Lesson

Read Data from Smart Contracts with Web3.js

In order to read data from smart contracts with Web3.js, we need two things:

  1. A JavaScript representation of the smart contract we want to interact with
  2. A way to call the functions on the smart contract when reading the data

We can get a JavaScript representation of an Ethereum smart contract with the web3.eth.Contract() function. This function expects two arguments: one for the smart contract ABI and one for the smart contract address.

A smart contract ABI stands for “Abstract Binary Interface”, and is a JSON array that describes how a specific smart contract works. Here is an example an ABI:

const abi = [{"constant":true,"inputs":[],"name":"mintingFinished","outputs":[{"name":"","type":"bool"}],"payable":false,"type":"function"},{"constant":true,"inputs":[],"name":"name","outputs":[{"name":"","type":"string"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_spender","type":"address"},{"name":"_value","type":"uint256"}],"name":"approve","outputs":[],"payable":false,"type":"function"},{"constant":true,"inputs":[],"name":"totalSupply","outputs":[{"name":"","type":"uint256"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_from","type":"address"},{"name":"_to","type":"address"},{"name":"_value","type":"uint256"}],"name":"transferFrom","outputs":[],"payable":false,"type":"function"},{"constant":true,"inputs":[],"name":"decimals","outputs":[{"name":"","type":"uint256"}],"payable":false,"type":"function"},{"constant":false,"inputs":[],"name":"unpause","outputs":[{"name":"","type":"bool"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_to","type":"address"},{"name":"_amount","type":"uint256"}],"name":"mint","outputs":[{"name":"","type":"bool"}],"payable":false,"type":"function"},{"constant":true,"inputs":[],"name":"paused","outputs":[{"name":"","type":"bool"}],"payable":false,"type":"function"},{"constant":true,"inputs":[{"name":"_owner","type":"address"}],"name":"balanceOf","outputs":[{"name":"balance","type":"uint256"}],"payable":false,"type":"function"},{"constant":false,"inputs":[],"name":"finishMinting","outputs":[{"name":"","type":"bool"}],"payable":false,"type":"function"},{"constant":false,"inputs":[],"name":"pause","outputs":[{"name":"","type":"bool"}],"payable":false,"type":"function"},{"constant":true,"inputs":[],"name":"owner","outputs":[{"name":"","type":"address"}],"payable":false,"type":"function"},{"constant":true,"inputs":[],"name":"symbol","outputs":[{"name":"","type":"string"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_to","type":"address"},{"name":"_value","type":"uint256"}],"name":"transfer","outputs":[],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_to","type":"address"},{"name":"_amount","type":"uint256"},{"name":"_releaseTime","type":"uint256"}],"name":"mintTimelocked","outputs":[{"name":"","type":"address"}],"payable":false,"type":"function"},{"constant":true,"inputs":[{"name":"_owner","type":"address"},{"name":"_spender","type":"address"}],"name":"allowance","outputs":[{"name":"remaining","type":"uint256"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"newOwner","type":"address"}],"name":"transferOwnership","outputs":[],"payable":false,"type":"function"},{"anonymous":false,"inputs":[{"indexed":true,"name":"to","type":"address"},{"indexed":false,"name":"value","type":"uint256"}],"name":"Mint","type":"event"},{"anonymous":false,"inputs":[],"name":"MintFinished","type":"event"},{"anonymous":false,"inputs":[],"name":"Pause","type":"event"},{"anonymous":false,"inputs":[],"name":"Unpause","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"}]

That’s a really long unbroken array, I know! Don’t worry if it looks overwhelming. 🙂 This example is the ABI for the OmiseGo token, which implements the ERC-20 token standard (if you’re unfamiliar with this standard, you can watch this video). You can find more details about this token, including its abi and address on Etherscan. We’ll use this smart contract ABI for the rest of the example.

While we’re here, I’ll go ahead and store the address to the OMG token from the Ethereum main net:

const address = "0xd26114cd6EE289AccF82350c8d8487fedB8A0C07"

Now that we have both of these values assigned, we can create a complete JavaScript representation of the OMG token smart contract like this:

const contract = new web3.eth.Contract(abi, address)

Now that the first part of this lesson is complete, we need to fulfill the second part: reading data from the smart contract by calling its functions. All of the smart contract functions are listed under the contract.methods namespace within the assigned Web3 contract. For example, we can call contract.methods.myFunction() if the contract implements myFunction().

