diff --git a/cli.js b/cli.js
index 5a792ab..e08690e 100755
--- a/cli.js
+++ b/cli.js
@@ -5,13 +5,13 @@ import {URL} from 'node:url'
 import {syllable} from './index.js'
 
 /** @type {Object.<string, unknown>} */
-var pack = JSON.parse(
+const pack = JSON.parse(
   String(fs.readFileSync(new URL('package.json', import.meta.url)))
 )
 
-var argv = process.argv.slice(2)
+const argv = process.argv.slice(2)
 
-var command = pack.name
+const command = pack.name
 
 if (argv.includes('--help') || argv.includes('-h')) {
   console.log(help())
@@ -29,7 +29,7 @@ if (argv.includes('--help') || argv.includes('-h')) {
  * @param {string} value
  */
 function getSyllables(value) {
-  var values = value
+  const values = value
     .split(/\s+/g)
     .map((/** @type {string} */ d) => d.trim())
     .filter(Boolean)
@@ -46,8 +46,8 @@ function getSyllables(value) {
  * @param {Array.<string>} values
  */
 function syllables(values) {
-  var sum = 0
-  var index = -1
+  let sum = 0
+  let index = -1
 
   while (++index < values.length) {
     sum += syllable(values[index])
diff --git a/index.js b/index.js
index 581d367..e934a37 100644
--- a/index.js
+++ b/index.js
@@ -3,11 +3,11 @@ import pluralize from 'pluralize'
 import normalize from 'normalize-strings'
 import {problematic} from './problematic.js'
 
-var own = {}.hasOwnProperty
+const own = {}.hasOwnProperty
 
 // Two expressions of occurrences which normally would be counted as two
 // syllables, but should be counted as one.
-var EXPRESSION_MONOSYLLABIC_ONE = new RegExp(
+const EXPRESSION_MONOSYLLABIC_ONE = new RegExp(
   [
     'awe($|d|so)',
     'cia(?:l|$)',
@@ -63,7 +63,7 @@ var EXPRESSION_MONOSYLLABIC_ONE = new RegExp(
   'g'
 )
 
-var EXPRESSION_MONOSYLLABIC_TWO = new RegExp(
+const EXPRESSION_MONOSYLLABIC_TWO = new RegExp(
   '[aeiouy](?:' +
     [
       '[bcdfgklmnprstvyz]',
@@ -84,7 +84,7 @@ var EXPRESSION_MONOSYLLABIC_TWO = new RegExp(
 
 // Four expression of occurrences which normally would be counted as one
 // syllable, but should be counted as two.
-var EXPRESSION_DOUBLE_SYLLABIC_ONE = new RegExp(
+const EXPRESSION_DOUBLE_SYLLABIC_ONE = new RegExp(
   '(?:' +
     [
       '([^aeiouy])\\1l',
@@ -111,7 +111,7 @@ var EXPRESSION_DOUBLE_SYLLABIC_ONE = new RegExp(
   'g'
 )
 
-var EXPRESSION_DOUBLE_SYLLABIC_TWO = new RegExp(
+const EXPRESSION_DOUBLE_SYLLABIC_TWO = new RegExp(
   [
     'creat(?!u)',
     '[^gq]ua[^auieo]',
@@ -123,7 +123,7 @@ var EXPRESSION_DOUBLE_SYLLABIC_TWO = new RegExp(
   'g'
 )
 
-var EXPRESSION_DOUBLE_SYLLABIC_THREE = new RegExp(
+const EXPRESSION_DOUBLE_SYLLABIC_THREE = new RegExp(
   [
     '[^aeiou]y[ae]',
     '[^l]lien',
@@ -142,10 +142,10 @@ var EXPRESSION_DOUBLE_SYLLABIC_THREE = new RegExp(
   'g'
 )
 
-var EXPRESSION_DOUBLE_SYLLABIC_FOUR = /[^s]ia/
+const EXPRESSION_DOUBLE_SYLLABIC_FOUR = /[^s]ia/
 
 // Expression to match single syllable pre- and suffixes.
-var EXPRESSION_SINGLE = new RegExp(
+const EXPRESSION_SINGLE = new RegExp(
   [
     '^(?:' +
       [
@@ -187,7 +187,7 @@ var EXPRESSION_SINGLE = new RegExp(
 )
 
 // Expression to match double syllable pre- and suffixes.
-var EXPRESSION_DOUBLE = new RegExp(
+const EXPRESSION_DOUBLE = new RegExp(
   [
     '^' +
       '(?:' +
@@ -223,7 +223,7 @@ var EXPRESSION_DOUBLE = new RegExp(
 )
 
 // Expression to match triple syllable suffixes.
-var EXPRESSION_TRIPLE = /(creations?|ology|ologist|onomy|onomist)$/g
+const EXPRESSION_TRIPLE = /(creations?|ology|ologist|onomy|onomist)$/g
 
