diff --git a/index.js b/index.js
index 8607c67..93fb758 100644
--- a/index.js
+++ b/index.js
@@ -1,10 +1,10 @@
 import {directive} from 'micromark-extension-directive'
 import {directiveFromMarkdown, directiveToMarkdown} from 'mdast-util-directive'
 
-var warningIssued
+let warningIssued
 
 export default function remarkDirective() {
-  var data = this.data()
+  const data = this.data()
 
   // Old remark.
   /* c8 ignore next 14 */
diff --git a/package.json b/package.json
index f6e6e90..d9321ec 100644
--- a/package.json
+++ b/package.json
@@ -45,7 +45,7 @@
     "tape": "^5.0.0",
     "to-vfile": "^7.0.0",
     "unified": "^10.0.0",
-    "xo": "^0.37.0"
+    "xo": "^0.39.0"
   },
   "scripts": {
     "format": "remark . -qfo --ignore-pattern test/ && prettier . -w --loglevel warn && xo --fix",
@@ -64,13 +64,6 @@
   },
   "xo": {
     "prettier": true,
-    "esnext": false,
-    "rules": {
-      "no-var": "off",
-      "prefer-arrow-callback": "off",
-      "unicorn/no-array-callback-reference": "off",
-      "unicorn/prefer-optional-catch-binding": "off"
-    },
     "ignores": [
       "types/"
     ]
diff --git a/test/index.js b/test/index.js
index 56fcdee..a8abbff 100644
--- a/test/index.js
+++ b/test/index.js
@@ -7,43 +7,40 @@ import {remark} from 'remark'
 import {isHidden} from 'is-hidden'
 import directive from '../index.js'
 
-test('directive()', function (t) {
-  t.doesNotThrow(function () {
+test('directive()', (t) => {
+  t.doesNotThrow(() => {
     remark().use(directive).freeze()
   }, 'should not throw if not passed options')
 
-  t.doesNotThrow(function () {
+  t.doesNotThrow(() => {
     unified().use(directive).freeze()
   }, 'should not throw if without parser or compiler')
 
   t.end()
 })
 
-test('fixtures', function (t) {
-  var base = path.join('test', 'fixtures')
-  var entries = fs.readdirSync(base).filter((d) => !isHidden(d))
+test('fixtures', (t) => {
+  const base = path.join('test', 'fixtures')
+  const entries = fs.readdirSync(base).filter((d) => !isHidden(d))
 
   t.plan(entries.length)
 
-  entries.forEach(each)
-
-  function each(fixture) {
-    t.test(fixture, function (st) {
-      var file = readSync(path.join(base, fixture, 'input.md'))
-      var input = String(file)
-      var outputPath = path.join(base, fixture, 'output.md')
-      var treePath = path.join(base, fixture, 'tree.json')
-      var proc
-      var actual
-      var output
-      var expected
-
-      proc = remark().use(directive).freeze()
-      actual = proc.parse(file)
+  let index = -1
+  while (++index < entries.length) {
+    const fixture = entries[index]
+    t.test(fixture, (st) => {
+      const file = readSync(path.join(base, fixture, 'input.md'))
+      const input = String(file)
+      const outputPath = path.join(base, fixture, 'output.md')
+      const treePath = path.join(base, fixture, 'tree.json')
+      const proc = remark().use(directive).freeze()
+      const actual = proc.parse(file)
+      let output
+      let expected
 
       try {
         expected = JSON.parse(fs.readFileSync(treePath))
-      } catch (_) {
+      } catch {
         // New fixture.
         fs.writeFileSync(treePath, JSON.stringify(actual, 0, 2) + '\n')
         expected = actual
@@ -51,7 +48,7 @@ test('fixtures', function (t) {
 
       try {
         output = fs.readFileSync(outputPath, 'utf8')
-      } catch (_) {
+      } catch {
         output = input
       }