Nuxt/lib/core/meta.js

import Vue from 'vue'
import VueMeta from 'vue-meta'
import VueServerRenderer from 'vue-server-renderer'
import LRU from 'lru-cache'

export default class MetaRenderer {
  constructor (nuxt, renderer) {
    this.nuxt = nuxt
    this.renderer = renderer
    this.options = nuxt.options
    this.vueRenderer = VueServerRenderer.createRenderer()
    this.cache = LRU({})

    // Add VueMeta to Vue (this is only for SPA mode)
    // See lib/app/index.js
    Vue.use(VueMeta, {
      keyName: 'head',
      attribute: 'data-n-head',
      ssrAttribute: 'data-n-head-ssr',
      tagIDKeyName: 'hid'
    })
  }

  getMeta (url) {
    return new Promise((resolve, reject) => {
      const vm = new Vue({
        render: (h) => h(), // Render empty html tag
        head: this.options.head || {}
      })
      this.vueRenderer.renderToString(vm, (err) => {
        if (err) return reject(err)
        resolve(vm.$meta().inject())
      })
    })
  }

  async render ({ url = '/' }) {
    let meta = this.cache.get(url)

    if (meta) {
      return meta
    }

    meta = {
      HTML_ATTRS: '',
      BODY_ATTRS: '',
      HEAD: ''
    }
    // Get vue-meta context
    const m = await this.getMeta(url)
    // HTML_ATTRS
    meta.HTML_ATTRS = m.htmlAttrs.text()
    // BODY_ATTRS
    meta.BODY_ATTRS = m.bodyAttrs.text()
    // HEAD tags
    meta.HEAD = m.meta.text() + m.title.text() + m.link.text() + m.style.text() + m.script.text() + m.noscript.text()
    // Resources Hints
    meta.resourceHints = ''
    // Resource Hints
    const clientManifest = this.renderer.resources.clientManifest
    if (this.options.render.resourceHints && clientManifest) {
      const publicPath = clientManifest.publicPath || '/_nuxt/'
      // Pre-Load initial resources
      if (Array.isArray(clientManifest.initial)) {
        meta.resourceHints += clientManifest.initial.map(r => `<link rel="preload" href="${publicPath}${r}" as="script" />`).join('')
      }
      // Pre-Fetch async resources
      if (Array.isArray(clientManifest.async)) {
        meta.resourceHints += clientManifest.async.map(r => `<link rel="prefetch" href="${publicPath}${r}" />`).join('')
      }
      // Add them to HEAD
      if (meta.resourceHints) {
        meta.HEAD += meta.resourceHints
      }
    }

    // Set meta tags inside cache
    this.cache.set(url, meta)

    return meta
  }
}