How to use the algosdk.Encoding.decode function in algosdk

return co(function*() {
      // take txHex first always, but as it might already be signed, take halfSigned second
      const txHex = params.txHex || (params.halfSigned && params.halfSigned.txHex);

      if (!txHex) {
        throw new Error('missing required param txHex or halfSigned.txHex');
      }
      let tx;
      try {
        const txToHex = Buffer.from(txHex, 'base64');
        const decodedTx = Encoding.decode(txToHex);

        // if we are a signed msig tx, the structure actually has the { msig, txn } as the root object
        // if we are not signed, the decoded tx is the txn - refer to partialSignTxn and MultiSig constructor
        //   in algosdk for more information
        const txnForDecoding = decodedTx.txn || decodedTx;

        tx = Multisig.MultiSigTransaction.from_obj_for_encoding(txnForDecoding);
      } catch (ex) {
        throw new Error('txHex needs to be a valid tx encoded as base64 string');
      }

      const id = tx.txID();
      const fee = { fee: tx.fee };

      const outputAmount = tx.amount || 0;
      const outputs: { amount: number; address: string }[] = [];

signTransaction(params: SignTransactionOptions): SignedTransaction {
    const { txHex, addressVersion, keys, sk, isHalfSigned } = this.verifySignTransactionParams(params);
    const encodedPublicKeys = _.map(keys, k => Address.decode(k).publicKey);

    // decode our unsigned/half-signed tx
    let transaction;
    let txToHex;
    try {
      txToHex = Buffer.from(txHex, 'base64');
      const initialDecodedTx = Encoding.decode(txToHex);

      // we need to scrub the txn of sigs for half-signed
      const decodedTx = isHalfSigned ? initialDecodedTx.txn : initialDecodedTx;

      transaction = Multisig.MultiSigTransaction.from_obj_for_encoding(decodedTx);
    } catch (e) {
      throw new Error('transaction needs to be a valid tx encoded as base64 string');
    }

    // sign our tx
    let signed = transaction.partialSignTxn({ version: addressVersion, threshold: 2, pks: encodedPublicKeys }, sk);

    // if we have already signed it, we'll have to merge that with our previous tx
    if (isHalfSigned) {
      signed = mergeMultisigTransactions([Buffer.from(signed), txToHex]);
    }