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/cli.js b/cli.js
index d13e56a..2df67ad 100755
--- a/cli.js
+++ b/cli.js
@@ -3,6 +3,7 @@ import fs from 'fs'
 import {URL} from 'url'
 import {syllable} from './index.js'
 
+/** @type {Object.<string, unknown>} */
 var pack = JSON.parse(
   String(fs.readFileSync(new URL('./package.json', import.meta.url)))
 )
@@ -23,20 +24,26 @@ if (argv.includes('--help') || argv.includes('-h')) {
   getSyllables(argv.join(' '))
 }
 
+/**
+ * @param {string} value
+ */
 function getSyllables(value) {
-  value = value
+  var values = value
     .split(/\s+/g)
-    .map((d) => d.trim())
+    .map((/** @type {string} */ d) => d.trim())
     .filter(Boolean)
 
-  if (value.length === 0) {
+  if (values.length === 0) {
     process.stderr.write(help())
     process.exit(1)
   } else {
-    console.log(syllables(value))
+    console.log(syllables(values))
   }
 }
 
+/**
+ * @param {Array.<string>} values
+ */
 function syllables(values) {
   var sum = 0
   var index = -1
diff --git a/index.js b/index.js
index bef999b..581d367 100644
--- a/index.js
+++ b/index.js
@@ -1,4 +1,5 @@
 import pluralize from 'pluralize'
+// @ts-ignore remove when typed.
 import normalize from 'normalize-strings'
 import {problematic} from './problematic.js'
 
@@ -225,6 +226,12 @@ var EXPRESSION_DOUBLE = new RegExp(
 var EXPRESSION_TRIPLE = /(creations?|ology|ologist|onomy|onomist)$/g
 
 // Wrapper to support multiple word-parts (GH-11).
+/**
+ * Syllable count
+ *
+ * @param {string} value
+ * @returns {number}
+ */
 export function syllable(value) {
   var values = normalize(String(value))
     .toLowerCase()
@@ -232,26 +239,34 @@ export function syllable(value) {
     .replace(/['’]/g, '')
     // Split on word boundaries.
     .split(/\b/g)
-  var length = values.length
   var index = -1
-  var total = 0
+  var sum = 0
 
-  while (++index < length) {
+  while (++index < values.length) {
     // Remove non-alphabetic characters from a given value.
-    total += one(values[index].replace(/[^a-z]/g, ''))
+    sum += one(values[index].replace(/[^a-z]/g, ''))
   }
 
-  return total
+  return sum
 }
 
-// Get syllables in a given value.
+/**
+ * Get syllables in a given value.
+ *
+ * @param {string} value
+ * @returns {number}
+ */
 function one(value) {
   var count = 0
+  /** @type {number} */
   var index
-  var length
+  /** @type {string} */
   var singular
+  /** @type {Array.<string>} */
   var parts
+  /** @type {ReturnType.<returnFactory>} */
   var addOne
+  /** @type {ReturnType.<returnFactory>} */
   var subtractOne
 
   if (value.length === 0) {
@@ -287,9 +302,8 @@ function one(value) {
   // Count multiple consonants.
   parts = value.split(/[^aeiouy]+/)
   index = -1
-  length = parts.length
 
-  while (++index < length) {
+  while (++index < parts.length) {
     if (parts[index] !== '') {
       count++
     }
@@ -312,20 +326,34 @@ function one(value) {
   // Make sure at least on is returned.
   return count || 1
 
-  // Define scoped counters, to be used in `String#replace()` calls.
-  // The scoped counter removes the matched value from the input.
+  /**
+   * Define scoped counters, to be used in `String#replace()` calls.
+   * The scoped counter removes the matched value from the input.
+   *
+   * @param {number} addition
+   */
   function countFactory(addition) {
     return counter
+    /**
+     * @returns {string}
+     */
     function counter() {
       count += addition
       return ''
     }
   }
 
-  // Define scoped counters, to be used in `String#replace()` calls.
-  // The scoped counter does not remove the matched value from the input.
+  /**
+   * This scoped counter does not remove the matched value from the input.
+   *
+   * @param {number} addition
+   */
   function returnFactory(addition) {
     return returner
+    /**
+     * @param {string} $0
+     * @returns {string}
+     */
     function returner($0) {
       count += addition
       return $0
diff --git a/package.json b/package.json
index 9317e5c..400d8b7 100644
--- a/package.json
+++ b/package.json
@@ -26,28 +26,37 @@
   "type": "module",
   "bin": "cli.js",
   "main": "index.js",
+  "types": "index.d.ts",
   "files": [
     "cli.js",
     "problematic.js",
+    "index.d.ts",
     "index.js"
   ],
   "dependencies": {
+    "@types/pluralize": "^0.0.29",
     "normalize-strings": "^1.1.0",
     "pluralize": "^8.0.0"
   },
   "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/index.js",
     "test-coverage": "c8 --check-coverage --branches 100 --functions 100 --lines 100 --statements 100 --reporter lcov node test/index.js",
-    "test": "npm run format && npm run test-coverage"
+    "test": "npm run build && npm run format && npm run test-coverage"
   },
   "prettier": {
     "tabWidth": 2,
@@ -69,5 +78,10 @@
     "plugins": [
       "preset-wooorm"
     ]
+  },
+  "typeCoverage": {
+    "atLeast": 100,
+    "detail": true,
+    "strict": true
   }
 }
diff --git a/test/index.js b/test/index.js
index 3dbd552..c040a74 100644
--- a/test/index.js
+++ b/test/index.js
@@ -7,10 +7,12 @@ import test from 'tape'
 
 var own = {}.hasOwnProperty
 
+/** @type {Object.<string, unknown>} */
 var pack = JSON.parse(
   String(fs.readFileSync(new URL('../package.json', import.meta.url)))
 )
 
+/** @type {Object.<string, number>} */
 var fixtures = JSON.parse(
   String(fs.readFileSync(new URL('./fixture.json', import.meta.url)))
 )
@@ -347,14 +349,16 @@ test('cli', function (t) {
 // This library focusses on the required Text-Statistics tests (the library
 // provides both optional and required tests).
 test('fixtures', function (t) {
-  var key
-  var expected
   var overwrite = {
     // GH-22: <https://github.com/words/syllable/issues/22>,
     // Barbed is one syllable as well:
     // <https://www.howmanysyllables.com/words/barbed>
     arbed: 1
   }
+  /** @type {string} */
+  var key
+  /** @type {number} */
+  var expected
 
   for (key in fixtures) {
     if (own.call(fixtures, key)) {
diff --git a/tsconfig.json b/tsconfig.json
new file mode 100644
index 0000000..1cb61bf
--- /dev/null
+++ b/tsconfig.json
@@ -0,0 +1,15 @@
+{
+  "include": ["*.js", "test/**/*.js"],
+  "compilerOptions": {
+    "target": "ES2020",
+    "lib": ["ES2020"],
+    "module": "ES2020",
+    "moduleResolution": "node",
+    "allowJs": true,
+    "checkJs": true,
+    "declaration": true,
+    "emitDeclarationOnly": true,
+    "allowSyntheticDefaultImports": true,
+    "skipLibCheck": true
+  }
+}