Querying the Blockchain with Web3.js

Web3.js is a powerful library for interacting with the Ethereum blockchain. In addition to sending transactions and reading data from the blockchain, you can also use Web3.js to query the blockchain for specific information. In this article, we will look at how to use Web3.js to query the Ethereum blockchain for specific data.

Querying Smart Contracts

One way to query the Ethereum blockchain is to query a smart contract. Smart contracts are self-executing contracts with the terms of the agreement between buyer and seller being directly written into lines of code. They are used to facilitate, verify, and enforce the negotiation or performance of a contract, and they are executed on the Ethereum Virtual Machine (EVM).

To query a smart contract with Web3.js, you will need to do the following:

1. Compile your smart contract with a Solidity compiler, such as Remix (https://remix.ethereum.org).
2. Deploy your smart contract to the Ethereum blockchain with a Web3 provider, such as Infura (https://infura.io).
3. Interact with your smart contract using the Web3.js API.

Here is an example of how to query a smart contract with Web3.js:

const Web3 = require('web3');

const web3 = new Web3('https://mainnet.infura.io/v3/YOUR-API-KEY');

const contract = new web3.eth.Contract(ABI, CONTRACT_ADDRESS);

contract.methods.functionName(functionArguments).call((error, result) => {
  console.log(result);
});

This code calls the functionName() function of the contract smart contract, with the specified functionArguments. The result of the function will be output to the console.

Querying the Ethereum Blockchain

In addition to querying smart contracts, you can also query the Ethereum blockchain directly. This can be useful for getting information such as the current block number, the current block hash, or the transaction history of an account.

To query the Ethereum blockchain with Web3.js, you can use the various methods provided by the web3.eth object. For example, you can use the web3.eth.getBlockNumber() method to get the current block number, and the web3.eth.getTransactionFromBlock() method to get the transaction history of an account.

Here is an example of how to get the transaction history of an Ethereum account with Web3.js:

const Web3 = require('web3');

const web3 = new Web3('https://mainnet.infura.io/v3/YOUR-API-KEY');

web3.eth.getTransactionCount('ACCOUNT-ADDRESS', 'latest', (error, transactionCount) => {
  for (let i = 0; i < transactionCount; i++) {
    web3.eth.getTransactionFromBlock(i, 'latest', (error, transaction) => {
      console.log(transaction);
    });
  }
});

This code gets the transaction count of the Ethereum account with the specified ACCOUNT-ADDRESS, and then loops through the transactions, outputting each one to the console.

Conclusion

Querying the Ethereum blockchain is an important function of any decentralized application (DApp) built on Ethereum. By using the Web3.js library, you can easily query smart contracts or the Ethereum blockchain, and you can build powerful DApps that are fast, secure, and transparent. Whether you are a seasoned blockchain developer or a beginner, Web3.js is a powerful tool for building on Ethereum.

In this article, we covered how to use Web3.js to query smart contracts and the Ethereum blockchain. We looked at how to compile and deploy smart contracts, how to interact with smart contracts using the Web3.js API, and how to use various methods provided by the web3.eth object to query the Ethereum blockchain. With this knowledge, you should be well-equipped to start querying the Ethereum blockchain with Web3.js.

Exercises

To review these concepts, we will go through a series of exercises designed to test your understanding and apply what you have learned.

const Web3 = require('web3');

const web3 = new Web3('https://mainnet.infura.io/v3/YOUR-API-KEY');

const callSmartContractFunction = (contractAddress, functionName) => {
  const contract = new web3.eth.Contract(ABI, contractAddress);
  contract.methods[functionName]().call((error, result) => {
    console.log(result);
  });
};

callSmartContractFunction('CONTRACT-ADDRESS', 'functionName');

const Web3 = require('web3');

const web3 = new Web3('https://mainnet.infura.io/v3/YOUR-API-KEY');

const getSmartContractFunctionTransactionHistory = (contractAddress, functionName) => {
  const contract = new web3.eth.Contract(ABI, contractAddress);
  web3.eth.getPastLogs({
    address: contractAddress,
    topics: [web3.utils.sha3(functionName)]
  }, (error, logs) => {
    console.log(logs);
  });
};

getSmartContractFunctionTransactionHistory('CONTRACT-ADDRESS', 'functionName');

const Web3 = require('web3');

const web3 = new Web3('https://mainnet.infura.io/v3/YOUR-API-KEY');

const callSmartContractFunctionWithArguments = (contractAddress, functionName, functionArguments) => {
  const contract = new web3.eth.Contract(ABI, contractAddress);
  contract.methods[functionName](...functionArguments).call((error, result) => {
    console.log(result);
  });
};

callSmartContractFunctionWithArguments('CONTRACT-ADDRESS', 'functionName', ['ARG1', 'ARG2']);

const Web3 = require('web3');

const web3 = new Web3('https://mainnet.infura.io/v3/YOUR-API-KEY');

const getEtherBalance = (address) => {
  web3.eth.getBalance(address, (error, balance) => {
    console.log(web3.utils.fromWei(balance, 'ether'));
  });
};

getEtherBalance('ACCOUNT-ADDRESS');

const Web3 = require('web3');

const web3 = new Web3('https://mainnet.infura.io/v3/YOUR-API-KEY');

const getBlockHash = (blockNumber) => {
  web3.eth.getBlock(blockNumber, (error, block) => {
    console.log(block.hash);
  });
};

getBlockHash(123456);