test: add test for external `Transaction` signing

web3.js/test/transaction.test.js

@@ -439,3 +439,22 @@ test('serialize unsigned transaction', () => {
     expectedTransaction,
   );
 });
+
+test('get sign data for transaction', () => {
+  const from_keypair = nacl.sign.keyPair.fromSeed(
+    Uint8Array.from(Array(32).fill(1)),
+  );
+  const from = new Account(Buffer.from(from_keypair.secretKey));
+  const to = new PublicKey(2);
+  const recentBlockhash = new PublicKey(3).toBuffer();
+  var tx = SystemProgram.transfer({
+    fromPubkey: from.publicKey,
+    toPubkey: to,
+    lamports: 42,
+  });
+  tx.recentBlockhash = bs58.encode(recentBlockhash);
+  const tx_bytes = tx.signData;
+  const signature = nacl.sign.detached(tx_bytes, from.secretKey);
+  tx.addSignature(from.publicKey, signature);
+  expect(tx.verifySignatures()).toBe(true);
+});