vuex报错'state' is already declared in the upper scope

Question

import Vuex from 'vuex';
import Vue from 'vue';

Vue.use(Vuex);
const state = {
  userName: '',
};
const mutations = {
  FETCH_USERNAME_SUCCESS(state) {
    state.userName = 'Jack';
  },
};
export default new Vuex.Store({
  state,
  mutations,
  strict: process.env.NODE_ENV !== 'production',
});

代码如上，运行性报错'state' is already declared in the upper scope
我必须要改一个写法,才能获取fetch回来的值：

import Vuex from 'vuex';
import Vue from 'vue';

Vue.use(Vuex);
const state = {
  userName: '',
};
const mutations = {
  FETCH_USERNAME_SUCCESS(a, res) {
    state.userName = res.userName ;
  },
};
export default new Vuex.Store({
  state,
  mutations,
  strict: process.env.NODE_ENV !== 'production',
});