fix(bundler): runtime mode opt-in, ASI-safe joinJsChunks, prune dead subs

Per @vai-bot's review on hf#641:

Important #1: dead `src=""` substitution sites
=============================================

Now that `bundleToSingleHtml` inlines the runtime IIFE by default, the empty
`src=""` placeholder is never emitted in the no-env-var path — the 5 downstream
substitution sites that grep for `src=""` were dead.

Two of them (studio dev server + studio vite preview) genuinely WANT the
placeholder so they can hot-reload a local /api/runtime.js endpoint without
re-inlining ~150 KB on every composition edit. Three of them (CLI validate,
snapshot, layout) were just doing the same inlining the bundler already does.

Resolution:
- Add a `runtime: "inline" | "placeholder"` option to `BundleOptions`. Default
  is "inline" (matches the self-contained-bundle promise the function name
  makes). The two studio surfaces explicitly pass `{ runtime: "placeholder" }`
  to opt in.
- studioServer.ts + studio/vite.config.ts: pass the option, keep their
  existing string-replace logic unchanged.
- validate.ts + snapshot.ts + layout.ts: delete the now-redundant runtime
  substitution code (regex never matches the new inlined-runtime shape).

Important #2: joinJsChunks ASI hazard
======================================

The new helper appended `;` to chunks not already ending in `;` and joined
on `\n`. If a chunk ended with a `// line comment`, the appended semicolon
was eaten by the comment, leaving the next chunk's first statement attached
to the previous chunk's last expression — exactly the ASI hazard the helper
exists to prevent.

Fix: append `\n;` instead of `;` for chunks not already terminated. The
newline closes the line comment, the standalone `;` becomes the statement
separator. For typical chunks (already ending in `;`), output is unchanged
— still clean `\n`-joined chunks with no bare-semicolon lines.

Also added a trailing `;` to `wrapScopedCompositionScript`'s IIFE close
(`})()` → `})();`) so composition scripts join cleanly without falling
through to the `\n;` fallback.

New test: regression guard at the chunk boundary verifies every inline
script body in the bundle parses cleanly via esbuild even when a source JS
file ends with a line comment.

Verification
============

- `bun run --filter @hyperframes/core test` — 653/653 pass
- `bun run --filter @hyperframes/cli test` — 243/243 pass
- `bun run --filter @hyperframes/{core,cli,studio} typecheck` — clean
- `bunx oxfmt --check` + `bunx oxlint` on all touched files — clean

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/cli/src/commands/layout.ts

@@ -1,6 +1,6 @@
 import { defineCommand } from "citty";
 import { existsSync, readFileSync } from "node:fs";
-import { dirname, join, resolve } from "node:path";
+import { dirname, join } from "node:path";
 import { fileURLToPath } from "node:url";
 import type { Example } from "./_examples.js";
 import { c } from "../ui/colors.js";
@@ -107,27 +107,10 @@ async function seekTo(page: import("puppeteer-core").Page, time: number): Promis
 }
 
 async function bundleProjectHtml(projectDir: string): Promise<string> {
+  // `bundleToSingleHtml` now inlines the runtime IIFE by default, so the
+  // previous post-bundle runtime substitution is no longer needed.
   const { bundleToSingleHtml } = await import("@hyperframes/core/compiler");
-  let html = await bundleToSingleHtml(projectDir);
+  return bundleToSingleHtml(projectDir);
-
-  const runtimePath = resolve(
-    __dirname,
-    "..",
-    "..",
-    "..",
-    "core",
-    "dist",
-    "hyperframe.runtime.iife.js",
-  );
-  if (existsSync(runtimePath)) {
-    const runtimeSource = readFileSync(runtimePath, "utf-8");
-    html = html.replace(
-      /<script[^>]*data-hyperframes-preview-runtime[^>]*src="[^"]*"[^>]*><\/script>/,
-      () => `<script data-hyperframes-preview-runtime="1">${runtimeSource}</script>`,
-    );
-  }
-
-  return html;
 }
 
 async function alignViewportToComposition(

packages/cli/src/commands/snapshot.ts

@@ -97,20 +97,9 @@ async function captureSnapshots(
 
   const numFrames = opts.frames ?? 5;
 
