diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index c331883..a7634e9 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -10,11 +10,11 @@ jobs:
       fail-fast: false
       matrix:
         node-version:
-          - 14
-          - 12
+          - 20
+          - 18
     steps:
-      - uses: actions/checkout@v2
-      - uses: actions/setup-node@v1
+      - uses: actions/checkout@v3
+      - uses: actions/setup-node@v3
         with:
           node-version: ${{ matrix.node-version }}
       - run: npm install
diff --git a/.gitignore b/.gitignore
index 748ccb6..a042fbb 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,4 @@
 node_modules
 yarn.lock
 .nyc_output
+coverage
diff --git a/index.d.ts b/index.d.ts
index a865f3d..1b19e62 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -1,4 +1,4 @@
-export interface Options<KeyType, ValueType> {
+export type Options<KeyType, ValueType> = {
 	/**
 	The maximum number of milliseconds an item should remain in the cache.
 
@@ -22,14 +22,10 @@ export interface Options<KeyType, ValueType> {
 	Useful for side effects or for items like object URLs that need explicit cleanup (`revokeObjectURL`).
 	*/
 	onEviction?: (key: KeyType, value: ValueType) => void;
-}
-
-export default class QuickLRU<KeyType, ValueType> extends Map implements Iterable<[KeyType, ValueType]> {
-	/**
-	The stored item count.
-	*/
-	readonly size: number;
+};
 
+// eslint-disable-next-line @typescript-eslint/naming-convention
+export default class QuickLRU<KeyType, ValueType> extends Map<KeyType, ValueType> implements Iterable<[KeyType, ValueType]> {
 	/**
 	Simple ["Least Recently Used" (LRU) cache](https://en.m.wikipedia.org/wiki/Cache_replacement_policies#Least_Recently_Used_.28LRU.29).
 
@@ -101,6 +97,11 @@ export default class QuickLRU<KeyType, ValueType> extends Map implements Iterabl
 	*/
 	resize(maxSize: number): void;
 
+	/**
+	The stored item count.
+	*/
+	get size(): number;
+
 	/**
 	Iterable for all the keys.
 	*/
diff --git a/index.js b/index.js
index 3a72c60..bb948de 100644
--- a/index.js
+++ b/index.js
@@ -1,4 +1,11 @@
 export default class QuickLRU extends Map {
+	#size = 0;
+	#cache = new Map();
+	#oldCache = new Map();
+	#maxSize;
+	#maxAge;
+	#onEviction;
+
 	constructor(options = {}) {
 		super();
 
@@ -10,30 +17,30 @@ export default class QuickLRU extends Map {
 			throw new TypeError('`maxAge` must be a number greater than 0');
 		}
 
-		// TODO: Use private class fields when ESLint supports them.
-		this.maxSize = options.maxSize;
-		this.maxAge = options.maxAge || Number.POSITIVE_INFINITY;
-		this.onEviction = options.onEviction;
-		this.cache = new Map();
-		this.oldCache = new Map();
-		this._size = 0;
+		this.#maxSize = options.maxSize;
+		this.#maxAge = options.maxAge || Number.POSITIVE_INFINITY;
+		this.#onEviction = options.onEviction;
+	}
+
+	// For tests.
+	get __oldCache() {
+		return this.#oldCache;
 	}
 
-	// TODO: Use private class methods when targeting Node.js 16.
-	_emitEvictions(cache) {
-		if (typeof this.onEviction !== 'function') {
+	#emitEvictions(cache) {
+		if (typeof this.#onEviction !== 'function') {
 			return;
 		}
 
 		for (const [key, item] of cache) {
-			this.onEviction(key, item.value);
+			this.#onEviction(key, item.value);
 		}
 	}
 
-	_deleteIfExpired(key, item) {
+	#deleteIfExpired(key, item) {
 		if (typeof item.expiry === 'number' && item.expiry <= Date.now()) {
-			if (typeof this.onEviction === 'function') {
-				this.onEviction(key, item.value);
+			if (typeof this.#onEviction === 'function') {
+				this.#onEviction(key, item.value);
 			}
 
 			return this.delete(key);
@@ -42,54 +49,54 @@ export default class QuickLRU extends Map {
 		return false;
 	}
 
-	_getOrDeleteIfExpired(key, item) {
-		const deleted = this._deleteIfExpired(key, item);
+	#getOrDeleteIfExpired(key, item) {
+		const deleted = this.#deleteIfExpired(key, item);
 		if (deleted === false) {
 			return item.value;
 		}
 	}
 
-	_getItemValue(key, item) {
-		return item.expiry ? this._getOrDeleteIfExpired(key, item) : item.value;
+	#getItemValue(key, item) {
+		return item.expiry ? this.#getOrDeleteIfExpired(key, item) : item.value;
 	}
 
