computed: {  
  value() {
   return this.$store.state.userData.xxx;
  }
 },

methods: { 
   fn() {   
       this.$store.commit('setValue',xxx)    
     }      
  }

 setValue(state,xxx){
  state.userData.xxx= xxx;
 },

const state = {
 
 userData: {
  
  "xxx":{}
 }
}