diff --git a/.gitignore b/.gitignore
index 704a7d4..3595bbc 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,5 @@
+*.d.ts
+*.d.ts.map
 *.log
 .DS_Store
 dest/
diff --git a/crawl.js b/crawl.js
index 604cf99..0808b6b 100644
--- a/crawl.js
+++ b/crawl.js
@@ -2,6 +2,7 @@ import fs from 'node:fs/promises'
 import {iso6393} from 'iso-639-3'
 import {fetch} from 'undici'
 
+/** @type {Record<string, string>} */
 const iso6393ToName = {}
 
 let index = -1
@@ -20,5 +21,9 @@ await fs.writeFile(new URL('src/fixtures.js', import.meta.url), body)
 
 await fs.writeFile(
   new URL('src/to-name.js', import.meta.url),
-  'export const toName = ' + JSON.stringify(iso6393ToName, undefined, 2) + '\n'
+  [
+    '/** @type {Record<string, string>} */',
+    'export const toName = ' + JSON.stringify(iso6393ToName, undefined, 2),
+    ''
+  ].join('\n')
 )
diff --git a/package.json b/package.json
index 79e4500..7de2b7b 100644
--- a/package.json
+++ b/package.json
@@ -20,6 +20,8 @@
     "stylelint": "^14.0.0",
     "stylelint-config-prettier": "^9.0.0",
     "stylelint-config-standard": "^28.0.0",
+    "type-coverage": "^2.27.1",
+    "typescript": "^5.3.3",
     "undici": "^6.0.0",
     "xo": "^0.56.0"
   },
@@ -48,13 +50,14 @@
   },
   "repository": "wooorm/franc",
   "scripts": {
+    "build": "tsc --build --clean && tsc --build && type-coverage",
     "crawl": "node --conditions development crawl.js",
     "format": "remark --frail --output --quiet -- . && prettier . --log-level warn --write && xo --fix && stylelint src/index.css --fix",
     "generate:script": "esbuild src/index.js --bundle --minify --outfile=dest/index.nomodule.js --target=es6 && esbuild src/index.js --bundle --format=esm --minify --outfile=dest/index.module.js --target=es2020",
     "generate:structure": "rehype -u rehype-preset-minify -u rehype-prevent-favicon-request src -o dest",
     "generate:style": "postcss src/index.css > dest/index.css",
     "generate": "npm run generate:script && npm run generate:style && npm run generate:structure",
-    "test": "npm run crawl && npm run format && npm run generate"
+    "test": "npm run crawl && npm run build && npm run format && npm run generate"
   },
   "stylelint": {
     "extends": [
@@ -63,6 +66,12 @@
     ]
   },
   "type": "module",
+  "typeCoverage": {
+    "atLeast": 100,
+    "detail": true,
+    "ignoreCatch": true,
+    "strict": true
+  },
   "xo": {
     "prettier": true,
     "ignore": [
diff --git a/src/index.js b/src/index.js
index 3185524..4c48b12 100644
--- a/src/index.js
+++ b/src/index.js
@@ -1,4 +1,5 @@
 /* eslint-env browser */
+
 import debounce from 'debounce'
 import {francAll} from 'franc'
 import {toName} from './to-name.js'
@@ -38,7 +39,7 @@ function oninputchange() {
     const c2 = document.createElement('td')
     c0.append(link)
     c1.textContent = toName[code]
-    c2.textContent = score
+    c2.textContent = String(score)
 
     $node.append(c0)
     $node.append(c1)
diff --git a/tsconfig.json b/tsconfig.json
new file mode 100644
index 0000000..c6d6d04
--- /dev/null
+++ b/tsconfig.json
@@ -0,0 +1,17 @@
+{
+  "compilerOptions": {
+    "checkJs": true,
+    "customConditions": ["development"],
+    "declaration": true,
+    "declarationMap": true,
+    "emitDeclarationOnly": true,
+    "exactOptionalPropertyTypes": true,
+    "jsx": "preserve",
+    "lib": ["dom", "es2022"],
+    "module": "node16",
+    "strict": true,
+    "target": "es2022"
+  },
+  "exclude": ["coverage/", "dest/", "node_modules/"],
+  "include": ["**/*.js"]
+}