UnhandledPromiseRejectionWarning Insufficient funds error in Ethereum NodeJS web3

Question

I am using web3.js v1.0.0-beta.34 & nodeJS v9.11.2 to execute a smart contract on the Kovantestnet. The same method works for me on Ropsten with another smart contract. Here are the two errors I get via the callback:

1.) UnhandledPromiseRejectionWarning: Error: Returned error: Insufficient funds. The account you tried to send transaction from does not have enough funds. Required 183675000000 and got: 0.

This is my smart contract:

pragma solidity ^0.4.24;

contract Test2 {
    address public customer;
    bytes32 public productName;

    struct Box {
        uint size;
    }
    Box public box;

    constructor() public {
        box.size = 3;
        customer = 0xDa3E3C75....;
        productName = "0x576...";
    }

    function changeBox(uint _change) public {
        box.size = _change;
    }

    function getBox() public returns (uint) {
        return box.size;
    }  
}

And here is the JavaScript code to make a transaction and execute the function changeBox with web3 and node:

const Tx = require('ethereumjs-tx');
var Web3 = require('web3'); 


var web3 = new Web3(new Web3.providers.HttpProvider('https://kovan.infura.io/api_key'));
const contractAddress = '0x36075430619b21Fff798454e2D5C81E9C18DEe81';
var contractABI = new web3.eth.Contract(
    [...json abi...], contractAddress);
var boxNum;


function changeBox(boxNum, callback) {
    web3.eth.defaultAccount = "0x002D189c25958c60...";
    const account = '0x002D189c2595...';
    const privateKey = Buffer.from('240462d5...', 'hex');
    const contractFunction = contractABI.methods.changeBox(Number(boxNum));
    const functionAbi = contractFunction.encodeABI();
    let estimatedGas;
    let nonce;

    contractFunction.estimateGas(function(error, gasAmount) {
        if(!error) {
            console.log('Estimated Gas : ' + gasAmount);
            estimatedGas = gasAmount + 10000;
            console.log('New Gas: ' + estimatedGas);

            web3.eth.getTransactionCount(account).then(_nonce => { 
                nonce = _nonce.toString(16);
                console.log("Nonce: " + nonce);

                const txParams = {
                    gasPrice: estimatedGas,
                    gasLimit: 5000000,
                    to: contractAddress,
                    data: functionAbi,
                    from: account,
                    nonce: '0x' + nonce
                };
                const tx = new Tx(txParams);
                tx.sign(privateKey);
                const serializedTx = tx.serialize();
                web3.eth.sendSignedTransaction('0x' + serializedTx.toString('hex')).on('receipt', receipt => {
                    callback(receipt);

                });
            });  
        } 
        else {
            callback(error);
        }
    });
}

//calling the contract with value 6
changeBox(6, function(err, data) {
    if (!err) {
        console.log(data);
    }
    else {
        console.log(err);
    }});

Answer 1

sendSignedTransaction returns a promiEvent onto which you can chain then and catch:

web3.eth.sendSignedTransaction('0x' + serializedTx.toString('hex'))
  .on('receipt', receipt => {
    callback(receipt);
  }).then(() => {
    // success
  }).catch(() => {
    // fail
  });

Unhandled promise rejection is thrown because the promise gets rejected but there is no catchhandler.

Hope this helps