rsnext/packages/eslint-plugin-next/lib/rules/no-page-custom-font.js

const NodeAttributes = require('../utils/nodeAttributes.js')

module.exports = {
  meta: {
    docs: {
      description:
        'Recommend adding custom font in a custom document and not in a specific page',
      recommended: true,
    },
  },
  create: function (context) {
    let documentImport = false
    return {
      ImportDeclaration(node) {
        if (node.source.value === 'next/document') {
          if (node.specifiers.some(({ local }) => local.name === 'Document')) {
            documentImport = true
          }
        }
      },
      JSXOpeningElement(node) {
        const documentClass = context
          .getAncestors()
          .find(
            (ancestorNode) =>
              ancestorNode.type === 'ClassDeclaration' &&
              ancestorNode.superClass &&
              ancestorNode.superClass.name === 'Document'
          )

        if ((documentImport && documentClass) || node.name.name !== 'link') {
          return
        }

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

        const hrefValue = attributes.value('href')
        const isGoogleFont =
          typeof hrefValue === 'string' &&
          hrefValue.startsWith('https://fonts.googleapis.com/css')

        if (isGoogleFont) {
          context.report({
            node,
            message:
              'Custom fonts should be added at the document level. See https://nextjs.org/docs/messages/no-page-custom-font.',
          })
        }
      },
    }
  },
}