A lot of things.

public/rom-patcher-js/RomPatcher.webworker.crc.js

@@ -0,0 +1,26 @@
+/* Rom Patcher JS v20240302 - Marc Robledo 2016-2024 - http://www.marcrobledo.com/license */
+
+self.importScripts(
+	'./RomPatcher.js',
+	'./modules/BinFile.js',
+	'./modules/HashCalculator.js'
+);
+
+
+self.onmessage = event => { // listen for messages from the main thread
+	const binFile=new BinFile(event.data.u8array);
+	binFile.fileName=event.data.fileName;
+	const startOffset=typeof event.data.checksumStartOffset==='number'? event.data.checksumStartOffset : 0;
+
+	self.postMessage(
+		{
+			action: event.data.action,
+			crc32:binFile.hashCRC32(startOffset),
+			md5:binFile.hashMD5(startOffset),
+			checksumStartOffset: startOffset,
+			rom:RomPatcher.getRomAdditionalChecksum(binFile),
+			u8array:event.data.u8array
+		},
+		[event.data.u8array.buffer]
+	);
+};