How does cardano-serialization-lib-asmjs generate transactions?

Developers Cardano Integration
1

Hi,I am using cardano-serialization-lib-asmjs, but I am having trouble generating transactions。
I run the following code txHex is empty：

 const txBuilder = CardanoWasm.TransactionBuilder.new(
            // all of these are taken from the mainnet genesis settings
            CardanoWasm.LinearFee.new(CardanoWasm.BigNum.from_str('44'), CardanoWasm.BigNum.from_str('155381')),
            CardanoWasm.BigNum.from_str('1000000'),
            CardanoWasm.BigNum.from_str('500000000'),
            CardanoWasm.BigNum.from_str('2000000')
        );

        const address = CardanoWasm.ByronAddress.from_base58("Ae2tdPwUPEZLs4HtbuNey7tK4hTKrwNwYtGqp7bDfCy2WdR3P6735W5Yfpe");
        txBuilder.add_bootstrap_input(
            address,
            CardanoWasm.TransactionInput.new(
                CardanoWasm.TransactionHash.from_bytes(
                    Buffer.from("488afed67b342d41ec08561258e210352fba2ac030c98a8199bc22ec7a27ccf1", "hex"),
                ),
                0, // index
            ),
            CardanoWasm.BigNum.from_str('3000000')
        );

        txBuilder.add_output(
            CardanoWasm.TransactionOutput.new(
                address.to_address(),
                // we can construct BigNum (Coin) from both a js BigInt (here) or from a string (below in fee)
                CardanoWasm.BigNum.from_str("1000000"),
            ),
        );

        txBuilder.set_ttl(410021);

        // calculate the min fee required and send any change to an address
        txBuilder.add_change_if_needed(CardanoWasm.ByronAddress.from_base58("Ae2tdPwUPEYxiWbAt3hUCJsZ9knze88qNhuTQ1MGCKqsVFo5ddNyoTDBymr").to_address())

        const txBody = txBuilder.build();

        const txHash = CardanoWasm.hash_transaction(txBody);

        const witnesses = CardanoWasm.TransactionWitnessSet.new();
        const bootstrapWitnesses = CardanoWasm.BootstrapWitnesses.new();
        const bootstrapWitness = CardanoWasm.make_icarus_bootstrap_witness(txHash,address,rootKey);
        bootstrapWitnesses.add(bootstrapWitness);
        witnesses.set_bootstraps(bootstrapWitnesses);
        const transaction = CardanoWasm.Transaction.new(
            txBody,
            witnesses,
            undefined, // transaction metadata
        );
        const txHex = Buffer.from(transaction.to_bytes()).toString("hex");
        console.log(txHex);