diff --git a/apps/vanilla-app/package.json b/apps/vanilla-app/package.json
index 63b8b02..ede0cbe 100644
--- a/apps/vanilla-app/package.json
+++ b/apps/vanilla-app/package.json
@@ -4,7 +4,7 @@
   "description": "browser example",
   "private": true,
   "scripts": {
-    "download": "node download-assets.js",
+    "prestart": "node ../../scripts/download-assets.js",
     "start": "node server.js"
   },
   "author": "Jerome Wu <jeromewus@gmail.com>",
diff --git a/apps/vanilla-app/download-assets.js b/scripts/download-assets.js
similarity index 88%
rename from apps/vanilla-app/download-assets.js
rename to scripts/download-assets.js
index 8ca1851..3cfd148 100644
--- a/apps/vanilla-app/download-assets.js
+++ b/scripts/download-assets.js
@@ -23,12 +23,18 @@ const mkdir = (dir) => {
 };
 
 const downloadAndUntar = async (url, tgzName, dst) => {
+  const dir = `${ROOT}/${dst}`;
+  if (fs.existsSync(dir)) {
+    console.log(`found @ffmpeg/${dst} assets.`);
+    return;
+  }
   console.log(`download and untar ${url}`);
-  mkdir(`${ROOT}/${dst}`);
+  mkdir(dir);
   const data = Buffer.from(await (await fetch(url)).arrayBuffer());
   fs.writeFileSync(tgzName, data);
 
   await tar.x({ file: tgzName, C: `${ROOT}/${dst}` });
+  fs.unlinkSync(tgzName);
 };
 
 mkdir(ROOT);