setDefaultMutation returns the right type

src/Updux.ts

@@ -210,7 +210,7 @@ export default class Updux<D extends DuxConfig> {
     setDefaultMutation(
         mutation: Mutation<any, DuxState<D>>,
         terminal?: boolean,
-    );
+    ): Updux<D>;
     setDefaultMutation(mutation, terminal = false) {
         this.#defaultMutation = { terminal, mutation };
         return this;

src/mutations.test.ts

@@ -98,3 +98,15 @@ test('actionType as string', () => {
         dux.addMutation('unknown', () => (x) => x);
     }).toThrow();
 });
+
+test('setDefaultMutation return value', () => {
+    const dux = new Updux({
+        initialState: 13,
+    });
+
+    let withDM = dux.setDefaultMutation(() => (state) => state);
+
+    expect(withDM).toEqual(dux);
+
+    expectTypeOf(withDM.initialState).toBeNumber();
+});