vue仓库状态丢失的问题

//在页面加载时读取sessionStorage里的状态信息
if (sessionStorage.getItem("store")) {
 this.$store.replaceState(Object.assign({},this.$store.state,
      JSON.parse(sessionStorage.getItem("store"))));
       sessionStorage.removeItem("store");}

//在页面刷新时将vuex里的信息保存到sessionStorage里
window.addEventListener("beforeunload", () => {
    sessionStorage.setItem("store", JSON.stringify(this.$store.state));
});