next.js/packages/eslint-plugin-next/src/rules/google-font-preconnect.ts

import { defineRule } from '../utils/define-rule'
import NodeAttributes from '../utils/node-attributes'

const url = 'https://nextjs.org/docs/messages/google-font-preconnect'

export = defineRule({
  meta: {
    docs: {
      description: 'Ensure `preconnect` is used with Google Fonts.',
      recommended: true,
      url,
    },
    type: 'problem',
    schema: [],
  },
  create(context) {
    return {
      JSXOpeningElement(node) {
        if (node.name.name !== 'link') {
          return
        }

        const attributes = new NodeAttributes(node)
        if (!attributes.has('href') || !attributes.hasValue('href')) {
          return
        }

        const hrefValue = attributes.value('href')
        const preconnectMissing =
          !attributes.has('rel') ||
          !attributes.hasValue('rel') ||
          attributes.value('rel') !== 'preconnect'

        if (
          typeof hrefValue === 'string' &&
          hrefValue.startsWith('https://fonts.gstatic.com') &&
          preconnectMissing
        ) {
          context.report({
            node,
            message: `\`rel="preconnect"\` is missing from Google Font. See: ${url}`,
          })
        }
      },
    }
  },
})