diff --git a/.npmrc b/.npmrc
index 9951b11..3757b30 100644
--- a/.npmrc
+++ b/.npmrc
@@ -1,2 +1,2 @@
-package-lock=false
 ignore-scripts=true
+package-lock=false
diff --git a/lib/index.js b/lib/index.js
index bc4853a..5171f3d 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -1,47 +1,42 @@
 /**
- * @typedef {import('mdast').Root} Root
- * @typedef {import('mdast').Content} Content
  * @typedef {import('mdast').Definition} Definition
+ * @typedef {import('mdast').Nodes} Nodes
  */
 
 /**
- * @typedef {Root | Content} Node
- *
  * @callback GetDefinition
  *   Get a definition by identifier.
  * @param {string | null | undefined} [identifier]
- *   Identifier of definition.
- * @returns {Definition | null}
+ *   Identifier of definition (optional).
+ * @returns {Definition | undefined}
  *   Definition corresponding to `identifier` or `null`.
  */
 
 import {visit} from 'unist-util-visit'
 
-const own = {}.hasOwnProperty
-
 /**
  * Find definitions in `tree`.
  *
  * Uses CommonMark precedence, which means that earlier definitions are
  * preferred over duplicate later definitions.
  *
- * @param {Node} tree
+ * @param {Nodes} tree
  *   Tree to check.
  * @returns {GetDefinition}
  *   Getter.
  */
 export function definitions(tree) {
-  /** @type {Record<string, Definition>} */
-  const cache = Object.create(null)
+  /** @type {Map<string, Definition>} */
+  const cache = new Map()
 
   if (!tree || !tree.type) {
     throw new Error('mdast-util-definitions expected node')
   }
 
-  visit(tree, 'definition', (definition) => {
+  visit(tree, 'definition', function (definition) {
     const id = clean(definition.identifier)
-    if (id && !own.call(cache, id)) {
-      cache[id] = definition
+    if (id && !cache.get(id)) {
+      cache.set(id, definition)
     }
   })
 
@@ -50,8 +45,7 @@ export function definitions(tree) {
   /** @type {GetDefinition} */
   function definition(identifier) {
     const id = clean(identifier)
-    // To do: next major: return `undefined` when not found.
-    return id && own.call(cache, id) ? cache[id] : null
+    return cache.get(id)
   }
 }
 
diff --git a/package.json b/package.json
index cc1ae71..04c8cf4 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "mdast-util-definitions",
-  "version": "5.1.2",
+  "version": "6.0.0",
   "description": "mdast utility to find definition nodes in a tree",
   "license": "MIT",
   "keywords": [
@@ -28,56 +28,56 @@
   ],
   "sideEffects": false,
   "type": "module",
-  "main": "index.js",
-  "types": "index.d.ts",
+  "exports": "./index.js",
   "files": [
     "lib/",
     "index.d.ts",
     "index.js"
   ],
   "dependencies": {
-    "@types/mdast": "^3.0.0",
-    "@types/unist": "^2.0.0",
-    "unist-util-visit": "^4.0.0"
+    "@types/mdast": "^4.0.0",
+    "@types/unist": "^3.0.0",
+    "unist-util-visit": "^5.0.0"
   },
   "devDependencies": {
-    "@types/node": "^18.0.0",
-    "c8": "^7.0.0",
+    "@types/node": "^20.0.0",
+    "c8": "^8.0.0",
     "mdast-util-from-markdown": "^1.0.0",
     "prettier": "^2.0.0",
     "remark-cli": "^11.0.0",
     "remark-preset-wooorm": "^9.0.0",
     "type-coverage": "^2.0.0",
-    "typescript": "^4.0.0",
-    "xo": "^0.53.0"
+    "typescript": "^5.0.0",
+    "xo": "^0.54.0"
   },
   "scripts": {
     "prepack": "npm run build && npm run format",
     "build": "tsc --build --clean && tsc --build && type-coverage",
     "format": "remark . -qfo && prettier . -w --loglevel warn && xo --fix",
     "test-api": "node --conditions development test.js",
-    "test-coverage": "c8 --check-coverage --100 --reporter lcov npm run test-api",
+    "test-coverage": "c8 --100 --reporter lcov npm run test-api",
     "test": "npm run build && npm run format && npm run test-coverage"
   },
   "prettier": {
-    "tabWidth": 2,
-    "useTabs": false,
-    "singleQuote": true,
     "bracketSpacing": false,
     "semi": false,
-    "trailingComma": "none"
-  },
-  "xo": {
-    "prettier": true
+    "singleQuote": true,
+    "tabWidth": 2,
+    "trailingComma": "none",
+    "useTabs": false
   },
   "remarkConfig": {
     "plugins": [
-      "preset-wooorm"
+      "remark-preset-wooorm"
     ]
   },
   "typeCoverage": {
     "atLeast": 100,
     "detail": true,
+    "ignoreCatch": true,
     "strict": true
+  },
+  "xo": {
+    "prettier": true
   }
 }
diff --git a/readme.md b/readme.md
index 078c402..171a89a 100644
--- a/readme.md
+++ b/readme.md
@@ -39,7 +39,7 @@ It’s small and protects against prototype pollution.
 ## Install
 