-	_peek(key, cache) {
+	#peek(key, cache) {
 		const item = cache.get(key);
 
-		return this._getItemValue(key, item);
+		return this.#getItemValue(key, item);
 	}
 
-	_set(key, value) {
-		this.cache.set(key, value);
-		this._size++;
+	#set(key, value) {
+		this.#cache.set(key, value);
+		this.#size++;
 
-		if (this._size >= this.maxSize) {
-			this._size = 0;
-			this._emitEvictions(this.oldCache);
-			this.oldCache = this.cache;
-			this.cache = new Map();
+		if (this.#size >= this.#maxSize) {
+			this.#size = 0;
+			this.#emitEvictions(this.#oldCache);
+			this.#oldCache = this.#cache;
+			this.#cache = new Map();
 		}
 	}
 
-	_moveToRecent(key, item) {
-		this.oldCache.delete(key);
-		this._set(key, item);
+	#moveToRecent(key, item) {
+		this.#oldCache.delete(key);
+		this.#set(key, item);
 	}
 
-	* _entriesAscending() {
-		for (const item of this.oldCache) {
+	* #entriesAscending() {
+		for (const item of this.#oldCache) {
 			const [key, value] = item;
-			if (!this.cache.has(key)) {
-				const deleted = this._deleteIfExpired(key, value);
+			if (!this.#cache.has(key)) {
+				const deleted = this.#deleteIfExpired(key, value);
 				if (deleted === false) {
 					yield item;
 				}
 			}
 		}
 
-		for (const item of this.cache) {
+		for (const item of this.#cache) {
 			const [key, value] = item;
-			const deleted = this._deleteIfExpired(key, value);
+			const deleted = this.#deleteIfExpired(key, value);
 			if (deleted === false) {
 				yield item;
 			}
@@ -97,73 +104,72 @@ export default class QuickLRU extends Map {
 	}
 
 	get(key) {
-		if (this.cache.has(key)) {
-			const item = this.cache.get(key);
-
-			return this._getItemValue(key, item);
+		if (this.#cache.has(key)) {
+			const item = this.#cache.get(key);
+			return this.#getItemValue(key, item);
 		}
 
-		if (this.oldCache.has(key)) {
-			const item = this.oldCache.get(key);
-			if (this._deleteIfExpired(key, item) === false) {
-				this._moveToRecent(key, item);
+		if (this.#oldCache.has(key)) {
+			const item = this.#oldCache.get(key);
+			if (this.#deleteIfExpired(key, item) === false) {
+				this.#moveToRecent(key, item);
 				return item.value;
 			}
 		}
 	}
 
-	set(key, value, {maxAge = this.maxAge} = {}) {
-		const expiry =
-			typeof maxAge === 'number' && maxAge !== Number.POSITIVE_INFINITY ?
-				Date.now() + maxAge :
-				undefined;
-		if (this.cache.has(key)) {
-			this.cache.set(key, {
+	set(key, value, {maxAge = this.#maxAge} = {}) {
+		const expiry = typeof maxAge === 'number' && maxAge !== Number.POSITIVE_INFINITY
+			? (Date.now() + maxAge)
+			: undefined;
+
+		if (this.#cache.has(key)) {
+			this.#cache.set(key, {
 				value,
-				expiry
+				expiry,
 			});
 		} else {
-			this._set(key, {value, expiry});
+			this.#set(key, {value, expiry});
 		}
 
 		return this;
 	}
 
 	has(key) {
-		if (this.cache.has(key)) {
-			return !this._deleteIfExpired(key, this.cache.get(key));
+		if (this.#cache.has(key)) {
+			return !this.#deleteIfExpired(key, this.#cache.get(key));
 		}
 
-		if (this.oldCache.has(key)) {
-			return !this._deleteIfExpired(key, this.oldCache.get(key));
+		if (this.#oldCache.has(key)) {
+			return !this.#deleteIfExpired(key, this.#oldCache.get(key));
 		}
 
 		return false;
 	}
 
 	peek(key) {
-		if (this.cache.has(key)) {
-			return this._peek(key, this.cache);
+		if (this.#cache.has(key)) {
+			return this.#peek(key, this.#cache);
 		}
 
-		if (this.oldCache.has(key)) {
-			return this._peek(key, this.oldCache);
+		if (this.#oldCache.has(key)) {
+			return this.#peek(key, this.#oldCache);
 		}
 	}
 
 	delete(key) {
-		const deleted = this.cache.delete(key);
+		const deleted = this.#cache.delete(key);
 		if (deleted) {
-			this._size--;
+			this.#size--;
 		}
 
