenciesMeta": {
+        "@types/react": {
+          "optional": true
+        },
+        "@types/react-dom": {
+          "optional": true
+        }
+      }
+    },
+    "node_modules/@testing-library/user-event": {
+      "version": "14.6.7",
+      "resolved": "https://registry.npmjs.org/@testing-library/user-event/-/user-event-14.6.7.tgz",
+      "integrity": "sha512-MPCpX8bxe8zS+JmmTwLp8jd0dy1rAm60Te/SL8JrQM3qvQJcBOs1d7IefJMyZzqM3EWBrDn/LWDt1BCGu4ASfg==",
+      "dev": true,
+      "license": "MIT",
+      "engines": {
+        "node": ">=12",
+        "npm": ">=6"
+      },
+      "peerDependencies": {
+        "@testing-library/dom": ">=7.21.4"
+      }
+    },
+    "node_modules/@types/aria-query": {
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+      "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/word-wrap": {
       "version": "1.2.5",
       "resolved": "https://registry.npmjs.org/word-wrap/-/word-wrap-1.2.5.tgz",
@@ -2809,6 +3823,23 @@
         "node": ">=0.10.0"
       }
     },
+    "node_modules/xml-name-validator": {
+      "version": "5.0.0",
+      "resolved": "https://registry.npmjs.org/xml-name-validator/-/xml-name-validator-5.0.0.tgz",
+      "integrity": "sha512-EvGK8EJ3DhaHfbRlETOWAS5pO9MZITeauHKJyb8wyajUfQUenkIg2MvLDTZ4T/TgIcm3HU0TFBgWWboAZ30UHg==",
+      "dev": true,
+      "license": "Apache-2.0",
+      "engines": {
+        "node": ">=18"
+      }
+    },
+    "node_modules/xmlchars": {
+      "version": "2.2.0",
+      "resolved": "https://registry.npmjs.org/xmlchars/-/xmlchars-2.2.0.tgz",
+      "integrity": "sha512-JZnDKK8B0RCDw84FNdDAIpZK+JuJw+s7Lz8nksI7SIuU3UXJJslUthsi+uWBUYOwPFwW7W7PRLRfUKpxjtjFCw==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/yallist": {
       "version": "3.1.1",
       "resolved": "https://registry.npmjs.org/yallist/-/yallist-3.1.1.tgz",
diff --git a/package.json b/package.json
index 01e715c..32c8de5 100644
--- a/package.json
+++ b/package.json
@@ -9,7 +9,9 @@
     "lint": "eslint .",
     "preview": "vite preview",
     "format": "prettier --write .",
-    "format:check": "prettier --check ."
+    "format:check": "prettier --check .",
+    "test": "vitest run",
+    "test:watch": "vitest"
   },
   "engines": {
     "node": ">=24.6.0",
@@ -22,6 +24,9 @@
   },
   "devDependencies": {
     "@eslint/js": "^10.0.1",
+    "@testing-library/jest-dom": "^7.0.1",
+    "@testing-library/react": "^16.3.3",
+    "@testing-library/user-event": "^14.6.7",
     "@types/node": "^24.13.3",
     "@types/react": "^19.2.18",
     "@types/react-dom": "^19.2.7",
@@ -31,9 +36,11 @@
     "eslint-plugin-react-hooks": "^7.1.1",
     "eslint-plugin-react-refresh": "^0.5.6",
     "globals": "^17.12.0",
+    "jsdom": "^29.1.1",
     "prettier": "^3.9.6",
     "typescript": "~6.0.2",
     "typescript-eslint": "^8.69.0",
-    "vite": "^8.3.0"
+    "vite": "^8.3.0",
+    "vitest": "^5.0.1"
   }
 }
diff --git a/src/components/FavoriteButton.test.tsx b/src/components/FavoriteButton.test.tsx
new file mode 100644
index 0000000..e6b9e50
--- /dev/null
+++ b/src/components/FavoriteButton.test.tsx
@@ -0,0 +1,38 @@
+import { render, screen } from '@testing-library/react';
+import userEvent from '@testing-library/user-event';
+import { describe, expect, it, vi } from 'vitest';
+import FavoriteButton from './FavoriteButton';
+
+describe('FavoriteButton', () => {
+  it('viser av-tilstand og kaller onToggle ved klikk', async () => {
+    const onToggle = vi.fn();
+    render(
+      <FavoriteButton
+        countryName="Norge"
+        isFavorite={false}
+        onToggle={onToggle}
+      />,
+    );
+
+    const button = screen.getByRole('button', { name: 'Favoritt: Norge' });
+    expect(button).toHaveAttribute('aria-pressed', 'false');
+
+    await userEvent.click(button);
+
+    expect(onToggle).toHaveBeenCalledTimes(1);
+  });
+
+  it('viser på-tilstand når landet er favoritt', () => {
+    render(
+      <FavoriteButton
+        countryName="Norge"
+        isFavorite={true}
+        onToggle={vi.fn()}
+      />,
+    );
+
+    expect(
+      screen.getByRole('button', { name: 'Favoritt: Norge' }),
+    ).toHaveAttribute('aria-pressed', 'true');
+  });
+});
diff --git a/src/hooks/useCountries.test.tsx b/src/hooks/useCountries.test.tsx
new file mode 100644
index 0000000..65c197f
--- /dev/null
+++ b/src/hooks/useCountries.test.tsx
@@ -0,0 +1,49 @@
+import { QueryClientProvider } from '@tanstack/react-query';
+import { renderHook, waitFor } from '@testing-library/react';
+import { describe, expect, it, vi } from 'vitest';
+import { createTestQueryClient } from '../test/queryClient';
+import { useCountries } from './useCountries';
+
+// Rått API-svar, med færre felter enn appen bruker, slik fetchCountries
+// selv må håndtere det.