 This package is [ESM only][esm].
-In Node.js (version 14.14+ and 16.0+), install with [npm][]:
+In Node.js (version 16+), install with [npm][]:
 
 ```sh
 npm install mdast-util-definitions
@@ -48,22 +48,22 @@ npm install mdast-util-definitions
 In Deno with [`esm.sh`][esmsh]:
 
 ```js
-import {definitions} from 'https://esm.sh/mdast-util-definitions@5'
+import {definitions} from 'https://esm.sh/mdast-util-definitions@6'
 ```
 
 In browsers with [`esm.sh`][esmsh]:
 
 ```html
 <script type="module">
-  import {definitions} from 'https://esm.sh/mdast-util-definitions@5?bundle'
+  import {definitions} from 'https://esm.sh/mdast-util-definitions@6?bundle'
 </script>
 ```
 
 ## Use
 
 ```js
-import {fromMarkdown} from 'mdast-util-from-markdown'
 import {definitions} from 'mdast-util-definitions'
+import {fromMarkdown} from 'mdast-util-from-markdown'
 
 const tree = fromMarkdown('[example]: https://example.com "Example"')
 
@@ -73,7 +73,7 @@ definition('example')
 // => {type: 'definition', 'title': 'Example', …}
 
 definition('foo')
-// => null
+// => undefined
 ```
 
 ## API
@@ -109,7 +109,7 @@ Get a definition by identifier (TypeScript type).
 ###### Returns
 
 Definition corresponding to `identifier` ([`Definition`][definition]) or
-`null`.
+`undefined`.
 
 ## Types
 
@@ -118,10 +118,13 @@ It exports the additional type  [`GetDefinition`][api-getdefinition].
 
 ## Compatibility
 
-Projects maintained by the unified collective are compatible with all maintained
+Projects maintained by the unified collective are compatible with maintained
 versions of Node.js.
-As of now, that is Node.js 14.14+ and 16.0+.
-Our projects sometimes work with older versions, but this is not guaranteed.
+
+When we cut a new major release, we drop support for unmaintained versions of
+Node.
+This means we try to keep the current release line,
+`mdast-util-definitions@^6`, compatible with Node.js 16.
 
 ## Security
 
@@ -162,9 +165,9 @@ abide by its terms.
 
 [downloads]: https://www.npmjs.com/package/mdast-util-definitions
 
-[size-badge]: https://img.shields.io/bundlephobia/minzip/mdast-util-definitions.svg
+[size-badge]: https://img.shields.io/badge/dynamic/json?label=minzipped%20size&query=$.size.compressedSize&url=https://deno.bundlejs.com/?q=mdast-util-definitions
 
-[size]: https://bundlephobia.com/result?p=mdast-util-definitions
+[size]: https://bundlejs.com/?q=mdast-util-definitions
 
 [sponsors-badge]: https://opencollective.com/unified/sponsors/badge.svg
 
diff --git a/test.js b/test.js
index ec76bc2..4291d3e 100644
--- a/test.js
+++ b/test.js
@@ -1,98 +1,115 @@
+/**
+ * @typedef {import('mdast').Root} Root
+ */
+
 import assert from 'node:assert/strict'
 import test from 'node:test'
+import {definitions} from 'mdast-util-definitions'
 import {fromMarkdown} from 'mdast-util-from-markdown'
-import {definitions} from './index.js'
-import * as mod from './index.js'
 
-test('definitions', () => {
-  assert.deepEqual(
-    Object.keys(mod).sort(),
-    ['definitions'],
-    'should expose the public api'
-  )
+test('definitions', async function (t) {
+  await t.test('should expose the public api', async function () {
+    assert.deepEqual(
+      Object.keys(await import('mdast-util-definitions')).sort(),
+      ['definitions']
+    )
+  })
 
-  assert.throws(
-    () => {
-      // @ts-expect-error runtime
+  await t.test('should fail without node', async function () {
+    assert.throws(function () {
+      // @ts-expect-error: check that an error is thrown at runtime.
       definitions()
-    },
-    /mdast-util-definitions expected node/,
-    'should fail without node'
-  )
+    }, /mdast-util-definitions expected node/)
+  })
 
-  assert.deepEqual(
-    definitions(fromMarkdown('[example]: https://example.com "Example"'))(
-      'example'
-    ),
-    {
-      type: 'definition',
-      identifier: 'example',
-      label: 'example',
-      title: 'Example',
-      url: 'https://example.com',
-      position: {
-        start: {line: 1, column: 1, offset: 0},
-        end: {line: 1, column: 41, offset: 40}
+  await t.test('should return a definition', async function () {
+    assert.deepEqual(
+      definitions(from('[example]: https://example.com "Example"'))('example'),
+      {
+        type: 'definition',
+        identifier: 'example',
+        label: 'example',
+        title: 'Example',
+        url: 'https://example.com',
+        position: {
+          start: {line: 1, column: 1, offset: 0},
