Web3js – How to Interact with Smart Contracts on Ropsten

You can do something like this:

const fs   = require("fs");
const Web3 = require("web3");

const NODE_ADDRESS  = process.argv[2];
const PRIVATE_KEY   = process.argv[3];
const CONTRACT_NAME = process.argv[4];

const ARTIFACTS_DIR = __dirname + "<path to your bin/abi folder relative to this script>";
const MIN_GAS_LIMIT = 100000; // increase this if necessary

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, gasPrice, transaction, value = 0) {
    while (true) {
        try {
            const options = {
                to      : transaction._parent._address,
                data    : transaction.encodeABI(),
                gas     : Math.max(await transaction.estimateGas({from: account.address}), MIN_GAS_LIMIT),
                gasPrice: gasPrice ? 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 deploy(web3, account, gasPrice, contractName, contractArgs) {
    const abi = fs.readFileSync(ARTIFACTS_DIR + contractName + ".abi", {encoding: "utf8"});
    const bin = fs.readFileSync(ARTIFACTS_DIR + contractName + ".bin", {encoding: "utf8"});
    const contract = new web3.eth.Contract(JSON.parse(abi));
    const options = {data: "0x" + bin, arguments: contractArgs};
    const transaction = contract.deploy(options);
    const receipt = await send(web3, account, gasPrice, transaction);
    return new web3.eth.Contract(JSON.parse(abi), receipt.contractAddress);
}

async function run() {
    const web3 = new Web3(NODE_ADDRESS);

    const gasPrice = await getGasPrice(web3);
    const account  = web3.eth.accounts.privateKeyToAccount(PRIVATE_KEY);

    const myContract = await deploy(web3, account, gasPrice, CONTRACT_NAME, [arg1, arg2]);
    console.log(CONTRACT_NAME, "deployed at", myContract._address);

    const receipt = await send(web3, account, gasPrice, myContract.mint(arg3, arg4));
    console.log("Mint receipt:", JSON.stringify(receipt, null, 4));

    const challengeNumber = await myContract.methods.getChallengeNumber().call();
    console.log(Web3.utils.hexToAscii(challengeNumber));

    if (web3.currentProvider.constructor.name == "WebsocketProvider")
        web3.currentProvider.connection.close();
}

run();

And then call it from command-line, for example:

node MyScript.js https://ropsten.infura.io/v3/<MyProjectId> <MyPrivateKey> <MyContractName>

Tested with:

• NodeJS v10.16.0
• Web3.js v1.2.1

Note that:

• I've used arg1/arg2 for arguments passed to your contract's constructor as an example, because you haven't specified how many there are and what values you want to set them to
• I've used arg3/arg4 for arguments passed to your contract's mint function as an example, because you haven't specified what values you want to set them to

Referring to your updated question:

const abi = json_obj['abi'];
const bin = json_obj['bytecode'];
const contract = new web3.eth.Contract(JSON.parse(abi));
const options = {data: "0x" + bin}; // , arguments: contractArgs};
// I got rid of this because my constructor doesn't accept any arguments

1. Cosmetic: in Javascript, you can do json_obj.abi and json_obj.bytecode
2. Optional: instead of ommiting arguments altogether, you can use arguments: []
3. Your problem: json_obj.bytecode already starts with 0x, so change "0x" + bin to bin