how to use private keys when working with smart contract

Question

Here is my scenario . We have wallets (accounts) with private keys . In case of raw transaction I know how I can use private keys for signing tx . How I can use private key for an account when I am calling smart contract methods ? Thanks

Answer 1

Here are three functions I use a lot to interact with a smart contract and a signed transaction:

function sendTransaction(tx) {
    return new Promise((resolve, reject) => {
        web3g.eth.sendSignedTransaction(tx.rawTransaction)
            .on('transactionHash', function (hash) {
                resolve(hash);
            });
    });
}
// tx = the transaction object from "generateTx"
function signTransaction(tx, private_key) {
    return new Promise((resolve, reject) => {
        web3g.eth.accounts.signTransaction(tx, private_key)
            .then(function (signed_tx) {
                resolve(signed_tx);
            }).catch(function (error) {
                console.log(error);
                reject("request failed");
            });
    });
}

// instance = contract instance
// body = some parameters
function generateTx(body, instance) {
    return new Promise((resolve, reject) => {
        try {
            let encoded_tx_data = instance.methods.transfer(body.destinationAddress, body.transferAmount).encodeABI();
            let tx_object = {
                gas: 2000000,
                data: encoded_tx_data,
                from: body.senderAddress,
                to: body.contractAddress
            };
            resolve(tx_object);
        } catch (error) {
            console.log(error);
            reject("request failed");
        }
    });
}