Extract snippets to external JS files with syntax highlighting component

All 48 JavaScript snippets are extracted from inline MDX code blocks into
standalone .js files under snippets/{category}/, making them individually
addressable by LLMs and agents.

- Add snippets/ directory with 48 .js files organized by category
  (CoreWebVitals, Interaction, Loading, Media, Resources)
- Add components/Snippet.jsx: renders raw JS with highlight.js syntax
  highlighting, copy button, max-height scroll, and dark/light mode support
- Add scripts/extract-snippets.js: one-time migration script that extracted
  code blocks from ### Snippet sections and rewrote MDX imports
- Update all 46 MDX pages to import snippets via ?raw and use <Snippet>
- Update next.config.js: webpack asset/source rule for ?raw imports,
  injected into oneOf to take precedence over SWC, with pre-loaders
  excluded to prevent React Fast Refresh injection
- Update styles/globals.css: hljs light/dark themes, override Nextra's
  `pre code { padding: 0 !important }`

Author: nucliweb

components/Snippet.jsx

@@ -0,0 +1,84 @@
+import { useState, useMemo } from "react";
+import hljs from "highlight.js/lib/core";
+import javascript from "highlight.js/lib/languages/javascript";
+
+hljs.registerLanguage("javascript", javascript);
+
+export function Snippet({ code }) {
+  const [copied, setCopied] = useState(false);
+  const [hovered, setHovered] = useState(false);
+
+  const highlighted = useMemo(() => hljs.highlight(code, { language: "javascript" }).value, [code]);
+
+  async function handleCopy() {
+    try {
+      await navigator.clipboard.writeText(code);
+    } catch {
+      const el = document.createElement("textarea");
+      el.value = code;
+      el.style.cssText = "position:fixed;top:0;left:0;opacity:0";
+      document.body.appendChild(el);
+      el.select();
+      document.execCommand("copy");
+      document.body.removeChild(el);
+    }
+    setCopied(true);
+    setTimeout(() => setCopied(false), 2000);
+  }
+
+  return (
+    <div
+      className="nextra-code-block nx-relative nx-mt-6 first:nx-mt-0"
+      onMouseEnter={() => setHovered(true)}
+      onMouseLeave={() => setHovered(false)}
+    >
+      <pre
+        data-language="js"
+        data-theme="default"
+        className="nx-bg-primary-700/5 nx-mb-4 nx-overflow-x-auto nx-rounded-xl nx-subpixel-antialiased dark:nx-bg-primary-300/10 nx-text-[13px] contrast-more:nx-border contrast-more:nx-border-primary-900/20 contrast-more:nx-contrast-150 contrast-more:dark:nx-border-primary-100/40"
+      >
+        <code
+          data-language="js"
+          data-theme="default"
+          className="hljs nx-border-black nx-border-opacity-[0.04] nx-bg-opacity-[0.03] nx-bg-black nx-break-words nx-rounded-md nx-border dark:nx-border-white/10 dark:nx-bg-white/10 nx-py-4 nx-px-4"
+          dangerouslySetInnerHTML={{ __html: highlighted }}
+        />
+      </pre>
+      <div
+        className="nx-transition nx-flex nx-gap-1 nx-absolute nx-m-[11px] nx-right-0 nx-top-0"
+        style={{ opacity: hovered || copied ? 1 : 0 }}
+      >
+        <button
+          onClick={handleCopy}
+          aria-label="Copy code"
+          className="nextra-button nx-transition-all active:nx-opacity-50 nx-bg-primary-700/5 nx-border nx-border-black/5 nx-text-gray-600 hover:nx-text-gray-900 nx-rounded-md nx-p-1.5 dark:nx-bg-gray-100/5 dark:nx-border-white/10 dark:nx-text-gray-400 dark:hover:nx-text-gray-50"
+        >
+          {copied ? (
+            <svg
+              width="16"
+              height="16"
+              viewBox="0 0 24 24"
+              fill="none"
+              stroke="currentColor"
+              strokeWidth="2"
+            >
+              <polyline points="20 6 9 17 4 12" />
+            </svg>
+          ) : (
+            <svg
+              width="16"
+              height="16"
+              viewBox="0 0 24 24"
+              fill="none"
+              stroke="currentColor"
+              strokeWidth="2"
+            >
+              <rect x="9" y="9" width="13" height="13" rx="2" ry="2" />
+              <path d="M5 15H4a2 2 0 0 1-2-2V4a2 2 0 0 1 2-2h9a2 2 0 0 1 2 2v1" />
+            </svg>
+          )}
+        </button>
+      </div>
+    </div>
+  );
+}

