Web3.js is an open source library of javascript built by the Ethereum foundation to allow end users to interact with an Ethereum node using HTTP, IPC, or WebSocket.
The web3.js library provides an eth package to interact with smart contracts. The eth package further contains a contract object with the functions required to interact with a smart deployed on the Ethereum network.
The functions need some arguments to interact with the required smart contracts successfully. The required arguments are the smart contract’s application binary interface (ABI) and address.
ABI is an interface between the front end and the back end of a Dapp. ABI contains information about the functions present in the smart contact along with their parameters and return type.
ABI of a smart contract can be taken from Etherscan.io by following the steps given below:
Go to Etherscan.io and click “ERC20 Top Tokens” in the drop-down of “Tokens” located in the top navigation bar.
Click any token from the list of ERC20 tokens.
Now, the details of that token will be shown. Click the “Contract” tab to see the contract details.
Then scroll down a little, and the contract ABI will be visible there. We'll use it as a parameter when selecting the smart contract.
Every account and smart contract in the Ethereum blockchain has an address. We need the address to locate the smart contract and interact with it.
The smart contract address can be fetched from the same page where we get the ABI. We just have to scroll up and see “Contract” written under the “Profile summary.” The smart contract address is written in front of the contract.
Note: We also need to establish a connection to the Ethereum node. For that, we use Infura API.
In the code example given below, we'll read the name, symbol, and total supply from the smart contract of the Tether USD token.
const Web3 = require('web3')
const jsonRpcURL = 'https://mainnet.infura.io/v3/917c4f75f17a4e28b78263cddd8f1b46'
const web3 = new Web3(jsonRpcURL)
const abi = [{"constant":true,"inputs":[],"name":"name","outputs":[{"name":"","type":"string"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"name":"_upgradedAddress","type":"address"}],"name":"deprecate","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":false,"inputs":[{"name":"_spender","type":"address"},{"name":"_value","type":"uint256"}],"name":"approve","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[],"name":"deprecated","outputs":[{"name":"","type":"bool"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"name":"_evilUser","type":"address"}],"name":"addBlackList","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[],"name":"totalSupply","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"name":"_from","type":"address"},{"name":"_to","type":"address"},{"name":"_value","type":"uint256"}],"name":"transferFrom","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[],"name":"upgradedAddress","outputs":[{"name":"","type":"address"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[{"name":"","type":"address"}],"name":"balances","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[],"name":"decimals","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[],"name":"maximumFee","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[],"name":"_totalSupply","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[],"name":"unpause","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[{"name":"_maker","type":"address"}],"name":"getBlackListStatus","outputs":[{"name":"","type":"bool"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[{"name":"","type":"address"},{"name":"","type":"address"}],"name":"allowed","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[],"name":"paused","outputs":[{"name":"","type":"bool"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[{"name":"who","type":"address"}],"name":"balanceOf","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[],"name":"pause","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[],"name":"getOwner","outputs":[{"name":"","type":"address"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[],"name":"owner","outputs":[{"name":"","type":"address"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[],"name":"symbol","outputs":[{"name":"","type":"string"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"name":"_to","type":"address"},{"name":"_value","type":"uint256"}],"name":"transfer","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":false,"inputs":[{"name":"newBasisPoints","type":"uint256"},{"name":"newMaxFee","type":"uint256"}],"name":"setParams","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":false,"inputs":[{"name":"amount","type":"uint256"}],"name":"issue","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":false,"inputs":[{"name":"amount","type":"uint256"}],"name":"redeem","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[{"name":"_owner","type":"address"},{"name":"_spender","type":"address"}],"name":"allowance","outputs":[{"name":"remaining","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[],"name":"basisPointsRate","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[{"name":"","type":"address"}],"name":"isBlackListed","outputs":[{"name":"","type":"bool"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"name":"_clearedUser","type":"address"}],"name":"removeBlackList","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[],"name":"MAX_UINT","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"name":"newOwner","type":"address"}],"name":"transferOwnership","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":false,"inputs":[{"name":"_blackListedUser","type":"address"}],"name":"destroyBlackFunds","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"inputs":[{"name":"_initialSupply","type":"uint256"},{"name":"_name","type":"string"},{"name":"_symbol","type":"string"},{"name":"_decimals","type":"uint256"}],"payable":false,"stateMutability":"nonpayable","type":"constructor"},{"anonymous":false,"inputs":[{"indexed":false,"name":"amount","type":"uint256"}],"name":"Issue","type":"event"},{"anonymous":false,"inputs":[{"indexed":false,"name":"amount","type":"uint256"}],"name":"Redeem","type":"event"},{"anonymous":false,"inputs":[{"indexed":false,"name":"newAddress","type":"address"}],"name":"Deprecate","type":"event"},{"anonymous":false,"inputs":[{"indexed":false,"name":"feeBasisPoints","type":"uint256"},{"indexed":false,"name":"maxFee","type":"uint256"}],"name":"Params","type":"event"},{"anonymous":false,"inputs":[{"indexed":false,"name":"_blackListedUser","type":"address"},{"indexed":false,"name":"_balance","type":"uint256"}],"name":"DestroyedBlackFunds","type":"event"},{"anonymous":false,"inputs":[{"indexed":false,"name":"_user","type":"address"}],"name":"AddedBlackList","type":"event"},{"anonymous":false,"inputs":[{"indexed":false,"name":"_user","type":"address"}],"name":"RemovedBlackList","type":"event"},{"anonymous":false,"inputs":[{"indexed":true,"name":"owner","type":"address"},{"indexed":true,"name":"spender","type":"address"},{"indexed":false,"name":"value","type":"uint256"}],"name":"Approval","type":"event"},{"anonymous":false,"inputs":[{"indexed":true,"name":"from","type":"address"},{"indexed":true,"name":"to","type":"address"},{"indexed":false,"name":"value","type":"uint256"}],"name":"Transfer","type":"event"},{"anonymous":false,"inputs":[],"name":"Pause","type":"event"},{"anonymous":false,"inputs":[],"name":"Unpause","type":"event"}]
const contractAddress = '0xdAC17F958D2ee523a2206206994597C13D831ec7'
const contract = new web3.eth.Contract(abi, contractAddress)
/*
This will not execute the function
instead it will return the function
as it is. Try to un comment line 16
and see the output.
