diff --git a/.gitignore b/.gitignore
index fdefc8c..735f4af 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,6 +1,5 @@
 .DS_Store
 *.log
-.nyc_output/
 coverage/
 node_modules/
 yarn.lock
diff --git a/.prettierignore b/.prettierignore
index e7939c4..cebe81f 100644
--- a/.prettierignore
+++ b/.prettierignore
@@ -1,3 +1,2 @@
 coverage/
-*.json
 *.md
diff --git a/index.js b/index.js
index 23b3f50..7c6ec16 100644
--- a/index.js
+++ b/index.js
@@ -1,19 +1,11 @@
-// LICENSE : MIT
-'use strict'
-
-var assign = require('object-assign')
-
-module.exports = map
-
-function map(tree, iteratee) {
+export function map(tree, iteratee) {
   return preorder(tree, null, null)
 
   function preorder(node, index, parent) {
-    var children = node.children
-    var newNode = assign({}, iteratee(node, index, parent))
+    var newNode = Object.assign({}, iteratee(node, index, parent))
 
-    if (children) {
-      newNode.children = children.map(function (child, index) {
+    if (node.children) {
+      newNode.children = node.children.map(function (child, index) {
         return preorder(child, index, node)
       })
     }
diff --git a/package.json b/package.json
index 9fb5782..a66223f 100644
--- a/package.json
+++ b/package.json
@@ -26,18 +26,18 @@
     "Titus Wormer <tituswormer@gmail.com> (https://wooorm.com)",
     "Christian Murphy <christian.murphy.42@gmail.com>"
   ],
+  "sideEffects": false,
+  "type": "module",
+  "main": "index.js",
+  "types": "types/index.d.ts",
   "files": [
     "index.js",
     "types/index.d.ts"
   ],
-  "types": "types/index.d.ts",
-  "dependencies": {
-    "@types/mdast": "^3.0.0",
-    "object-assign": "^4.0.0"
-  },
+  "dependencies": {},
   "devDependencies": {
+    "c8": "^7.0.0",
     "dtslint": "^4.0.0",
-    "nyc": "^15.0.0",
     "prettier": "^2.0.0",
     "remark-cli": "^9.0.0",
     "remark-preset-wooorm": "^8.0.0",
@@ -49,17 +49,11 @@
   },
   "scripts": {
     "format": "remark . -qfo && prettier . -w --loglevel warn && xo --fix",
-    "test-api": "node test",
-    "test-coverage": "nyc --reporter lcov tape test.js",
+    "test-api": "node test.js",
+    "test-coverage": "c8 --check-coverage --branches 100 --functions 100 --lines 100 --statements 100 --reporter lcov node test.js",
     "test-types": "dtslint types",
     "test": "npm run format && npm run test-coverage && npm run test-types"
   },
-  "nyc": {
-    "check-coverage": true,
-    "lines": 100,
-    "functions": 100,
-    "branches": 100
-  },
   "prettier": {
     "tabWidth": 2,
     "useTabs": false,
@@ -70,13 +64,10 @@
   },
   "xo": {
     "prettier": true,
-    "esnext": false,
     "rules": {
-      "unicorn/no-fn-reference-in-iterator": "off"
-    },
-    "ignore": [
-      "types/"
-    ]
+      "no-var": "off",
+      "prefer-arrow-callback": "off"
+    }
   },
   "remarkConfig": {
     "plugins": [
diff --git a/readme.md b/readme.md
index 44f4f5b..1af0b69 100644
--- a/readme.md
+++ b/readme.md
@@ -13,6 +13,9 @@ with the given function.
 
 ## 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
@@ -22,8 +25,8 @@ npm install unist-util-map
 ## Use
 
 ```js
-var u = require('unist-builder')
-var map = require('unist-util-map')
+import u from 'unist-builder'
+import {map} from 'unist-util-map'
 
 var tree = u('tree', [
   u('leaf', 'leaf 1'),
@@ -59,6 +62,9 @@ Yields:
 
 ## API
 
+This package exports the following identifiers: `map`.
+There is no default export.
+
 ### `map(tree, mapFn)`
 
 Create a new [tree][] by mapping all [node][]s with the given function.
diff --git a/test.js b/test.js
index eb38807..0ffc59f 100644
--- a/test.js
+++ b/test.js
@@ -1,9 +1,6 @@
-'use strict'
-
-var test = require('tape')
-var assign = require('object-assign')
-var u = require('unist-builder')
-var map = require('.')
+import test from 'tape'
+import u from 'unist-builder'
+import {map} from './index.js'
 
 test('unist-util-map', function (t) {
   t.deepEqual(
@@ -27,7 +24,9 @@ test('unist-util-map', function (t) {
   t.end()
 
   function changeLeaf(node) {
-    return node.type === 'leaf' ? assign({}, node, {value: 'CHANGED'}) : node
+    return node.type === 'leaf'
+      ? Object.assign({}, node, {value: 'CHANGED'})
+      : node
   }
 
   function nullLeaf(node) {