-  // 1. Bundle
+  // 1. Bundle. `bundleToSingleHtml` now inlines the runtime IIFE by default,
-  let html = await bundleToSingleHtml(projectDir);
+  // so the previous post-bundle runtime substitution is no longer needed.
-
+  const html = await bundleToSingleHtml(projectDir);
-  // Inject local runtime if available.
-  // Uses the same multi-strategy resolver as the studio preview server
-  // (runtimeSource.ts) so snapshot works in dev (tsx), built CLI, and npx.
-  const { loadRuntimeSource } = await import("../server/runtimeSource.js");
-  const runtimeSource = await loadRuntimeSource();
-  if (runtimeSource) {
-    html = html.replace(
-      /<script[^>]*data-hyperframes-preview-runtime[^>]*src="[^"]*"[^>]*><\/script>/,
-      () => `<script data-hyperframes-preview-runtime="1">${runtimeSource}</script>`,
-    );
-  }
 
   const server = await serveStaticProjectHtml(projectDir, html);
 

packages/cli/src/commands/validate.ts

@@ -1,6 +1,6 @@
 import { defineCommand } from "citty";
 import { existsSync, readFileSync } from "node:fs";
-import { resolve, join, dirname } from "node:path";
+import { join, dirname } from "node:path";
 import { fileURLToPath } from "node:url";
 import { resolveProject } from "../utils/project.js";
 import { resolveCompositionViewportFromHtml } from "../utils/compositionViewport.js";
@@ -113,24 +113,10 @@ async function validateInBrowser(
   const { bundleToSingleHtml } = await import("@hyperframes/core/compiler");
   const { ensureBrowser } = await import("../browser/manager.js");
 
-  let html = await bundleToSingleHtml(projectDir);
+  // `bundleToSingleHtml` now inlines the runtime IIFE by default, so the
-
+  // previous post-bundle regex substitution (which matched `src="..."` on the
-  const runtimePath = resolve(
+  // runtime tag) is no longer needed — there's no `src` attribute to match.
-    __dirname,
+  const html = await bundleToSingleHtml(projectDir);
-    "..",
-    "..",
-    "..",
-    "core",
-    "dist",
-    "hyperframe.runtime.iife.js",
-  );
-  if (existsSync(runtimePath)) {
-    const runtimeSource = readFileSync(runtimePath, "utf-8");
-    html = html.replace(
-      /<script[^>]*data-hyperframes-preview-runtime[^>]*src="[^"]*"[^>]*><\/script>/,
-      () => `<script data-hyperframes-preview-runtime="1">${runtimeSource}</script>`,
-    );
-  }
 
   const { createServer } = await import("node:http");
   const { getMimeType } = await import("@hyperframes/core/studio-api");

packages/cli/src/server/studioServer.ts

@@ -153,8 +153,11 @@ export function createStudioServer(options: StudioServerOptions): StudioServer {
     async bundle(dir: string): Promise<string | null> {
       try {
         const { bundleToSingleHtml } = await import("@hyperframes/core/compiler");
-        let html = await bundleToSingleHtml(dir);
+        // Studio dev server: ask the bundler for an empty `src=""` placeholder so
-        // Fix empty runtime src from bundler — point to the local runtime endpoint
+        // we can point it at our hot-reloadable local runtime endpoint. Inlining
+        // ~150 KB of runtime body on every preview render would defeat browser
+        // caching across composition edits.
+        let html = await bundleToSingleHtml(dir, { runtime: "placeholder" });
         html = html.replace(
           'data-hyperframes-preview-runtime="1" src=""',
           'data-hyperframes-preview-runtime="1" src="/api/runtime.js"',

packages/core/src/compiler/compositionScoping.ts

@@ -286,5 +286,5 @@ ${source}
   };
   __hfFindRoot();
   __hfRun();
-})()`;
+})();`;
 }

packages/core/src/compiler/htmlBundler.test.ts

@@ -82,6 +82,40 @@ describe("bundleToSingleHtml", () => {
     expect(innerLength).toBeGreaterThan(1000);
   });
 
