window or document undefined?

This is due to the server-side rendering. If you need to specify that you want to import a resource only on the client-side, you need to use the process.browser variable.

For example, in your .vue file:

if (process.browser) {
  require('external_library')
}

If you are using this library within multiple files, we recommend that you add it into your
vendor bundle

nuxt.config.js:

  build: {
    vendor: ['external_library']
  }

在服务端渲染，找不到浏览器的window对象，alert也属于浏览器的api，
需要添加条件来判断是否在浏览器端
if (typeof window !== 'undefined'){
}

参考：https://nuxtjs.org/faq/window-document-undefined