Sending transaction with web3.js

Question

I'm trying to use web3.js with infura.io to interact with my smartcontract. here is my simple greeter contract code

pragma solidity ^0.5.7;
contract greeter{
    string greeting;

    function greet(string memory _greeting)public {
        greeting=_greeting;
    }
    function getGreeting() public view returns(string memory) {
        return greeting;
    }
}

Here is what I found from web3.js documentation.

var tx = web3.eth.sendTransaction({
             from: '0x3dCDe57Ad49d639c4D702b607Dd5eBc0aB54A671',
             nonce: web3.toHex(txCount),
             gasLimit: web3.toHex(1000000),
             gasPrice: web3.toHex(web3.toWei('10', 'gwei')),
             data:0x756774689538938966626728299293638893o
         });
web3.eth.accounts.signTransaction(tx, privateKey [, callback]);

But it doesn't take me anywhere. Because,where can I pass greeting data? All the documentation talks about sending ether. Then what about sending string data like this? with web3.js 0.20 I can read greeting from chain. I wanted to add new greeting string with greet function. but somehow I have no idea about signing transaction. could anyone help me in understanding how to sign transacion and adding greeting to the contract using web3.js?

Answer 1

You are not providing too much informations for me to give you an easy way to do that :) , anyway this is one way to how you can send transaction:

Better to use web3 1.0, you need also install ethereumjs-tx: npm install ethereumjs-tx

const Tx = require('ethereumjs-tx');

to get instance of your contract:

new web3.eth.Contract(jsonInterface, address, options)

Example:

const myContract = new web3.eth.Contract([...], '..address of your contract..', {
    defaultAccount: '....', // default from address
    defaultGasPrice: '20000000000' // default gas price in wei, 20 gwei in this case
});

and to get the data for the transaction parameter:

const mydata = myContract.methods.greet("<your-greeting.string>").encodeABI()));

to Sign and Send

const privateKey = Buffer('<your privatekey>', 'hex')
    
const rawTx = {
    nonce: <your-tx-nonce.. you can also get it with web3 command >,
    gasLimit: web3.toHex(1000000),
    gasPrice: web3.toHex(web3.toWei('10', 'gwei')),
    to: <your contract address>,
    value: '0x00',
    data: mydata 
}
    
const tx = new Tx(rawTx);
tx.sign(privateKey);
    
const serializedTx = tx.serialize();
    
web3.eth.sendSignedTransaction('0x' + serializedTx.toString('hex'))
        .on('receipt', console.log);

https://web3js.readthedocs.io/en/1.0/web3-eth.html#sendsignedtransaction https://web3js.readthedocs.io/en/1.0/web3-eth-abi.html?highlight=encode