vercel · timneutkens · May 10, 2021 · May 3, 2021 · May 3, 2021 · May 3, 2021
diff --git a/errors/google-font-display.md b/errors/google-font-display.md
@@ -0,0 +1,36 @@
+# Google Font Display
+
+### Why This Error Occurred
+
+For a Google Font, the `display` descriptor was either not assigned or set to `auto`, `fallback`, or `block`.
+
+### Possible Ways to Fix It
+
+For most cases, the best font display strategy for custom fonts is `optional`.
+
+```jsx
+import Head from 'next/head'
+
+export default function IndexPage() {
+  return (
+    <div>
+      <Head>
+        <link
+          href="https://fonts.googleapis.com/css2?family=Krona+One&display=optional"
+          rel="stylesheet"
+        />
+      </Head>
+    </div>
+  )
+}
+```
+
+Specifying `display=optional` minimizes the risk of invisible text or layout shift. If swapping to the custom font after it has loaded is important to you, then use `display=swap` instead.
+
+### When Not To Use It
+
+If you want to specifically display a font using a `block` or `fallback` strategy, then you can disable this rule.
+
+### Useful Links
+
+- [Font-display](https://font-display.glitch.me/)
diff --git a/errors/google-font-preconnect.md b/errors/google-font-preconnect.md
@@ -0,0 +1,17 @@
+# Google Font Preconnect
+
+### Why This Error Occurred
+
+A preconnect resource hint was not used with a request to the Google Fonts domain. Adding `preconnect` is recommended to initiate an early connection to the origin.
+
+### Possible Ways to Fix It
+
+Add `rel="preconnect"` to the Google Font domain `<link>` tag:
+
+```jsx
+<link rel="preconnect" href="https://fonts.gstatic.com" />
+```
+
+### Useful Links
+
+- [Preconnect to required origins](https://web.dev/uses-rel-preconnect/)
diff --git a/errors/manifest.json b/errors/manifest.json
@@ -106,6 +106,14 @@
           "title": "generatebuildid-not-a-string",
           "path": "/errors/generatebuildid-not-a-string.md"
         },
+        {
+          "title": "google-font-display",
+          "path": "/errors/google-font-display.md"
+        },
+        {
+          "title": "google-font-preconnect",
+          "path": "/errors/google-font-preconnect.md"
+        },
         {
           "title": "get-initial-props-as-an-instance-method",
           "path": "/errors/get-initial-props-as-an-instance-method.md"

diff --git a/packages/eslint-plugin-next/lib/index.js b/packages/eslint-plugin-next/lib/index.js
@@ -5,6 +5,8 @@ module.exports = {
     'no-html-link-for-pages': require('./rules/no-html-link-for-pages'),
     'no-unwanted-polyfillio': require('./rules/no-unwanted-polyfillio'),
     'no-title-in-document-head': require('./rules/no-title-in-document-head'),
+    'google-font-display': require('./rules/google-font-display'),
+    'google-font-preconnect': require('./rules/google-font-preconnect'),
   },
   configs: {
     recommended: {
@@ -15,6 +17,8 @@ module.exports = {
         '@next/next/no-html-link-for-pages': 1,
         '@next/next/no-unwanted-polyfillio': 1,
         '@next/next/no-title-in-document-head': 1,
+        '@next/next/google-font-display': 1,
+        '@next/next/google-font-preconnect': 1,
       },
     },
   },

diff --git a/packages/eslint-plugin-next/lib/rules/google-font-display.js b/packages/eslint-plugin-next/lib/rules/google-font-display.js
@@ -0,0 +1,56 @@
+const NodeAttributes = require('../utils/nodeAttributes.js')
+
+module.exports = {
+  meta: {
+    docs: {
+      description:
+        'Ensure correct font-display property is assigned for Google Fonts',
+      recommended: true,
+    },
+  },
+  create: function (context) {
+    return {
+      JSXOpeningElement(node) {
+        let message
+
+        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 isGoogleFont = hrefValue.includes(
+          'https://fonts.googleapis.com/css'
+        )
+
+        if (isGoogleFont) {
+          const params = new URLSearchParams(hrefValue.split('?')[1])
+          const displayValue = params.get('display')
+
+          if (!params.has('display')) {
+            message = 'Display parameter is missing.'