+const RAW_RESPONSE = [
+  { alpha3Code: 'NOR', name: 'Norway', population: 5_400_000 },
+];
+
+describe('useCountries', () => {
+  it('henter land via mocket fetch, uten å kontakte det ekte API-et', async () => {
+    const fetchMock = vi.fn().mockResolvedValue({
+      ok: true,
+      status: 200,
+      json: async () => RAW_RESPONSE,
+    });
+    vi.stubGlobal('fetch', fetchMock);
+
+    const queryClient = createTestQueryClient();
+    const { result } = renderHook(() => useCountries(), {
+      wrapper: ({ children }) => (
+        <QueryClientProvider client={queryClient}>
+          {children}
+        </QueryClientProvider>
+      ),
+    });
+
+    await waitFor(() => expect(result.current.isPending).toBe(false));
+
+    expect(result.current.data).toEqual([
+      {
+        code: 'NOR',
+        name: 'Norway',
+        flagUrl: undefined,
+        capital: undefined,
+        region: undefined,
+        population: 5_400_000,
+        area: undefined,
+        languages: [],
+      },
+    ]);
+    // Bekrefter at ingen ekte nettverkskall gjøres, og at data kommer fra
+    // den mockede fetch-en.
+    expect(fetchMock).toHaveBeenCalledTimes(1);
+  });
+});
diff --git a/src/test/queryClient.ts b/src/test/queryClient.ts
new file mode 100644
index 0000000..c2bd468
--- /dev/null
+++ b/src/test/queryClient.ts
@@ -0,0 +1,16 @@
+import { QueryClient } from '@tanstack/react-query';
+
+// Egen QueryClient per test, slik at cache fra én test aldri påvirker en
+// annen. retry er av som standard, og retryDelay er satt til 0 for de
+// tilfellene en hook uansett setter sin egen retry (som useCountries).
+// Uten dette ville en feiltest ventet på ekte forsinkelse mellom forsøk.
+export function createTestQueryClient(): QueryClient {
+  return new QueryClient({
+    defaultOptions: {
+      queries: {
+        retry: false,
+        retryDelay: 0,
+      },
+    },
+  });
+}
diff --git a/src/test/setup.ts b/src/test/setup.ts
new file mode 100644
index 0000000..cd93927
--- /dev/null
+++ b/src/test/setup.ts
@@ -0,0 +1,24 @@
+import { afterEach, vi } from 'vitest';
+import { cleanup } from '@testing-library/react';
+// Utvider Vitests expect med matchere som toBeInTheDocument.
+import '@testing-library/jest-dom/vitest';
+
+// Fjerner rendrede komponenter fra DOM-et etter hver test. Uten dette ville
+// komponenter fra en test kunne bli liggende og påvirke den neste.
+afterEach(() => {
+  cleanup();
+});
+
+// Nullstiller nettleserlagring etter hver test, slik at en test aldri leser
+// verdier en annen test har lagret.
+afterEach(() => {
+  localStorage.clear();
+  sessionStorage.clear();
+});
+
+// Fjerner mocks og stubbede globaler (for eksempel window.fetch) etter hver
+// test, slik at en test aldri påvirkes av oppsett fra en annen.
+afterEach(() => {
+  vi.restoreAllMocks();
+  vi.unstubAllGlobals();
+});
diff --git a/vite.config.ts b/vite.config.ts
index a74ec84..051fdd7 100644
--- a/vite.config.ts
+++ b/vite.config.ts
@@ -1,7 +1,13 @@
 import react from '@vitejs/plugin-react';
-import { defineConfig } from 'vite';
+// defineConfig fra vitest/config gir typer for test-feltet under,
+// i tillegg til alt Vite selv støtter.
+import { defineConfig } from 'vitest/config';
 
 // https://vite.dev/config/
 export default defineConfig({
   plugins: [react()],
+  test: {
+    environment: 'jsdom',
+    setupFiles: ['./src/test/setup.ts'],
+  },
 });