fix(vuex): correct return type of useStore

Author: fanwei

README.md

@@ -33,7 +33,7 @@ router.push('/login');
 |Declaration|Description|
 |-|-|
 |`createStore`<S>(options: StoreOptions<S>): Store<S>||
-|`useStore`<S = any>(): Store<S> \| undefined||
+|`useStore`<S = any>(): Store<S>|
 
 ### vue2-helpers/vue-router
 |Declaration|Description|

src/vuex.ts

@@ -10,11 +10,12 @@ export function createStore<S>(options: StoreOptions<S>) {
 }
 
 
-export function useStore<S = any>() {
+export function useStore<S = any>(): Store<S> {
     const inst = getCurrentInstance()
     if (inst) {
-        return inst.proxy.$store as Store<S>
+        return inst.proxy.$store
     } else {
         warn(OUT_OF_SCOPE)
     }
+    return undefined as any
 }