fix(types): allow a function type for root state option (#1132)

Author: ktsn

types/index.d.ts

@@ -79,7 +79,7 @@ export interface CommitOptions {
 }
 
 export interface StoreOptions<S> {
-  state?: S;
+  state?: S | (() => S);
   getters?: GetterTree<S, S>;
   actions?: ActionTree<S, S>;
   mutations?: MutationTree<S>;

types/test/index.ts

@@ -98,6 +98,15 @@ namespace RootDefaultModule {
   });
 }
 
+namespace InitialStateFunction {
+  const store = new Vuex.Store({
+    state: () => ({
+      value: 1
+    })
+  });
+  const n: number = store.state.value;
+}
+
 namespace NestedModules {
   interface RootState {
     a: {