+          end: {line: 1, column: 41, offset: 40}
+        }
       }
-    },
-    'should return a definition'
-  )
+    )
+  })
 
-  assert.equal(
-    definitions(fromMarkdown('[example]: https://example.com "Example"'))(
-      'foo'
-    ),
-    null,
-    'should return null when not found'
-  )
+  await t.test('should return `undefined` when not found', async function () {
+    assert.equal(
+      definitions(from('[example]: https://example.com "Example"'))('foo'),
+      undefined
+    )
+  })
 
-  assert.deepEqual(
-    definitions(fromMarkdown('[__proto__]: https://proto.com "Proto"'))(
-      '__proto__'
-    ),
-    {
-      type: 'definition',
-      identifier: '__proto__',
-      label: '__proto__',
-      title: 'Proto',
-      url: 'https://proto.com',
-      position: {
-        start: {line: 1, column: 1, offset: 0},
-        end: {line: 1, column: 39, offset: 38}
+  await t.test('should work on weird identifiers', async function () {
+    assert.deepEqual(
+      definitions(from('[__proto__]: https://proto.com "Proto"'))('__proto__'),
+      {
+        type: 'definition',
+        identifier: '__proto__',
+        label: '__proto__',
+        title: 'Proto',
+        url: 'https://proto.com',
+        position: {
+          start: {line: 1, column: 1, offset: 0},
+          end: {line: 1, column: 39, offset: 38}
+        }
       }
-    },
-    'should work on weird identifiers'
-  )
+    )
+  })
 
-  /* eslint-disable no-use-extend-native/no-use-extend-native */
-  // @ts-expect-error: yes.
-  // type-coverage:ignore-next-line
-  assert.equal({}.type, undefined, 'should not polute the prototype')
-  /* eslint-enable no-use-extend-native/no-use-extend-native */
+  await t.test('should not polute the prototype', async function () {
+    /* eslint-disable no-use-extend-native/no-use-extend-native */
+    // @ts-expect-error: yes.
+    // type-coverage:ignore-next-line
+    assert.equal({}.type, undefined)
+    /* eslint-enable no-use-extend-native/no-use-extend-native */
+  })
 
-  assert.deepEqual(
-    definitions(fromMarkdown('[__proto__]: https://proto.com "Proto"'))(
-      'toString'
-    ),
-    null,
-    'should work on weird identifiers when not found'
+  await t.test(
+    'should work on weird identifiers when not found',
+    async function () {
+      assert.deepEqual(
+        definitions(from('[__proto__]: https://proto.com "Proto"'))('toString'),
+        undefined
+      )
+    }
   )
 
-  const example = definitions(
-    fromMarkdown('[example]: https://one.com\n[example]: https://two.com')
-  )('example')
+  await t.test(
+    'should prefer the first of duplicate definitions',
+    async function () {
+      const example = definitions(
+        from('[example]: https://one.com\n[example]: https://two.com')
+      )('example')
 
-  assert.deepEqual(
-    example && example.url,
-    'https://one.com',
-    'should prefer the first of duplicate definitions'
+      assert.deepEqual(example && example.url, 'https://one.com')
+    }
   )
 
-  assert.deepEqual(
-    definitions(
-      fromMarkdown('[example]: https://one.com\n[example]: https://two.com')
-    )(''),
-    null,
-    'should not return something for a missing identifier'
+  await t.test(
+    'should not return something for a missing identifier',
+    async function () {
+      assert.deepEqual(
+        definitions(
+          from('[example]: https://one.com\n[example]: https://two.com')
+        )(''),
+        undefined
+      )
+    }
   )
 })
+
+/**
+ * @param {string} value
+ * @returns {Root}
+ */
+function from(value) {
+  // @ts-expect-error: To do: remove cast when `from-markdown` is released.
+  return fromMarkdown(value)
+}
diff --git a/tsconfig.json b/tsconfig.json
index ebe8889..82cc749 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -1,17 +1,15 @@
 {
-  "include": ["**/*.js"],
-  "exclude": ["coverage/", "node_modules/"],
   "compilerOptions": {
     "checkJs": true,
+    "customConditions": ["development"],
     "declaration": true,
     "emitDeclarationOnly": true,
     "exactOptionalPropertyTypes": true,
-    "forceConsistentCasingInFileNames": true,
-    "lib": ["es2020"],
+    "lib": ["es2022"],
     "module": "node16",
-    "newLine": "lf",
-    "skipLibCheck": true,
     "strict": true,
-    "target": "es2020"
-  }
+    "target": "es2022"
+  },
+  "exclude": ["coverage/", "node_modules/"],
+  "include": ["**/*.js"]
 }