+          } else if (
+            displayValue === 'block' ||
+            displayValue === 'fallback' ||
+            displayValue === 'auto'
+          ) {
+            message = `${
+              displayValue[0].toUpperCase() + displayValue.slice(1)
+            } behavior is not recommended.`
+          }
+        }
+
+        if (message) {
+          context.report({
+            node,
+            message: `${message} See https://nextjs.org/docs/messages/google-font-display.`,
+          })
+        }
+      },
+    }
+  },
+}
diff --git a/packages/eslint-plugin-next/lib/rules/google-font-preconnect.js b/packages/eslint-plugin-next/lib/rules/google-font-preconnect.js
@@ -0,0 +1,40 @@
+const NodeAttributes = require('../utils/nodeAttributes.js')
+
+module.exports = {
+  meta: {
+    docs: {
+      description: 'Ensure preconnect is used with Google Fonts',
+      recommended: true,
+    },
+  },
+  create: function (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 (
+          hrefValue.includes('https://fonts.gstatic.com') &&
+          preconnectMissing
+        ) {
+          context.report({
+            node,
+            message: `Preconnect is missing. See https://nextjs.org/docs/messages/google-font-preconnect.`,
+          })
+        }
+      },
+    }
+  },
+}
diff --git a/packages/eslint-plugin-next/lib/utils/nodeAttributes.js b/packages/eslint-plugin-next/lib/utils/nodeAttributes.js
@@ -0,0 +1,52 @@
+// Return attributes and values of a node in a convenient way:
+/* example: 
+    <ExampleElement attr1="15" attr2>
+    {   attr1: {
+            hasValue: true,
+            value: 15
+        },
+        attr2: {
+            hasValue: false
+        }
+Inclusion of hasValue is in case an eslint rule cares about boolean values
+explicitely assigned to attribute vs the attribute being used as a flag
+*/
+class NodeAttributes {
+  constructor(ASTnode) {
+    this.attributes = {}
+    ASTnode.attributes.forEach((attribute) => {
+      if (!attribute.type || attribute.type !== 'JSXAttribute') {
+        return
+      }
+      this.attributes[attribute.name.name] = {
+        hasValue: !!attribute.value,
+      }
+      if (attribute.value) {
+        if (attribute.value.value) {
+          this.attributes[attribute.name.name].value = attribute.value.value
+        } else if (attribute.value.expression) {
+          this.attributes[attribute.name.name].value =
+            attribute.value.expression.value
+        }
+      }
+    })
+  }
+  hasAny() {
+    return !!Object.keys(this.attributes).length
+  }
+  has(attrName) {
+    return !!this.attributes[attrName]
+  }
+  hasValue(attrName) {
+    return !!this.attributes[attrName].hasValue
+  }
+  value(attrName) {
+    if (!this.attributes[attrName]) {
+      return true
+    }
+
+    return this.attributes[attrName].value
+  }
+}
+
+module.exports = NodeAttributes
diff --git a/test/eslint-plugin-next/google-font-display.unit.test.js b/test/eslint-plugin-next/google-font-display.unit.test.js
@@ -0,0 +1,143 @@
+const rule = require('@next/eslint-plugin-next/lib/rules/google-font-display')
+const RuleTester = require('eslint').RuleTester
+
+RuleTester.setDefaultConfig({
+  parserOptions: {
+    ecmaVersion: 2018,
+    sourceType: 'module',
+    ecmaFeatures: {
+      modules: true,
+      jsx: true,
+    },
+  },
+})
+
+var ruleTester = new RuleTester()
+ruleTester.run('google-font-display', rule, {
+  valid: [
+    `import Head from "next/head";
+
+     export default Test = () => {
+      return (
+        <Head>
+          <link
+            href="https://fonts.googleapis.com/css2?family=Krona+One&display=optional"
+            rel="stylesheet"
+          />
+        </Head>
+      );
+     };
+    `,
+
+    `import Document, { Html, Head } from "next/document";
+
+     class MyDocument extends Document {
+      render() {
+        return (
+          <Html>
+            <Head>
+              <link
+                href="https://fonts.googleapis.com/css?family=Krona+One&display=swap"
+                rel="stylesheet"
+              />
+            </Head>
+          </Html>
+        );
+      }
+     }
+
+     export default MyDocument;
+    `,
+  ],
+
+  invalid: [
+    {
+      code: `import Head from "next/head";
+
+      export default Test = () => {
+       return (
+         <Head>
+           <link
+             href="https://fonts.googleapis.com/css2?family=Krona+One"
+             rel="stylesheet"
+           />
+         </Head>
+       );
+      };
+     `,
+      errors: [
+        {
+          message:
+            'Display parameter is missing. See https://nextjs.org/docs/messages/google-font-display.',
+          type: 'JSXOpeningElement',
+        },
+      ],
+    },
+    {
+      code: `import Head from "next/head";
+
+      export default Test = () => {
+       return (
+         <Head>
+           <link
+             href="https://fonts.googleapis.com/css2?family=Krona+One&display=block"
+             rel="stylesheet"
+           />
+         </Head>
+       );
+      };
+     `,
+      errors: [
+        {
+          message:
+            'Block behavior is not recommended. See https://nextjs.org/docs/messages/google-font-display.',
+          type: 'JSXOpeningElement',
+        },
+      ],
+    },
+    {
+      code: `import Head from "next/head";
+
+      export default Test = () => {
+       return (
+         <Head>
+           <link
+             href="https://fonts.googleapis.com/css2?family=Krona+One&display=auto"
+             rel="stylesheet"
+           />
+         </Head>
+       );
+      };
+     `,
+      errors: [
+        {
+          message:
+            'Auto behavior is not recommended. See https://nextjs.org/docs/messages/google-font-display.',
+          type: 'JSXOpeningElement',
+        },
+      ],
+    },
+    {
+      code: `import Head from "next/head";
+
+      export default Test = () => {
+       return (
+         <Head>
+           <link
+             href="https://fonts.googleapis.com/css2?display=fallback&family=Krona+One"
+             rel="stylesheet"
+           />
+         </Head>
+       );
+      };
+     `,
+      errors: [
+        {
+          message:
+            'Fallback behavior is not recommended. See https://nextjs.org/docs/messages/google-font-display.',
+          type: 'JSXOpeningElement',
+        },
+      ],
+    },
+  ],
+})