Executing an contract via code and without metamask / link an account to JS code to automatically wirte on Ethereum

Question

I'm trying to get my JS-code to call a smart contract on Ethereum (kovan testnet) that will write something on the blockchain and therefore needs ether (actually gas). I called a view function which worked fine but I don't know how to get my address and private key into the script so that my function call will be executed by using ether from my address.

var mqtt = require('mqtt');
var client = mqtt.connect('mqtt://localhost:1234');
var topic = 'test123';
const Web3 = require('web3');
const web3 = new Web3(new Web3.providers.HttpProvider("https://kovan.infura.io/v3/---address---"));
var contract;
var address = "0x---address";
var abi = ---abi---;

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

client.on('message', (topic, message)=>{
    message = message.toString()
    console.log("The following data was sent: " + message);
    if (message == '5202'){
        contract.methods.viewNewestComment().call().then(console.log); // this is the function that views only and which works just fine
        contract.methods.addCommentToNewest("hello").call().then(console.log);//this is function that writes on the blockchain
    } else {
        console.log('Number not defined!');
    }
});

client.on('connect', ()=>{
    client.subscribe(topic)
});

So what my code does it takes an MQTT signal and if it is '5202' the code should be executed. Is my problem that I enter with infura and I can't pay with infura? How do I link my address with test ether on it to my JS-code?

I know this question seems pretty basic but I searched many sites and many questions on stackexchange but I can't find a solution. I would love to hear your solution! :)

EDIT with suggestion of goodvibration:

const Web3 = require('web3');
var Tx = require('ethereumjs-tx')
const web3 = new Web3(new Web3.providers.HttpProvider("https://kovan.infura.io/v3/---XXX---"));
var contract;
var address = "---XXX---";
var abi = [---XXX---];



const account = "---XXX---";
const privateKey = process.env.PRIVATE_KEY;
const privateKey2 = "---XXX---";
var comment = "test1234";

contract = new web3.eth.Contract(abi, address);
contract.methods.viewNewestComment().call().then(console.log);

async function addCommentToNewest(web3, privateKey2, comment) {
    const account = web3.eth.accounts.privateKeyToAccount(privateKey2);
    const receipt = await send(web3, account, contract.methods.addCommentToNewest(comment));
    console.log(JSON.stringify(receipt, null, 4));
}

addCommentToNewest()

edit2: I added my smart contract that I deployed on remix, so it may help to understand what I do with my "addCommentToNewest"

pragma solidity ^0.5.12;

contract NewContract {

    struct Location {
        string JSON;
        uint time;
        string comment;
    }

    Location[] locations;

    function createLocation(string memory _json) public {
        locations.push(Location(_json, now, 'no comment'));
    }

    function addComment(string memory _comment, uint _id) public {
        locations[_id].comment = _comment;
    }

    function addCommentToNewest(string memory _comment) public {
        uint _id = locations.length - 1;
        locations[_id].comment = _comment;
    }

    function viewLocation(uint _id) public view returns(string memory) {
        string memory result = locations[_id].JSON;
        return result;
    }

    function viewTimestamp(uint _id) public view returns(uint) {
        uint result = locations[_id].time;
        return result;
    }

    function viewComment(uint _id) public view returns(string memory) {
        string memory result = locations[_id].comment;
        return result;
    }

    function viewEverything(uint _id) public view returns(string memory, uint, string memory) {
        string memory result1 = locations[_id].JSON;
        uint result2 = locations[_id].time;
        string memory result3 = locations[_id].comment;
        return (result1, result2, result3);
    }

    function viewNewestLocation() public view returns(string memory) {
        uint _id = locations.length - 1;
        string memory result = locations[_id].JSON;
        return result;
    } 

    function viewNewestTimestamp() public view returns(uint) {
        uint _id = locations.length - 1;
        uint result = locations[_id].time;
        return result;
    }

    function viewNewestComment() public view returns(string memory) {
        uint _id = locations.length - 1;
        string memory result = locations[_id].comment;
        return result;
    }

    function viewNewestEverything() public view returns(string memory, uint, string memory) {
        uint _id = locations.length - 1;
        string memory result1 = locations[_id].JSON;
        uint result2 = locations[_id].time;
        string memory result3 = locations[_id].comment;
        return (result1, result2, result3);
    }   
}

Answer 1

Try this (tested with web3.js v1.2.1):

async function scan(message) {
    process.stdout.write(message);
    return await new Promise(function(resolve, reject) {
        process.stdin.resume();
        process.stdin.once("data", function(data) {
            process.stdin.pause();
            resolve(data.toString().trim());
        });
    });
}

async function getGasPrice(web3) {
    while (true) {
        const nodeGasPrice = await web3.eth.getGasPrice();
        const userGasPrice = await scan(`Enter gas-price or leave empty to use ${nodeGasPrice}: `);
        if (/^\d+$/.test(userGasPrice))
            return userGasPrice;
        if (userGasPrice == "")
            return nodeGasPrice;
        console.log("Illegal gas-price");
    }
}

async function getTransactionReceipt(web3) {
    while (true) {
        const hash = await scan("Enter transaction-hash or leave empty to retry: ");
        if (/^0x([0-9A-Fa-f]{64})$/.test(hash)) {
            const receipt = await web3.eth.getTransactionReceipt(hash);
            if (receipt)
                return receipt;
            console.log("Invalid transaction-hash");
        }
        else if (hash) {
            console.log("Illegal transaction-hash");
        }
        else {
            return null;
        }
    }
}

async function send(web3, account, transaction, value = 0) {
    while (true) {
        try {
            const options = {
                to      : transaction._parent._address,
                data    : transaction.encodeABI(),
                gas     : await transaction.estimateGas({from: account.address}),
                gasPrice: await getGasPrice(web3),
                value   : value,
            };
            const signed  = await web3.eth.accounts.signTransaction(options, account.privateKey);
            const receipt = await web3.eth.sendSignedTransaction(signed.rawTransaction);
            return receipt;
        }
        catch (error) {
            console.log(error.message);
            const receipt = await getTransactionReceipt(web3);
            if (receipt)
                return receipt;
        }
    }
}

async function addCommentToNewest(web3, privateKey, comment) {
    const account = web3.eth.accounts.privateKeyToAccount(privateKey);
    const receipt = await send(web3, account, contract.methods.addCommentToNewest(comment));
    console.log(JSON.stringify(receipt, null, 4));
}