*/
//console.log(contract.methods.name())
/*
This will execute the function
and return a promise as it is an
asynchronous function. Try to un
comment line 24 and see the output.
*/
//console.log(contract.methods.name().call())
async function readFromSmartContract()
{
tokenName = await contract.methods.name().call()
console.log("Name: ", tokenName)
tokenSymbol = await contract.methods.symbol().call()
console.log("Symbol: ", tokenSymbol)
tokenTotalSupply = await contract.methods.totalSupply().call()
console.log("Total supply: ", tokenTotalSupply)
}
readFromSmartContract() // Function call to fetch the informationThe explanation of the code above is as follows:
Line 8: We declare contract and initialize with the smart contract specified using the abi and the contractAddress.
Line 28: We fetch the name of the token using name() and call(), as name() returns the function itself. And call() helps execute that function. Furthermore, we use await because call() is asynchronous and returns a promise. So to get the actual answer, the await stops the execution of the function until call() returns.
Line 31: We fetch the token’s symbol using symbol() and call(). The await keyword is used as call() (an asynchronous function).
Line 34: We fetch the token’s total supply using totalSupply() and call(). The await keyword is used as call() (an asynchronous function).
In the code example below, we'll read the balance of an account holding the Tether USD token.
To get the balance, we first need the address of the account that holds the specified token. We can get the address by following the simple steps below. We'll continue from the example above.
Go to the “Holders” tab next to the “Contract” tab.
Click any address from the list.
Copy the address next to “Contract” under “Profile Summary.”
const Web3 = require('web3')
const jsonRpcURL = 'https://mainnet.infura.io/v3/917c4f75f17a4e28b78263cddd8f1b46'
const web3 = new Web3(jsonRpcURL)
const abi = 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onstant":false,"inputs":[{"name":"newOwner","type":"address"}],"name":"transferOwnership","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":false,"inputs":[{"name":"_blackListedUser","type":"address"}],"name":"destroyBlackFunds","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"inputs":[{"name":"_initialSupply","type":"uint256"},{"name":"_name","type":"string"},{"name":"_symbol","type":"string"},{"name":"_decimals","type":"uint256"}],"payable":false,"stateMutability":"nonpayable","type":"constructor"},{"anonymous":false,"inputs":[{"indexed":false,"name":"amount","type":"uint256"}],"name":"Issue","type":"event"},{"anonymous":false,"inputs":[{"indexed":false,"name":"amount","type":"uint256"}],"name":"Redeem","type":"event"},{"anonymous":false,"inputs":[{"indexed":false,"name":"newAddress","type":"address"}],"name":"Deprecate","type":"event"},{"anonymous":false,"inputs":[{"indexed":false,"name":"feeBasisPoints","type":"uint256"},{"indexed":false,"name":"maxFee","type":"uint256"}],"name":"Params","type":"event"},{"anonymous":false,"inputs":[{"indexed":false,"name":"_blackListedUser","type":"address"},{"indexed":false,"name":"_balance","type":"uint256"}],"name":"DestroyedBlackFunds","type":"event"},{"anonymous":false,"inputs":[{"indexed":false,"name":"_user","type":"address"}],"name":"AddedBlackList","type":"event"},{"anonymous":false,"inputs":[{"indexed":false,"name":"_user","type":"address"}],"name":"RemovedBlackList","type":"event"},{"anonymous":false,"inputs":[{"indexed":true,"name":"owner","type":"address"},{"indexed":true,"name":"spender","type":"address"},{"indexed":false,"name":"value","type":"uint256"}],"name":"Approval","type":"event"},{"anonymous":false,"inputs":[{"indexed":true,"name":"from","type":"address"},{"indexed":true,"name":"to","type":"address"},{"indexed":false,"name":"value","type":"uint256"}],"name":"Transfer","type":"event"},{"anonymous":false,"inputs":[],"name":"Pause","type":"event"},{"anonymous":false,"inputs":[],"name":"Unpause","type":"event"}]
const contractAddress = '0xdAC17F958D2ee523a2206206994597C13D831ec7'
const address = '0x69166e49d2fd23e4cbea767d7191be423a7733a5'
const contract = new web3.eth.Contract(abi, contractAddress)
async function readFromSmartContract()
{
balanceOfAccount = await contract.methods.balanceOf(address).call()
console.log("Balance: ", balanceOfAccount)
}
readFromSmartContract() // Function call to fetch the informationThe explanation of the code above is as follows:
Line 13: We fetch the token holder’s account balance using balanceOf() and call(). The balanceOf() is sent an address of the account of the token holder as a parameter. The await keyword is used as call() (an asynchronous function).
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