错误写法

router.beforeEach((to, from, next) => {
  var token = sessionStorage.getItem('token')
  if (!token) {
      next({ path: '/login' })
  } else {
    next()
  }
})

正确写法

router.beforeEach((to, from, next) => {
  var token = sessionStorage.getItem('token')
  if (!token) {
    if (to.path === '/login' || to.path === '/') {
      next()
    } else {
      next({ path: '/login' })
    }
  } else {
    next()
  }
})

next带参会重新回到beforeEach，所以还需要进行一步判断