diff --git a/packages/core-data/src/types.ts b/packages/core-data/src/types.ts
new file mode 100644
index 0000000000000..b1664d8ab8dbf
--- /dev/null
+++ b/packages/core-data/src/types.ts
@@ -0,0 +1,3 @@
+export interface AnyFunction {
+	( ...args: any[] ): any;
+}
diff --git a/packages/core-data/src/utils/on-sub-key.js b/packages/core-data/src/utils/on-sub-key.js
index 24adf06b773eb..c867187f822c3 100644
--- a/packages/core-data/src/utils/on-sub-key.js
+++ b/packages/core-data/src/utils/on-sub-key.js
@@ -1,10 +1,12 @@
+/** @typedef {import('../types').AnyFunction} AnyFunction */
+
 /**
  * Higher-order reducer creator which creates a combined reducer object, keyed
  * by a property on the action object.
  *
  * @param {string} actionProperty Action property by which to key object.
  *
- * @return {Function} Higher-order reducer.
+ * @return {AnyFunction} Higher-order reducer.
  */
 export const onSubKey = ( actionProperty ) => ( reducer ) => (
 	state = {},