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":{}
  }
}