Basic Transactions

In the code snippet, you will find txBuilder, which is an instance of MeshTxBuilder, a powerful low-level APIs that allows you to build transactions. Learn how to initialize MeshTxBuilder.

const txBuilder = new MeshTxBuilder({
  fetcher: provider, // get a provider https://meshjs.dev/providers
  verbose: true,
});

The MeshTxBuilder is a powerful interface where the higher level Transaction class is indeed a pre-built combination of the MeshTxBuilder APIs. With these lower level APIs, it builds the object to be passing to the serialization libraries like cardano-sdk and Whisky SDK to construct transactions.

In this page, we will cover how to initialize the MeshTxBuilder and the basic operations of building a transaction.

Initialize Tx Builder

To start building an customized transaction, you need to first initialize MeshTxBuilder:

import { BlockfrostProvider, MeshTxBuilder } from "@meshsdk/core";

const provider = new BlockfrostProvider('<Your-API-Key>');

const txBuilder = new MeshTxBuilder({
  fetcher: provider,
  verbose: true,
});

The MeshTxBuilder instance has the following signature:

{
  fetcher?: IFetcher;
  submitter?: ISubmitter;
  evaluator?: IEvaluator;
  serializer?: IMeshTxSerializer;
  isHydra?: boolean;
  params?: Partial<Protocol>;
  verbose?: boolean;
}

There are 6 optional fields to pass in to initialized the lower level APIs instance:

• serializer: The default serializer is CSLSerializer. You can pass in your own serializer instance.
• fetcher: When you build the transaction without sufficient fields as required by the serialization library, we would index the blockchain to fill the information for you. Affected APIs are txIn, txInCollateral, spendingTxInReference.
• submitter: It is used if you would like to use the submitter submitTx API directly from the instance.
• evaluator: It would perform redeemer execution unit optimization, returning error message in case of invalid transaction.
• isHydra: Use another set of default protocol parameters for building transactions.
• params: You can pass in the protocol parameters directly.
• verbose: Set to true to enable verbose logging.

Send Value

Sending value with MeshTxBuilder come with the .txOut() endpoint:

.txOut(address: string, amount: Asset[])

In order to send values (so as every transaction), we have to fund the transaction to do so. There are 2 ways to provide values in a transaction:

• Specifying which input to spend with

.txIn(txHash: string, txIndex: number, amount?: Asset[], address?: string)

• Providing an array of UTxOs, and perform auto UTxO selection:

.selectUtxosFrom(extraInputs: UTxO[])

Since the input and output values might not be the same, we have to specify the address (usually own's address) to receive change:

.changeAddress(addr: string)

The following shows a simple example of building a transaction to send values with UTxO selection:

txBuilder
  .txOut(address, [{ unit: "lovelace", quantity: amount }])
  .changeAddress(changeAddress)
  .selectUtxosFrom(utxos)
  .complete();

Example code:

const utxos = await wallet.getUtxos();
const changeAddress = await wallet.getChangeAddress();

const txBuilder = new MeshTxBuilder({
  fetcher: provider, // get a provider https://meshjs.dev/providers
  verbose: true,
});

const unsignedTx = await txBuilder
  .txOut('addr_test1vpvx0sacufuypa2k4sngk7q40zc5c4npl337uusdh64kv0c7e4cxr', [{ unit: "lovelace", quantity: '1000000' }])
  .changeAddress(changeAddress)
  .selectUtxosFrom(utxos)
  .complete();

const signedTx = await wallet.signTx(unsignedTx);
const txHash = await wallet.submitTx(signedTx);

Check out the Mesh website to see the full list of APIs.