Hello Folks,

So in this session we will be discussing about How to call smart contract functions using web3 from nodejs.

To do that we need to have web3 Node module installed on our system.

We can do this by running the following code on the cmd

npm install web3

Then let's run our node server. Just run the following code on cmd.

node

Then we need web3 module in our server so run the following code

var Web3 = require('web3')

we will be needing a Blockchain network to create an account  so get an rpc url from here

Just click on the link -> Login to your account ->Click on ethereum on left side bar ->create project ->settings->keys->endpoints 

and choose your respective testnet rpc url

rpc url it will look like this - https://mainnet.infura.io/v3/your_api_key

 var url = 'your RPC url here'

Then run this one

var web3 = new Web3(url)

For testing purpose we have chosen an address from ethereum mainnet and we look at it's balance to convert it's units to other ether units

let's use this to get mainnet transactions of an address

so for this session we have choosen OMG Token as an example.

We will be needing it's address and abi.That can be found here

Let's create variables to store contract address and abi; that will look like this

var address = '0xd26114cd6EE289AccF82350c8d8487fedB8A0C07'

var contractAddress = [{"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"}]

Now to get an instance of the contract.

Try the following code

var contract = new web3.eth.Contract(abi,contractAddress) 

Now we have the instance of contract inside "contract" variable 

To see all the methods/ functions defined inside the "contract" variable.

Try this code

contract.methods

And it will show all the listed methods stored in the smart contract.

Let's call a method named "name( )" listed in the smart contract

contract.methods.name().call()

This will return a promise.

To read the value we need to use a callback function inside "the call()" method

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

Now this will return you the name of the token 

Let's know the symbol of this ERC Token by similar manner using "symbol( )" method

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

Let's know how many OMG tokens exists by using "totalSupply( )" method in similar manner

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

Let's get balance of someone who owns it by calling "balanceOf(address)" method.Yo can know who owns it from etherscan itself.

var accountAddress ='0x24BA1542F8a0a20e8251d096213384Cfb0eE3dbC'

contract.methods.balanceOf(accountAddress).call((err,result)=>{console.log(result)})

For test ethers and link tokens. Click Here

For many more exciting tutorials keep following my username @BlockTalks_Raj

!!!!!HAPPY LEARNING!!!!