 // Wrapper to support multiple word-parts (GH-11).
 /**
@@ -233,14 +233,14 @@ var EXPRESSION_TRIPLE = /(creations?|ology|ologist|onomy|onomist)$/g
  * @returns {number}
  */
 export function syllable(value) {
-  var values = normalize(String(value))
+  const values = normalize(String(value))
     .toLowerCase()
     // Remove apostrophes.
     .replace(/['’]/g, '')
     // Split on word boundaries.
     .split(/\b/g)
-  var index = -1
-  var sum = 0
+  let index = -1
+  let sum = 0
 
   while (++index < values.length) {
     // Remove non-alphabetic characters from a given value.
@@ -257,17 +257,7 @@ export function syllable(value) {
  * @returns {number}
  */
 function one(value) {
-  var count = 0
-  /** @type {number} */
-  var index
-  /** @type {string} */
-  var singular
-  /** @type {Array.<string>} */
-  var parts
-  /** @type {ReturnType.<returnFactory>} */
-  var addOne
-  /** @type {ReturnType.<returnFactory>} */
-  var subtractOne
+  let count = 0
 
   if (value.length === 0) {
     return count
@@ -284,14 +274,14 @@ function one(value) {
   }
 
   // Additionally, the singular word might be in `problematic`.
-  singular = pluralize(value, 1)
+  const singular = pluralize(value, 1)
 
   if (own.call(problematic, singular)) {
     return problematic[singular]
   }
 
-  addOne = returnFactory(1)
-  subtractOne = returnFactory(-1)
+  const addOne = returnFactory(1)
+  const subtractOne = returnFactory(-1)
 
   // Count some prefixes and suffixes, and remove their matched ranges.
   value = value
@@ -300,8 +290,8 @@ function one(value) {
     .replace(EXPRESSION_SINGLE, countFactory(1))
 
   // Count multiple consonants.
-  parts = value.split(/[^aeiouy]+/)
-  index = -1
+  const parts = value.split(/[^aeiouy]+/)
+  let index = -1
 
   while (++index < parts.length) {
     if (parts[index] !== '') {
diff --git a/package.json b/package.json
index 4cb51c6..bf521ac 100644
--- a/package.json
+++ b/package.json
@@ -67,12 +67,7 @@
     "trailingComma": "none"
   },
   "xo": {
-    "prettier": true,
-    "rules": {
-      "import/no-mutable-exports": "off",
-      "no-var": "off",
-      "prefer-arrow-callback": "off"
-    }
+    "prettier": true
   },
   "remarkConfig": {
     "plugins": [
diff --git a/problematic.js b/problematic.js
index f723b18..d5b0c84 100644
--- a/problematic.js
+++ b/problematic.js
@@ -1,4 +1,4 @@
-export var problematic = {
+export const problematic = {
   abalone: 4,
   abare: 3,
   abbruzzese: 4,
diff --git a/test/index.js b/test/index.js
index 9fcb11f..55a7b2d 100644
--- a/test/index.js
+++ b/test/index.js
@@ -5,20 +5,20 @@ import {PassThrough} from 'node:stream'
 import test from 'tape'
 import {syllable} from '../index.js'
 
-var own = {}.hasOwnProperty
+const own = {}.hasOwnProperty
 
 /** @type {Object.<string, unknown>} */
-var pack = JSON.parse(
+const pack = JSON.parse(
   String(fs.readFileSync(new URL('../package.json', import.meta.url)))
 )
 
 /** @type {Object.<string, number>} */
-var fixtures = JSON.parse(
+const fixtures = JSON.parse(
   String(fs.readFileSync(new URL('fixture.json', import.meta.url)))
 )
 
 test('api', function (t) {
-  var result = syllable('syllables')
+  const result = syllable('syllables')
 
   t.equal(syllable('SYLLABLES'), result, 'should be case insensitive (1)')
   t.equal(syllable('SyLlABlEs'), result, 'should be case insensitive (2)')
@@ -249,7 +249,7 @@ test('api', function (t) {
 })
 
 test('cli', function (t) {
-  var input = new PassThrough()
+  const input = new PassThrough()
 
   t.plan(8)
 
@@ -269,7 +269,7 @@ test('cli', function (t) {
     )
   })
 
-  var subprocess = exec('./cli.js', function (error, stdout, stderr) {
+  const subprocess = exec('./cli.js', function (error, stdout, stderr) {
     t.deepEqual([error, stdout, stderr], [null, '6\n', ''], 'stdin')
   })
 
@@ -322,20 +322,18 @@ 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 overwrite = {
+  const 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
+  let key
 
   for (key in fixtures) {
     if (own.call(fixtures, key)) {
-      expected = (key in overwrite ? overwrite : fixtures)[key]
+      const expected = (key in overwrite ? overwrite : fixtures)[key]
       t.equal(syllable(key), expected, key)
     }
   }