Error in created hook: "TypeError: this.getCookie is not a function"

new Vue({
  data:{},
  el: '#app',
  router,
  components: { App },
  template: '<App/>',
  methods:{
    checkLogin:function() {
      if(!this.getCookie('session')){
        this.$router.push('/login')
      }else{
        this.$router.push('/userinfo')
      }
    },
  },
  created:function () {
    this.checkLogin();
  },
})
为什么会报这个错误？