diff --git a/.gitignore b/.gitignore
index b957bb5..a136532 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,3 +6,4 @@ node_modules/
 /lib/
 /index.js
 yarn.lock
+!dev/index.d.ts
diff --git a/dev/index.d.ts b/dev/index.d.ts
new file mode 100644
index 0000000..a578824
--- /dev/null
+++ b/dev/index.d.ts
@@ -0,0 +1,12 @@
+export {gfmStrikethroughHtml} from './lib/html.js'
+export {gfmStrikethrough} from './lib/syntax.js'
+
+declare module 'micromark-util-types' {
+  // eslint-disable-next-line @typescript-eslint/consistent-type-definitions
+  interface TokenTypeMap {
+    strikethroughSequence: 'strikethroughSequence'
+    strikethroughSequenceTemporary: 'strikethroughSequenceTemporary'
+    strikethrough: 'strikethrough'
+    strikethroughText: 'strikethroughText'
+  }
+}
diff --git a/dev/lib/syntax.js b/dev/lib/syntax.js
index c3c6332..082adab 100644
--- a/dev/lib/syntax.js
+++ b/dev/lib/syntax.js
@@ -1,7 +1,9 @@
 /**
+ * @typedef {import('micromark-util-types').Event} Event
  * @typedef {import('micromark-util-types').Extension} Extension
  * @typedef {import('micromark-util-types').Resolver} Resolver
  * @typedef {import('micromark-util-types').State} State
+ * @typedef {import('micromark-util-types').Token} Token
  * @typedef {import('micromark-util-types').TokenizeContext} TokenizeContext
  * @typedef {import('micromark-util-types').Tokenizer} Tokenizer
  *
@@ -81,12 +83,14 @@ export function gfmStrikethrough(options) {
             events[index][1].type = 'strikethroughSequence'
             events[open][1].type = 'strikethroughSequence'
 
+            /** @type {Token} */
             const strikethrough = {
               type: 'strikethrough',
               start: Object.assign({}, events[open][1].start),
               end: Object.assign({}, events[index][1].end)
             }
 
+            /** @type {Token} */
             const text = {
               type: 'strikethroughText',
               start: Object.assign({}, events[open][1].end),
@@ -94,6 +98,7 @@ export function gfmStrikethrough(options) {
             }
 
             // Opening.
+            /** @type {Array<Event>} */
             const nextEvents = [
               ['enter', strikethrough, context],
               ['enter', events[open][1], context],
@@ -109,7 +114,6 @@ export function gfmStrikethrough(options) {
                 nextEvents,
                 nextEvents.length,
                 0,
-                // @ts-expect-error: to do: update `mdast-util-types` to allow explicit `undefined`s.
                 resolveAll(insideSpan, events.slice(open + 1, index), context)
               )
             }
diff --git a/tsconfig.json b/tsconfig.json
index 397ca4a..bbc8fb2 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -1,5 +1,5 @@
 {
-  "include": ["**/*.js"],
+  "include": ["**/*.js", "dev/index.d.ts"],
   "exclude": ["coverage/", "lib/", "node_modules/", "index.js"],
   "compilerOptions": {
     "customConditions": ["development"],