⚡ Bolt: Optimize large array serialization in CodeBlock.tsx

.jules/bolt.md

@@ -5,3 +5,7 @@
 ## 2026-10-15 - [Performance] Prevent unnecessary UI array recalculations
 **Learning:** In the chat component (`useThreadList`), derived state that requires filtering and sorting large arrays (like generating nested agent lists) was running synchronously on every render. Given that chat polling triggers frequent re-renders, this caused unnecessary CPU strain on lists that only rarely change.
 **Action:** Use `useMemo` for derived states like `rows` and `current` in custom hooks when they involve `.filter()`, `.map()`, or `.sort()` and depend on data that updates less frequently than the component renders.
+
+## 2025-02-23 - Optimizing Large Array Serialization in Preact Components
+**Learning:** In Preact components, deriving state on hot paths (like `CodeBlock`) using nested `.map().join()` operations creates unnecessary intermediate array allocations, causing memory pressure and CPU overhead from garbage collection when dealing with large datasets (e.g. string concatenation of large code blocks).
+**Action:** Wrap expensive derived states in `useMemo` and replace `.map().join()` with manual `for` loops. This prevents intermediate allocations and significantly reduces overhead during render loops.

apps/web/src/components/chat/CodeBlock.tsx

@@ -1,4 +1,4 @@
-import { useEffect, useRef, useState } from 'preact/hooks';
+import { useEffect, useRef, useState, useMemo } from 'preact/hooks';
 import { Check, Copy } from 'lucide-preact';
 import { copyText } from '../../lib/clipboard';
 import { highlightCode, normalizeLang, type CodeToken } from '../../lib/highlight';
@@ -26,7 +26,24 @@ export function CodeBlock({ code, lang }: { code: string; lang?: string }) {
       window.clearTimeout(t);
     };
   }, [code, lang]);
-  const shown = tokens && tokens.map((l) => l.map((t) => t.content).join('')).join('\n') === code ? tokens : null;
+
+  // Performance optimization: Avoid intermediate array allocations for large code blocks.
+  // We use useMemo and manual `for` loops instead of nested `.map().join()` chains
+  // to reduce memory allocations and CPU overhead during hot path renders.
+  const shown = useMemo(() => {
+    if (!tokens) return null;
+    let text = "";
+    for (let i = 0; i < tokens.length; i++) {
+      const line = tokens[i];
+      for (let j = 0; j < line.length; j++) {
+        text += line[j].content;
+      }
+      if (i < tokens.length - 1) {
+        text += '\n';
+      }
+    }
+    return text === code ? tokens : null;
+  }, [tokens, code]);
 
   async function onCopy() {
     const ok = await copyText(code);