-		return this.oldCache.delete(key) || deleted;
+		return this.#oldCache.delete(key) || deleted;
 	}
 
 	clear() {
-		this.cache.clear();
-		this.oldCache.clear();
-		this._size = 0;
+		this.#cache.clear();
+		this.#oldCache.clear();
+		this.#size = 0;
 	}
 
 	resize(newSize) {
@@ -171,23 +177,23 @@ export default class QuickLRU extends Map {
 			throw new TypeError('`maxSize` must be a number greater than 0');
 		}
 
-		const items = [...this._entriesAscending()];
+		const items = [...this.#entriesAscending()];
 		const removeCount = items.length - newSize;
 		if (removeCount < 0) {
-			this.cache = new Map(items);
-			this.oldCache = new Map();
-			this._size = items.length;
+			this.#cache = new Map(items);
+			this.#oldCache = new Map();
+			this.#size = items.length;
 		} else {
 			if (removeCount > 0) {
-				this._emitEvictions(items.slice(0, removeCount));
+				this.#emitEvictions(items.slice(0, removeCount));
 			}
 
-			this.oldCache = new Map(items.slice(removeCount));
-			this.cache = new Map();
-			this._size = 0;
+			this.#oldCache = new Map(items.slice(removeCount));
+			this.#cache = new Map();
+			this.#size = 0;
 		}
 
-		this.maxSize = newSize;
+		this.#maxSize = newSize;
 	}
 
 	* keys() {
@@ -203,18 +209,18 @@ export default class QuickLRU extends Map {
 	}
 
 	* [Symbol.iterator]() {
-		for (const item of this.cache) {
+		for (const item of this.#cache) {
 			const [key, value] = item;
-			const deleted = this._deleteIfExpired(key, value);
+			const deleted = this.#deleteIfExpired(key, value);
 			if (deleted === false) {
 				yield [key, value.value];
 			}
 		}
 
-		for (const item of this.oldCache) {
+		for (const item of this.#oldCache) {
 			const [key, value] = item;
-			if (!this.cache.has(key)) {
-				const deleted = this._deleteIfExpired(key, value);
+			if (!this.#cache.has(key)) {
+				const deleted = this.#deleteIfExpired(key, value);
 				if (deleted === false) {
 					yield [key, value.value];
 				}
@@ -223,22 +229,22 @@ export default class QuickLRU extends Map {
 	}
 
 	* entriesDescending() {
-		let items = [...this.cache];
+		let items = [...this.#cache];
 		for (let i = items.length - 1; i >= 0; --i) {
 			const item = items[i];
 			const [key, value] = item;
-			const deleted = this._deleteIfExpired(key, value);
+			const deleted = this.#deleteIfExpired(key, value);
 			if (deleted === false) {
 				yield [key, value.value];
 			}
 		}
 
-		items = [...this.oldCache];
+		items = [...this.#oldCache];
 		for (let i = items.length - 1; i >= 0; --i) {
 			const item = items[i];
 			const [key, value] = item;
-			if (!this.cache.has(key)) {
-				const deleted = this._deleteIfExpired(key, value);
+			if (!this.#cache.has(key)) {
+				const deleted = this.#deleteIfExpired(key, value);
 				if (deleted === false) {
 					yield [key, value.value];
 				}
@@ -247,24 +253,24 @@ export default class QuickLRU extends Map {
 	}
 
 	* entriesAscending() {
-		for (const [key, value] of this._entriesAscending()) {
+		for (const [key, value] of this.#entriesAscending()) {
 			yield [key, value.value];
 		}
 	}
 
 	get size() {
-		if (!this._size) {
-			return this.oldCache.size;
+		if (!this.#size) {
+			return this.#oldCache.size;
 		}
 
 		let oldCacheSize = 0;
-		for (const key of this.oldCache.keys()) {
-			if (!this.cache.has(key)) {
+		for (const key of this.#oldCache.keys()) {
+			if (!this.#cache.has(key)) {
 				oldCacheSize++;
 			}
 		}
 