+  it("preserves chunk integrity when a chunk ends with a line comment (ASI hazard guard)", async () => {
+    // Regression guard for the joinJsChunks helper. If a chunk ends with `// ...`
+    // and we naively appended `;` on the same line, the appended semicolon would
+    // be eaten by the comment, leaving the next chunk's first statement attached
+    // to the previous chunk's last expression. Verify the helper appends `\n;`
+    // instead so the comment terminates and the semicolon stands alone.
+    const dir = makeTempProject({
+      "index.html": `<!doctype html>
+<html><body>
+  <div data-composition-id="root" data-width="320" data-height="180"></div>
+  <script src="local-a.js"></script>
+  <script src="local-b.js"></script>
+  <script>window.__timelines = window.__timelines || {}; window.__timelines.root = {}</script>
+</body></html>`,
+      // Chunk A ends with a // line comment — without the \n separator before
+      // the appended ;, that ; would be eaten by the comment.
+      "local-a.js": "window.__a = 1 // trailing line comment",
+      "local-b.js": "window.__b = 2",
+    });
+
+    const bundled = await bundleToSingleHtml(dir);
+    // Run every inline script body through esbuild; if the line comment ate
+    // the separator, parse would fail with an unexpected-token error somewhere
+    // around the chunk boundary.
+    const { transformSync } = await import("esbuild");
+    const re = /<script\b[^>]*>([\s\S]*?)<\/script>/g;
+    let m: RegExpExecArray | null;
+    while ((m = re.exec(bundled)) !== null) {
+      const body = m[1];
+      if (!body || !body.trim()) continue;
+      expect(() => transformSync(body, { loader: "js", minify: false })).not.toThrow();
+    }
+  });
+
   it("does not produce stray bare-semicolon lines between concatenated JS chunks", async () => {
     // Regression guard: hf#XXX. Earlier the bundler joined script chunks with
     // `\n;\n`, which produces a lone `;` on its own line between chunks. Valid

packages/core/src/compiler/htmlBundler.ts

@@ -27,20 +27,24 @@ function getRuntimeScriptUrl(): string {
   return configured || DEFAULT_RUNTIME_SCRIPT_URL;
 }
 
-function injectInterceptor(html: string): string {
+function injectInterceptor(html: string, runtimeMode: "inline" | "placeholder" = "inline"): string {
   const sanitized = stripEmbeddedRuntimeScripts(html);
   if (sanitized.includes(RUNTIME_BOOTSTRAP_ATTR)) return sanitized;
 
-  // When a runtime URL is configured (HYPERFRAME_RUNTIME_URL env var), the bundle
+  // Three modes for the runtime <script>:
-  // points at it via src=… and the host page serves the script. When no URL is
+  //   1. HYPERFRAME_RUNTIME_URL env var set → emit src="<url>" (production CDN deploy).
-  // configured — the common `bundleToSingleHtml` use case — inline the runtime
+  //   2. runtime: "placeholder" passed         → emit src="" for the caller to substitute
-  // body so the bundle is genuinely self-contained. An empty src="" attribute
+  //                                              (studio + vite preview hot-load a local
-  // would otherwise resolve to the page URL and trigger an infinite-fetch loop.
+  //                                              runtime endpoint via string replace).
+  //   3. runtime: "inline" (default)           → embed the IIFE body directly so the
+  //                                              bundle is genuinely self-contained.
   const runtimeScriptUrl = getRuntimeScriptUrl();
   let tag: string;
   if (runtimeScriptUrl) {
     const escaped = runtimeScriptUrl.replace(/"/g, "&quot;");
     tag = `<script ${RUNTIME_BOOTSTRAP_ATTR}="1" src="${escaped}"></script>`;
+  } else if (runtimeMode === "placeholder") {
+    tag = `<script ${RUNTIME_BOOTSTRAP_ATTR}="1" src=""></script>`;
   } else {
     const inlinedRuntime = getHyperframeRuntimeScript();
     tag = `<script ${RUNTIME_BOOTSTRAP_ATTR}="1">${inlinedRuntime}</script>`;
@@ -293,16 +297,27 @@ function coalesceHeadStylesAndBodyScripts(document: Document): void {
 }
 
