Implement redux.

redux/actions/user.js

@@ -0,0 +1,16 @@
+import { createSlice } from '@reduxjs/toolkit';
+
+const user = createSlice({
+  name: 'user',
+  initialState: null,
+  reducers: {
+    updateUser(state, action) {
+      state = action.payload;
+      return state;
+    },
+  },
+});
+
+export const { updateUser } = user.actions;
+
+export default user.reducer;