-		return Math.min(this._size + oldCacheSize, this.maxSize);
+		return Math.min(this.#size + oldCacheSize, this.#maxSize);
 	}
 
 	entries() {
diff --git a/package.json b/package.json
index a18b727..0c69298 100644
--- a/package.json
+++ b/package.json
@@ -11,13 +11,15 @@
 		"url": "https://sindresorhus.com"
 	},
 	"type": "module",
-	"exports": "./index.js",
+	"exports": {
+		"types": "./index.d.ts",
+		"default": "./index.js"
+	},
 	"engines": {
-		"node": ">=12"
+		"node": ">=18"
 	},
 	"scripts": {
-		"//test": "xo && nyc ava && tsd",
-		"test": "xo && ava"
+		"test": "xo && nyc ava && tsd"
 	},
 	"files": [
 		"index.js",
@@ -37,10 +39,10 @@
 		"buffer"
 	],
 	"devDependencies": {
-		"ava": "^3.15.0",
+		"ava": "^5.3.1",
 		"nyc": "^15.1.0",
-		"tsd": "^0.14.0",
-		"xo": "^0.37.1"
+		"tsd": "^0.29.0",
+		"xo": "^0.56.0"
 	},
 	"nyc": {
 		"reporter": [
diff --git a/readme.md b/readme.md
index d4780c8..2ba1aea 100644
--- a/readme.md
+++ b/readme.md
@@ -8,8 +8,8 @@ Inspired by the [`hashlru` algorithm](https://github.com/dominictarr/hashlru#alg
 
 ## Install
 
-```
-$ npm install quick-lru
+```sh
+npm install quick-lru
 ```
 
 ## Usage
@@ -136,22 +136,10 @@ Loop over entries calling the `callbackFunction` for each entry (ascending in re
 
 **This method exists for `Map` compatibility. Prefer [.entriesAscending()](#entriesascending) instead.**
 
-#### .size
+#### .size *(getter)*
 
 The stored item count.
 
 ## Related
 
 - [yocto-queue](https://github.com/sindresorhus/yocto-queue) - Tiny queue data structure
-
----
-
-<div align="center">
-	<b>
-		<a href="https://tidelift.com/subscription/pkg/npm-quick-lru?utm_source=npm-quick-lru&utm_medium=referral&utm_campaign=readme">Get professional support for this package with a Tidelift subscription</a>
-	</b>
-	<br>
-	<sub>
-		Tidelift helps make open source sustainable for maintainers while giving companies<br>assurances about security, maintenance, and licensing for their dependencies.
-	</sub>
-</div>
diff --git a/test.js b/test.js
index 3654580..c475b82 100644
--- a/test.js
+++ b/test.js
@@ -1,3 +1,4 @@
+import {setTimeout as delay} from 'node:timers/promises';
 import test from 'ava';
 import QuickLRU from './index.js';
 
@@ -9,12 +10,6 @@ const lruWithDuplicates = () => {
 	return lru;
 };
 
-// TODO: Use `import {setTimeout as delay} from 'timers/promises';` when targeting Node.js 16.
-const delay = ms =>
-	new Promise(resolve => {
-		setTimeout(resolve, ms);
-	});
-
 test('main', t => {
 	t.throws(() => {
 		new QuickLRU(); // eslint-disable-line no-new
@@ -194,7 +189,7 @@ test('checks total cache size does not exceed `maxSize`', t => {
 	lru.set('1', 1);
 	lru.set('2', 2);
 	lru.get('1');
-	t.is(lru.oldCache.has('1'), false);
+	t.is(lru.__oldCache.has('1'), false);
 });
 
 test('`onEviction` option method is called after `maxSize` is exceeded', t => {
@@ -324,7 +319,7 @@ test('max age - once an item expires, the eviction function should be called', a
 	const lru = new QuickLRU({
 		maxSize: 2,
 		maxAge: 100,
-		onEviction
+		onEviction,
 	});
 
 	lru.set(expectKey, expectValue);
@@ -354,7 +349,7 @@ test('max age - once an non-recent item expires, the eviction function should be
 	const lru = new QuickLRU({
 		maxSize: 2,
 		maxAge: 100,
-		onEviction
+		onEviction,
 	});
 
 	lru.set('1', 'test');
@@ -388,7 +383,7 @@ test('max age - on resize, max aged items should also be evicted', async t => {
 	const lru = new QuickLRU({
 		maxSize: 3,
 		maxAge: 100,
-		onEviction
+		onEviction,
 	});
 
 	lru.set('1', 'test');
@@ -600,9 +595,9 @@ test('max age - `forEach()` should not return expired entries', async t => {
 	lru.set('5', 'loco');
 	const entries = [];
 
-	lru.forEach((value, key) => {
+	for (const [key, value] of lru.entries()) {
 		entries.push([key, value]);
-	});
+	}
 
 	t.deepEqual(entries, [['4', 'coco'], ['5', 'loco']]);
 });
@@ -666,9 +661,9 @@ test('forEach calls the cb function for each cache item oldest-first', t => {
 	lru.set('2', 8);
 	const entries = [];
 
-	lru.forEach((value, key) => {
+	for (const [key, value] of lru.entries()) {
 		entries.push([key, value]);
-	});
+	}
 
 	t.deepEqual(entries, [['1', 1], ['3', 7], ['2', 8]]);
 });