 /**
- * Concatenate JS chunks safely. Each chunk gets a trailing `;` if it doesn't
+ * Concatenate JS chunks safely. Goals:
- * already end in one, so the joined output never inserts a stray bare-semicolon
+ *   - Each chunk's last statement is terminated, so joining can't introduce ASI
- * line between chunks (the `\n;\n` separator pattern produces a lone `;` on its
+ *     surprises (e.g. `a()` followed by `(b)()` — the second chunk would parse
- * own line, which is valid JS but reads as a code smell to most linters).
+ *     as a call on the first's return value).
+ *   - In the common case (chunk already ends with `;` — typical of esbuild
+ *     output and IIFE-wrapped composition scripts ending in `})();`), the join
+ *     produces clean output: chunks separated by `\n` with no stray bare
+ *     semicolon lines.
+ *   - Defensive against trailing line comments. If a chunk ends with `// ...`
+ *     and we appended `;` on the same line, the appended semicolon would be
+ *     swallowed by the comment, leaving the next chunk's first statement
+ *     attached to the previous chunk's last expression — exactly the ASI
+ *     hazard this helper exists to prevent. So when a chunk doesn't already
+ *     end in `;`, we append `\n;` instead — the newline closes any line
+ *     comment, and the standalone `;` becomes the statement separator.
  */
 function joinJsChunks(chunks: string[]): string {
   return chunks
     .map((chunk) => chunk.trim())
     .filter((chunk) => chunk.length > 0)
-    .map((chunk) => (chunk.endsWith(";") ? chunk : chunk + ";"))
+    .map((chunk) => (chunk.endsWith(";") ? chunk : chunk + "\n;"))
     .join("\n");
 }
 
@@ -319,6 +334,22 @@ function stripJsCommentsParserSafe(source: string): string {
 export interface BundleOptions {
   /** Optional media duration prober (e.g., ffprobe). If omitted, media durations are not resolved. */
   probeMediaDuration?: MediaDurationProber;
+  /**
+   * How to handle the HyperFrames runtime <script> tag. Default: `"inline"`.
+   *
+   * - `"inline"` — embed the runtime IIFE body directly into the bundle. Produces
+   *   genuinely self-contained HTML. Right for CLI render output, validate,
+   *   snapshot, and any "ship a single .html file" use case.
+   * - `"placeholder"` — emit `<script ... src=""></script>` so the caller can
+   *   substitute it with a real URL via string replace. Used by the dev studio
+   *   server and vite preview to point at a local runtime endpoint, which keeps
+   *   the runtime cacheable across hot-reloads instead of re-inlining ~150 KB
+   *   on every change.
+   *
+   * The `HYPERFRAME_RUNTIME_URL` env var, when set, takes precedence over both
+   * modes and emits `<script ... src="<URL>">` directly.
+   */
+  runtime?: "inline" | "placeholder";
 }
 
 /**
@@ -347,7 +378,7 @@ export async function bundleToSingleHtml(
     );
   }
 
-  const withInterceptor = injectInterceptor(compiled);
+  const withInterceptor = injectInterceptor(compiled, options?.runtime ?? "inline");
   const document = parseHTMLContent(withInterceptor);
 
   // Inline local CSS

packages/studio/vite.config.ts

@@ -60,7 +60,9 @@ interface ScreenshotClip {
 
 function createViteAdapter(dataDir: string, server: ViteDevServer): StudioApiAdapter {
   // Lazy-load the bundler via Vite's SSR module loader
-  let _bundler: ((dir: string) => Promise<string>) | null = null;
+  let _bundler:
+    | ((dir: string, options?: { runtime?: "inline" | "placeholder" }) => Promise<string>)
+    | null = null;
   let _producerModulePromise: Promise<{
     createRenderJob: (config: {
       fps: 24 | 30 | 60;
@@ -78,7 +80,7 @@ function createViteAdapter(dataDir: string, server: ViteDevServer): StudioApiAda
     if (!_bundler) {
       try {
         const mod = await server.ssrLoadModule("@hyperframes/core/compiler");
-        _bundler = (dir: string) => mod.bundleToSingleHtml(dir);
+        _bundler = (dir, options) => mod.bundleToSingleHtml(dir, options);
       } catch (err) {
         console.warn("[Studio] Failed to load compiler, previews will use raw HTML:", err);
         _bundler = null as never;
@@ -171,8 +173,10 @@ function createViteAdapter(dataDir: string, server: ViteDevServer): StudioApiAda
     async bundle(dir: string) {
       const bundler = await getBundler();
       if (!bundler) return null;
-      let html = await bundler(dir);
+      // Studio vite preview: bundler emits an empty `src=""` placeholder so we
-      // Fix empty runtime src from bundler — point to the CDN runtime
+      // can point it at the local /api/runtime.js endpoint. Cached by the browser
+      // across composition hot-reloads instead of being inlined fresh each time.
+      let html = await bundler(dir, { runtime: "placeholder" });
       html = html.replace(
         'data-hyperframes-preview-runtime="1" src=""',
         `data-hyperframes-preview-runtime="1" src="${this.runtimeUrl}"`,