diff --git a/.gitignore b/.gitignore
index 735f4af..c977c85 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,5 @@
 .DS_Store
+*.d.ts
 *.log
 coverage/
 node_modules/
diff --git a/index.js b/index.js
index 95eb4bc..73bbe72 100644
--- a/index.js
+++ b/index.js
@@ -1,10 +1,19 @@
+/**
+ * @typedef {Object} SmogFormulaCounts
+ * @property {number} sentence
+ * @property {number} [polysillabicWord]
+ */
+
 var sentenceSize = 30
 var weight = 1.043
 var base = 3.1291
 
-// Get the grade level of a given value according to the SMOG formula.
-// More information is available at WikiPedia:
-// <https://en.wikipedia.org/wiki/SMOG>
+/**
+ * Get the grade level of a given value according to the SMOG formula. More information is available at WikiPedia: <https://en.wikipedia.org/wiki/SMOG>.
+ *
+ * @param {SmogFormulaCounts} counts
+ * @returns {number}
+ */
 export function smogFormula(counts) {
   if (!counts || !counts.sentence) {
     return Number.NaN
diff --git a/package.json b/package.json
index 479fad5..31ee13c 100644
--- a/package.json
+++ b/package.json
@@ -25,22 +25,30 @@
   "sideEffects": false,
   "type": "module",
   "main": "index.js",
+  "types": "index.d.ts",
   "files": [
+    "index.d.ts",
     "index.js"
   ],
   "devDependencies": {
+    "@types/tape": "^4.0.0",
     "c8": "^7.0.0",
     "prettier": "^2.0.0",
     "remark-cli": "^9.0.0",
     "remark-preset-wooorm": "^8.0.0",
+    "rimraf": "^3.0.0",
     "tape": "^5.0.0",
+    "type-coverage": "^2.0.0",
+    "typescript": "^4.0.0",
     "xo": "^0.38.0"
   },
   "scripts": {
+    "prepack": "npm run build && npm run format",
+    "build": "rimraf \"*.d.ts\" && tsc && type-coverage",
     "format": "remark . -qfo && prettier . -w --loglevel warn && xo --fix",
     "test-api": "node test.js",
     "test-coverage": "c8 --check-coverage --branches 100 --functions 100 --lines 100 --statements 100 --reporter lcov node test.js",
-    "test": "npm run format && npm run test-coverage"
+    "test": "npm run build && npm run format && npm run test-coverage"
   },
   "prettier": {
     "tabWidth": 2,
@@ -61,5 +69,10 @@
     "plugins": [
       "preset-wooorm"
     ]
+  },
+  "typeCoverage": {
+    "atLeast": 100,
+    "detail": true,
+    "strict": true
   }
 }
diff --git a/test.js b/test.js
index 4f67b7f..f7e815b 100644
--- a/test.js
+++ b/test.js
@@ -2,12 +2,17 @@ import test from 'tape'
 import {smogFormula} from './index.js'
 
 test('smogFormula', function (t) {
+  // @ts-ignore runtime.
   t.ok(Number.isNaN(smogFormula()), 'NaN when an invalid value is given')
   t.equal(round(smogFormula({sentence: 1})), 3.1291)
   t.equal(round(smogFormula({sentence: 1, polysillabicWord: 4})), 14.554593)
   t.end()
 })
 
+/**
+ * @param {number} value
+ * @returns {number}
+ */
 function round(value) {
   return Math.round(value * 1e6) / 1e6
 }
diff --git a/tsconfig.json b/tsconfig.json
new file mode 100644
index 0000000..be08abe
--- /dev/null
+++ b/tsconfig.json
@@ -0,0 +1,15 @@
+{
+  "include": ["*.js"],
+  "compilerOptions": {
+    "target": "ES2020",
+    "lib": ["ES2020"],
+    "module": "ES2020",
+    "moduleResolution": "node",
+    "allowJs": true,
+    "checkJs": true,
+    "declaration": true,
+    "emitDeclarationOnly": true,
+    "allowSyntheticDefaultImports": true,
+    "skipLibCheck": true
+  }
+}