next.config.js

@@ -1,9 +1,36 @@
+const path = require('path')
 const withNextra = require('nextra')({
   theme: 'nextra-theme-docs',
   themeConfig: './theme.config.jsx'
 })
 
 module.exports = withNextra({
+  webpack(config) {
+    const snippetsDir = path.join(__dirname, 'snippets')
+
+    // Exclude snippets from pre-loaders (React Fast Refresh, etc.)
+    for (const rule of config.module.rules) {
+      if (rule.enforce === 'pre') {
+        rule.exclude = [].concat(rule.exclude || [], snippetsDir)
+      }
+    }
+
+    // Inject into oneOf so our rule takes precedence over the SWC loader
+    const rawRule = {
+      test: /\.js$/,
+      include: snippetsDir,
+      resourceQuery: /raw/,
+      type: 'asset/source',
+    }
+    const oneOfRule = config.module.rules.find(r => Array.isArray(r.oneOf))
+    if (oneOfRule) {
+      oneOfRule.oneOf.unshift(rawRule)
+    } else {
+      config.module.rules.unshift(rawRule)
+    }
+
+    return config
+  },
   async redirects() {
     return [
       {

pages/CoreWebVitals/CLS.mdx

@@ -1,3 +1,6 @@
+import snippet from '../../snippets/CoreWebVitals/CLS.js?raw'
+import { Snippet } from '../../components/Snippet'
+
 # Cumulative Layout Shift (CLS)
 
 ### Overview
@@ -20,64 +23,7 @@ Unexpected layout shifts are frustrating and can cause users to click the wrong
 
 ### Snippet
 
-```js copy
-// CLS Quick Check
-// https://webperf-snippets.nucliweb.net
-
-(() => {
-  let cls = 0;
-
-  const valueToRating = (score) =>
-    score <= 0.1 ? "good" : score <= 0.25 ? "needs-improvement" : "poor";
-
-  const RATING = {
-    good: { icon: "🟢", color: "#0CCE6A" },
-    "needs-improvement": { icon: "🟡", color: "#FFA400" },
-    poor: { icon: "🔴", color: "#FF4E42" },
-  };
-
-  const logCLS = () => {
-    const rating = valueToRating(cls);
-    const { icon, color } = RATING[rating];
-    console.log(
-      `%cCLS: ${icon} ${cls.toFixed(4)} (${rating})`,
-      `color: ${color}; font-weight: bold; font-size: 14px;`
-    );
-  };
-
-  const observer = new PerformanceObserver((list) => {
-    for (const entry of list.getEntries()) {
-      if (!entry.hadRecentInput) {
-        cls += entry.value;
-      }
-    }
-    logCLS();
-  });
-
-  observer.observe({ type: "layout-shift", buffered: true });
-
-  // Update on visibility change (final CLS)
-  document.addEventListener("visibilitychange", () => {
-    if (document.visibilityState === "hidden") {
-      observer.takeRecords();
-      console.log("%c📊 Final CLS (on page hide):", "font-weight: bold;");
-      logCLS();
-    }
-  });
-
-  // Expose function for manual check
-  window.getCLS = () => {
-    logCLS();
-    return cls;
-  };
-
-  console.log(
-    "   Call %cgetCLS()%c anytime to check current value.",
-    "font-family: monospace; background: #f3f4f6; padding: 2px 4px;",
-    ""
-  );
-})();
-```
+<Snippet code={snippet} />
 
 ### Understanding CLS