diff --git a/lib/traverse.js b/lib/traverse.js
index b6b7d60..74904c4 100644
--- a/lib/traverse.js
+++ b/lib/traverse.js
@@ -1,9 +1,14 @@
+/**
+ * @typedef { import('./types.js').EnterFn } EnterFn
+ * @typedef { import('./types.js').LeaveFn } LeaveFn
+ */
+
 /**
  * Traverse a moddle tree, depth first from top to bottom
  * and call the passed visitor fn.
  *
  * @param {ModdleElement} element
- * @param {{ enter?: Function; leave?: Function }} options
+ * @param {{ enter?: EnterFn; leave?: LeaveFn }} options
  */
 module.exports = function traverse(element, options) {
 
diff --git a/lib/types.d.ts b/lib/types.d.ts
new file mode 100644
index 0000000..b39fbf5
--- /dev/null
+++ b/lib/types.d.ts
@@ -0,0 +1,5 @@
+export type ModdleElement = any;
+
+export type EnterFn = (element: ModdleElement) => boolean | null;
+
+export type LeaveFn = (element: ModdleElement) => void;
\ No newline at end of file