diff --git a/index.js b/index.js
index aef2e29..17b57c3 100644
--- a/index.js
+++ b/index.js
@@ -1,25 +1 @@
-/**
- * @typedef {import('hast').Root} Root
- * @typedef {import('hast').Content} Content
- */
-
-/**
- * @typedef {Root | Content} Node
- */
-
-// To do next major: return `undefined`.
-/**
- * Rank of a heading: H1 -> 1, H2 -> 2, etc.
- *
- * @param {Node} node
- * @returns {number | null}
- */
-export function headingRank(node) {
-  const name =
-    (node && node.type === 'element' && node.tagName.toLowerCase()) || ''
-  const code =
-    name.length === 2 && name.charCodeAt(0) === 104 /* `h` */
-      ? name.charCodeAt(1)
-      : 0
-  return code > 48 /* `0` */ && code < 55 /* `7` */ ? code - 48 /* `0` */ : null
-}
+export {headingRank} from './lib/index.js'
diff --git a/lib/index.js b/lib/index.js
new file mode 100644
index 0000000..aef2e29
--- /dev/null
+++ b/lib/index.js
@@ -0,0 +1,25 @@
+/**
+ * @typedef {import('hast').Root} Root
+ * @typedef {import('hast').Content} Content
+ */
+
+/**
+ * @typedef {Root | Content} Node
+ */
+
+// To do next major: return `undefined`.
+/**
+ * Rank of a heading: H1 -> 1, H2 -> 2, etc.
+ *
+ * @param {Node} node
+ * @returns {number | null}
+ */
+export function headingRank(node) {
+  const name =
+    (node && node.type === 'element' && node.tagName.toLowerCase()) || ''
+  const code =
+    name.length === 2 && name.charCodeAt(0) === 104 /* `h` */
+      ? name.charCodeAt(1)
+      : 0
+  return code > 48 /* `0` */ && code < 55 /* `7` */ ? code - 48 /* `0` */ : null
+}
diff --git a/package.json b/package.json
index 8bb5a9e..a0b25bf 100644
--- a/package.json
+++ b/package.json
@@ -30,6 +30,7 @@
   "main": "index.js",
   "types": "index.d.ts",
   "files": [
+    "lib/",
     "index.d.ts",
     "index.js"
   ],