02-web3js-transaction

中文 / English

Abstract

The demo code provides developers with an overview of how to sign, send, and receive receipt of transactions, and verify the results of their execution. The sample also provides the event monitoring code so that the developer can understand how to listen to an event one or more times.

Getting Started

Understanding The Functions of the Smart Contract

• Constructor: The constructor function of the smart contract, which is called when the contract is deploying, at the same time it will initialize the number to _initialNumber;
• increment: The function of incrementing the number by given _value;
• rest: The function of resetting the number to 0;
• getNumber: The function of getting the number.

How to run it

1. Install dependencies: npm install
2. Copy the configuration file: cp .env.example .env
3. Edit the configuration file: vim .env, copy your project ID and private key to the .env file.

   PRIVATE_KEY=YOUR_PRIVATE_KEY
   INFURA_ID=YOUR_PROJECT_ID
   

4. Run the index.js file: node index.js

Interpret Source Code

compile.js

You can't use .sol files directly, you need to compile it to binary file firstly.

Load the smart contract file Incrementer.sol into source variable.

// Load contract
const source = fs.readFileSync("Incrementer.sol", "utf8");

Compile the smart contract file

const input = {
  language: "Solidity",
  sources: {
    "Incrementer.sol": {
      content: source,
    },
  },
  settings: {
    outputSelection: {
      "*": {
        "*": ["*"],
      },
    },
  },
};

const tempFile = JSON.parse(solc.compile(JSON.stringify(input)));

| Note: The version of solidity in this task is 0.8.0, different versions may have different compile ways.

Get the Contract Object

const contractFile = tempFile.contracts["Incrementer.sol"]["Incrementer"];

Export contractFile Object

If you want to use the contractFile object in other js files, you need to export it.

module.exports = contractFile;

index.js

1. Load the Incrementer smart contract from compile file

const contractOfIncrementer = require("./compile");

2. Read private key from environment variables

For security’s sake, the private key is not hard-coded, but it can be read as environment variables. When run this task, the dotenv plugin will automatically read the configurations in the .env file and load them as environment variables, and then you can use the private key and other environment variables via process.env.

require("dotenv").config();
const privatekey = process.env.PRIVATE_KEY;

3. Create the web3 instance

web3 is the main API of the web3js library. It is used to interact with the blockchain.

// Provider
const providerRPC = {
  development: "https://goerli.infura.io/v3/" + process.env.INFURA_ID,
  moonbase: "https://rpc.testnet.moonbeam.network",
};
const web3 = new Web3(providerRPC.development); //Change to correct network

| Note: The INFURA_ID is the PROJECT ID of the Infura project you created in last task

4. Get the account address

On blockchain, each user has a address, which is unique for others, and you can get the address by the private key. In this task, you can use the we3.eth.accounts.privateKeyToAccount API to get your account address by passing the private key as a parameter.

const account = web3.eth.accounts.privateKeyToAccount(privatekey);
const account_from = {
  privateKey: privatekey,
  accountAddress: account.address,
};

5. Get the bytecode and abi

When deploying the smart contract, you need to specify the bytecode and abi of the smart contract. The bytecode is the compiled binary code of the smart contract, and the abi (Application Binary Interface) is the interface of the smart contract.

const bytecode = contractOfIncrementer.evm.bytecode.object;
const abi = contractOfIncrementer.abi;

6. Get contract instance

In the last step, you got the bytecode and abi, so you can create the contract instance by the abi.

// Create contract instance
  const deployContract = new web3.eth.Contract(abi);

7. Create the transaction of the deployContract

// Create Tx
const deployTx = deployContract.deploy({
    data: bytecode,
    arguments: [5],
});

8. Sign the transaction

Use your private key to sign the transaction.

// Sign Tx
const deployTransaction = await web3.eth.accounts.signTransaction(
    {
        data: deployTx.encodeABI(),
        gas: 8000000,
    },
    account_from.privateKey
);

9. Send the transaction / Deploy your smart contract

Send your deploy transaction to the blockchain. You will receive a receipt, and get this contract address from the receipt.

const deployReceipt = await web3.eth.sendSignedTransaction(
    deployTransaction.rawTransaction
);
console.log(`Contract deployed at address: ${deployReceipt.contractAddress}`);

10. Load the contract instance from blockchain through above address

In previous steps, you built a contract instance, and then deployed the transaction by sending the contract to the blockchain so that you can operate the transaction later. Besides, you could also load a contract instance that is already on the blockchain so that you can operate it directly and avoid the process of deploying it.

let incrementer = new web3.eth.Contract(abi, deployReceipt.contractAddress);

11. Use the view function of a contract

Whether a contract instance is create by deploying, or by loading, you can interact with the contract once you have an instance of the contract already on the blockchain.
There are two types of contract functions: view and without view. The functions with view promise not to modify the state, while the functions without view will generate the corresponding block data on the blockchain. For example, after calling the getNumber function of the contract, you will get the public variable number of the contract, and this operation will not charge any gas.

let number = await incrementer.methods.getNumber().call();

12. Build a transaction

Before you send the transaction to the blockchain, you need to build the transaction, it means you need to specify the parameters of the transaction.

let incrementTx = incrementer.methods.increment(_value);

// Sign with Pk
let incrementTransaction = await web3.eth.accounts.signTransaction(
  {
    to: createReceipt.contractAddress,
    data: incrementTx.encodeABI(),
    gas: 8000000,
  },
  account_from.privateKey
);

13. Send the transaction and get the receipt

You can use sendSignedTransaction function to send above transaction to the blockchain, and got the receipt to check result.

const incrementReceipt = await web3.eth.sendSignedTransaction(
  incrementTransaction.rawTransaction
);

Listen to Event Increment

When invoking the interfaces of the contract, the only way to get information about the processing details, apart from the results returned by the interface, is through events.
In the interfaces, you retrieve the corresponding internal information by triggering an event, and then capturing this event generated by the external block.

const web3Socket = new Web3(
new Web3.providers.WebsocketProvider(
    'wss://goerli.infura.io/ws/v3/' + process.env.INFURA_ID
));
incrementer = new web3Socket.eth.Contract(abi, createReceipt.contractAddress);

| goerli don't support http protocol to event listen, need to use websocket. More details , please refer to this blog

Listen to Increment event only once

incrementer.once('Increment', (error, event) => {
    console.log('I am a onetime event listener, I am going to die now');
});

Listen to Increment event continuously

incrementer.events.Increment(() => {
    console.log("I am a longlive event listener, I get a event now");
});

References

• Code part: https://docs.moonbeam.network/getting-started/local-node/deploy-contract/
• web3socket of Goerli: https://medium.com/blockcentric/listening-for-smart-contract-events-on-public-blockchains-fdb5a8ac8b9a