data () {
  return {
    user: {
      email: '',
      password: ''
    }
  }
}

watch: {
  user (newValues, oldValues) {
    window.sessionStorage.setItem('userData', JSON.stringify(newValues))
  }
}

created () { // created hook
  const userData = window.sessionStorage.getItem('userData')

  if (userData) {
    try {
      userData = JSON.parse(userData)

      // now initiate the model
      this.user.email = userData.email
      this.user.password = userData.password
    } catch (err) {
      // userData was not a valid json string
    }
  }
}