Great! So we can theoretically call any function that the smart contract implements. But how do we know which functions it implements? For one, we could log contract.methods to the console, and see what’s returned (you can watch me do this in the video above). However, since this smart contract implements the ERC-20 standard, we know that it implements several functions like totalSupply()name()symbol(), and balanceOf(). We can read each of those values individually, like this:

First, the total supply of all OMG tokens in existence:

contract.methods.totalSupply().call((err, result) => { console.log(result) })
// > 140245398

Second, the name of the OMG token:

contract.methods.name().call((err, result) => { console.log(result) })
// > OMG Token

Third, the symbol of the OMG token:

contract.methods.symbol().call((err, result) => { console.log(result) })
// > OMG

Last, we can check the balance for a given account. I looked for a rich OMG holder on Etherscan, and found this address 0xd26114cd6EE289AccF82350c8d8487fedB8A0C07. We can check the balance for this account like this:

contract.methods.balanceOf('0xd26114cd6EE289AccF82350c8d8487fedB8A0C07').call((err, result) => { console.log(result) })
// > A very large number...

And that’s it! That’s how easy it is to read data from smart contracts with Web3.js. Here is a summary of all the code from this lesson:

const Web3 = require('web3')
const rpcURL = '' // Your RCP URL goes here
const web3 = new Web3(rpcURL)

const abi = [{"constant":true,"inputs":[],"name":"mintingFinished","outputs":[{"name":"","type":"bool"}],"payable":false,"type":"function"},{"constant":true,"inputs":[],"name":"name","outputs":[{"name":"","type":"string"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_spender","type":"address"},{"name":"_value","type":"uint256"}],"name":"approve","outputs":[],"payable":false,"type":"function"},{"constant":true,"inputs":[],"name":"totalSupply","outputs":[{"name":"","type":"uint256"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_from","type":"address"},{"name":"_to","type":"address"},{"name":"_value","type":"uint256"}],"name":"transferFrom","outputs":[],"payable":false,"type":"function"},{"constant":true,"inputs":[],"name":"decimals","outputs":[{"name":"","type":"uint256"}],"payable":false,"type":"function"},{"constant":false,"inputs":[],"name":"unpause","outputs":[{"name":"","type":"bool"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_to","type":"address"},{"name":"_amount","type":"uint256"}],"name":"mint","outputs":[{"name":"","type":"bool"}],"payable":false,"type":"function"},{"constant":true,"inputs":[],"name":"paused","outputs":[{"name":"","type":"bool"}],"payable":false,"type":"function"},{"constant":true,"inputs":[{"name":"_owner","type":"address"}],"name":"balanceOf","outputs":[{"name":"balance","type":"uint256"}],"payable":false,"type":"function"},{"constant":false,"inputs":[],"name":"finishMinting","outputs":[{"name":"","type":"bool"}],"payable":false,"type":"function"},{"constant":false,"inputs":[],"name":"pause","outputs":[{"name":"","type":"bool"}],"payable":false,"type":"function"},{"constant":true,"inputs":[],"name":"owner","outputs":[{"name":"","type":"address"}],"payable":false,"type":"function"},{"constant":true,"inputs":[],"name":"symbol","outputs":[{"name":"","type":"string"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_to","type":"address"},{"name":"_value","type":"uint256"}],"name":"transfer","outputs":[],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_to","type":"address"},{"name":"_amount","type":"uint256"},{"name":"_releaseTime","type":"uint256"}],"name":"mintTimelocked","outputs":[{"name":"","type":"address"}],"payable":false,"type":"function"},{"constant":true,"inputs":[{"name":"_owner","type":"address"},{"name":"_spender","type":"address"}],"name":"allowance","outputs":[{"name":"remaining","type":"uint256"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"newOwner","type":"address"}],"name":"transferOwnership","outputs":[],"payable":false,"type":"function"},{"anonymous":false,"inputs":[{"indexed":true,"name":"to","type":"address"},{"indexed":false,"name":"value","type":"uint256"}],"name":"Mint","type":"event"},{"anonymous":false,"inputs":[],"name":"MintFinished","type":"event"},{"anonymous":false,"inputs":[],"name":"Pause","type":"event"},{"anonymous":false,"inputs":[],"name":"Unpause","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"}]
const address = '0xd26114cd6EE289AccF82350c8d8487fedB8A0C07'

const contract = new web3.eth.Contract(abi, address)

contract.methods.totalSupply().call((err, result) => { console.log(result) })
contract.methods.name().call((err, result) => { console.log(result) })
contract.methods.symbol().call((err, result) => { console.log(result) })
contract.methods.balanceOf('0xd26114cd6EE289AccF82350c8d8487fedB8A0C07').call((err, result) => { console.log(result) })

You can also download all the code examples in this tutorial series from github.

Course Discussion