diff --git a/lib/babel-packages.js.flow b/lib/babel-packages.js.flow
index 11c681b051ee..e500b5fd93e5 100644
--- a/lib/babel-packages.js.flow
+++ b/lib/babel-packages.js.flow
@@ -43,3 +43,9 @@ declare module "@babel/helper-optimise-call-expression" {
     optional: boolean
   ): BabelNodeCallExpression | BabelNodeOptionalCallExpression;
 }
+
+declare module "@babel/helper-get-function-arity" {
+  declare export default function helperGetFunctionArity(
+    node: BabelNodeFunction
+  ): number;
+}
diff --git a/packages/babel-helper-get-function-arity/src/index.js b/packages/babel-helper-get-function-arity/src/index.ts
similarity index 72%
rename from packages/babel-helper-get-function-arity/src/index.js
rename to packages/babel-helper-get-function-arity/src/index.ts
index 6b00da3c3557..e4ce063da1f8 100644
--- a/packages/babel-helper-get-function-arity/src/index.js
+++ b/packages/babel-helper-get-function-arity/src/index.ts
@@ -1,7 +1,7 @@
 import * as t from "@babel/types";
 
-export default function (node): number {
-  const params: Array<Object> = node.params;
+export default function (node: t.Function): number {
+  const params = node.params;
   for (let i = 0; i < params.length; i++) {
     const param = params[i];
     if (t.isAssignmentPattern(param) || t.isRestElement(param)) {