diff --git a/.gitignore b/.gitignore
index f1053d1..735f4af 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,8 +1,5 @@
 .DS_Store
 *.log
-.nyc_output/
 coverage/
 node_modules/
-mdast-util-definitions.js
-mdast-util-definitions.min.js
 yarn.lock
diff --git a/.prettierignore b/.prettierignore
index 05e7944..cebe81f 100644
--- a/.prettierignore
+++ b/.prettierignore
@@ -1,5 +1,2 @@
 coverage/
-mdast-util-definitions.js
-mdast-util-definitions.min.js
-*.json
 *.md
diff --git a/index.js b/index.js
index 17cc583..56a50a1 100644
--- a/index.js
+++ b/index.js
@@ -1,13 +1,9 @@
-'use strict'
-
-var visit = require('unist-util-visit')
-
-module.exports = createGetDefinition
+import {visit} from 'unist-util-visit'
 
 var own = {}.hasOwnProperty
 
 // Get a definition in `node` by `identifier`.
-function createGetDefinition(node) {
+export function definitions(node) {
   var cache = {}
 
   if (!node || !node.type) {
diff --git a/package.json b/package.json
index 81c487d..627485e 100644
--- a/package.json
+++ b/package.json
@@ -26,42 +26,32 @@
   "contributors": [
     "Titus Wormer <tituswormer@gmail.com> (https://wooorm.com)"
   ],
+  "sideEffects": false,
+  "type": "module",
+  "main": "index.js",
   "types": "types/index.d.ts",
   "files": [
     "types",
     "index.js"
   ],
   "dependencies": {
-    "unist-util-visit": "^2.0.0"
+    "unist-util-visit": "^3.0.0"
   },
   "devDependencies": {
     "@types/mdast": "^3.0.0",
-    "browserify": "^17.0.0",
-    "dtslint": "^4.0.0",
-    "nyc": "^15.0.0",
+    "c8": "^7.0.0",
     "prettier": "^2.0.0",
     "remark": "^13.0.0",
     "remark-cli": "^9.0.0",
     "remark-preset-wooorm": "^8.0.0",
     "tape": "^5.0.0",
-    "tinyify": "^3.0.0",
-    "xo": "^0.38.0"
+    "xo": "^0.39.0"
   },
   "scripts": {
-    "format": "remark . -qfo && prettier . --write && xo --fix --ignore types",
-    "build-bundle": "browserify . -s mdastUtilDefinitions -o mdast-util-definitions.js",
-    "build-mangle": "browserify . -s mdastUtilDefinitions -o mdast-util-definitions.min.js -p tinyify",
-    "build": "npm run build-bundle && npm run build-mangle",
-    "test-api": "node test",
-    "test-coverage": "nyc --reporter lcov tape test.js",
-    "test-types": "dtslint types",
-    "test": "npm run format && npm run build && npm run test-coverage && npm run test-types"
-  },
-  "nyc": {
-    "check-coverage": true,
-    "lines": 100,
-    "functions": 100,
-    "branches": 100
+    "format": "remark . -qfo && prettier . -w --loglevel warn && xo --fix",
+    "test-api": "node test.js",
+    "test-coverage": "c8 --check-coverage --branches 100 --functions 100 --lines 100 --statements 100 --reporter lcov node test.js",
+    "test": "npm run format && npm run test-coverage"
   },
   "prettier": {
     "tabWidth": 2,
@@ -73,10 +63,12 @@
   },
   "xo": {
     "prettier": true,
-    "esnext": false,
+    "rules": {
+      "no-var": "off",
+      "prefer-arrow-callback": "off"
+    },
     "ignore": [
-      "types",
-      "mdast-util-definitions.js"
+      "types/"
     ]
   },
   "remarkConfig": {
diff --git a/readme.md b/readme.md
index 6233ecf..a9b11ae 100644
--- a/readme.md
+++ b/readme.md
@@ -14,30 +14,27 @@ Supports funky keys, like `__proto__` or `toString`.
 
 ## Install
 
+This package is [ESM only](https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c):
+Node 12+ is needed to use it and it must be `import`ed instead of `require`d.
+
 [npm][]:
 
 ```sh
 npm install mdast-util-definitions
 ```
 
-[npm][] with [TypeScript][] support:
-
-```sh
-npm install mdast-util-definitions @types/mdast
-```
-
 ## Use
 
 ```js
-var remark = require('remark')
-var definitions = require('mdast-util-definitions')
+import remark from 'remark'
+import {definitions} from 'mdast-util-definitions'
 
-var ast = remark().parse('[example]: https://example.com "Example"')
+var tree = remark().parse('[example]: https://example.com "Example"')
 
-var definition = definitions(ast)
+var definition = definitions(tree)
 
 definition('example')
-// => {type: 'definition', 'title': 'Example', ...}
+// => {type: 'definition', 'title': 'Example', …}
 
 definition('foo')
 // => null
@@ -45,6 +42,9 @@ definition('foo')
 
 ## API
 
+This package exports the following identifiers: `definitions`.
+There is no default export.
+
 ### `definitions(tree)`
 
 Create a cache of all [definition][]s in [`tree`][node].
@@ -126,8 +126,6 @@ abide by its terms.
 
 [npm]: https://docs.npmjs.com/cli/install
 
-[typescript]: https://www.typescriptlang.org/
-
 [contributing]: https://github.com/syntax-tree/.github/blob/HEAD/contributing.md
 
 [support]: https://github.com/syntax-tree/.github/blob/HEAD/support.md
diff --git a/test.js b/test.js
index 7809cc0..82d574a 100644
--- a/test.js
+++ b/test.js
@@ -1,8 +1,6 @@
-'use strict'
-
-var test = require('tape')
-var remark = require('remark')
-var definitions = require('.')
+import test from 'tape'
+import remark from 'remark'
+import {definitions} from './index.js'
 
 test('mdast-util-definitions', function (t) {
   var tree