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Ethereum Hello World Example Using solc and web3

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Ethereum Hello World Example Using solc and web3

Looking for an example of how to get an Ethereum Smart Contract up and running quickly? Then look no further and come check out this post!

· Security Zone ·
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I’ve been trying to find an Ethereum Hello World example and came across Thomas Conté’s excellent post that shows how to compile and deploy an Ethereum smart contract with solc and web3.

In the latest version of web3, the API has changed to be based on promises so I decided to translate Thomas’s example.

Let’s get started.

Install npm Libraries

We need to install these libraries before we start:

npm install web3
npm install abi-decoder
npm install ethereumjs-testrpc

What do these libraries do?

  • web3 is a client library for interacting with an Ethereum blockchain.
  • abi-decoder is used to decode the hash of a smart contract so that we can work out what was in it.
  • ethereum-testrpc lets us spin up a local test version of Ethereum.

Smart Contract

We’ll still use the same smart contract as Thomas did. Token.sol is a smart contract written in the Solidity language and describes money being transferred between addresses:

contracts/Token.sol

pragma solidity ^0.4.0;

contract Token {
    mapping (address => uint) public balances;

    function Token() {
        balances[msg.sender] = 1000000;
    }

    function transfer(address _to, uint _amount) {
        if (balances[msg.sender] < _amount) {
            throw;
        }

        balances[msg.sender] -= _amount;
        balances[_to] += _amount;
    }
}

Whenever somebody tries to transfer some money we’ll put 1,000,000 in their account and then transfer the appropriate amount, assuming there’s enough money in the account.

Start Local Ethereum Node

Let’s start a local Ethereum node. We’ll reduce the gas price – the amount you ‘pay’ to execute a transaction – so we don’t run out.

$ ./node_modules/.bin/testrpc --gasPrice 20000
EthereumJS TestRPC v6.0.3 (ganache-core: 2.0.2)

Listening on localhost:8545

Pre-Requisites

We need to load a few Node.js modules:

const fs = require("fs"),
      abiDecoder = require('abi-decoder'),
      Web3 = require('web3'),
      solc = require('solc');

Compile Smart Contract

Next, we’ll compile our smart contract:

const input = fs.readFileSync('contracts/Token.sol');
const output = solc.compile(input.toString(), 1);
const bytecode = output.contracts[':Token'].bytecode;
const abi = JSON.parse(output.contracts[':Token'].interface);

Connect to Ethereum and Create Contract Object

Now that we’ve got the ABI (Application Binary Interface), we’ll connect to our local Ethereum node and create a contract object based on the ABI:

let provider = new Web3.providers.HttpProvider("http://localhost:8545");
const web3 = new Web3(provider);
let Voting = new web3.eth.Contract(abi);

Add ABI to Decoder

Before we interact with the blockchain, we’ll first add the ABI to our ABI decoder to use later:

abiDecoder.addABI(abi);

Find (Dummy) Ethereum Accounts

Now we’re ready to create some transactions! We’ll need some Ethereum accounts to play with and if we call web3.eth.getAccounts we can get a collection of accounts that the node controls. Since our node is a test one, these are all dummy accounts.

web3.eth.getAccounts().then(accounts => {
  accounts.forEach(account => {
    console.log(account)
  })
});
0xefeaE7B180c7Af4Dfd23207422071599c7dfd2f7
0x3a54BaAFDe6747531a28491FDD2F36Cb61c83663
0x367e1ac67b9a85E438C7fab7648964E5ed12061e
0xB34ECD20Be6eC99e8e9fAF641A343BAc826FFFf1
0xE65587a2951873efE3325793D5729Ef91b15d5b5
0xdA232aEe954a31179E2F5b40E6efbEa27bB89c87
0x7119fEbab069d440747589b0f1fCDDBAdBDd105d
0xCacB2b61dE0Ca12Fd6FECe230d2f956c8Cdfed34
0x4F33BF93612D1B89C8C8872D4Af30Fa2A9CbfaAf
0xA1Ebc0D19dB41A96B5278720F47C2B6Ab2506ccF

Transfer Money between Accounts

Now that we have some accounts, let’s transfer some money between them.

var allAccounts;
web3.eth.getAccounts().then(accounts => {
  allAccounts = accounts;
  Voting.deploy({data: bytecode}).send({
    from: accounts[0],
    gas: 1500000,
    gasPrice: '30000000000000'
  }).on('receipt', receipt => {
    Voting.options.address = receipt.contractAddress;
    Voting.methods.transfer(accounts[1], 10).send({from: accounts[0]}).then(transaction => {
      console.log("Transfer lodged. Transaction ID: " + transaction.transactionHash);
      let blockHash = transaction.blockHash
      return web3.eth.getBlock(blockHash, true);
    }).then(block => {
      block.transactions.forEach(transaction => {
        console.log(abiDecoder.decodeMethod(transaction.input));
      });

      allAccounts.forEach(account => {
          Voting.methods.balances(account).call({from: allAccounts[0]}).then(amount => {
            console.log(account + ": " + amount);
          });
      });
    });
  });
});

Let’s run it:

Transfer lodged. Transaction ID: 0x699cbe40121d6c2da7b36a107cd5f28b35a71aff2a0d584f8e734b10f4c49de4

{ name: 'transfer',
  params: 
   [ { name: '_to',
       value: '0xeb25dbd0931386eeab267981626ae3908d598404',
       type: 'address' },
     { name: '_amount', value: '10', type: 'uint256' } ] }

0x084181d6fDe8bA802Ee85396aB1d25Ddf1d7D061: 999990
0xEb25dbD0931386eEaB267981626AE3908D598404: 10
0x7deB2487E6Ac40f85fB8f5A3bC6896391bf2570F: 0
0xA15ad4371B62afECE5a7A70457F82A30530630a3: 0
0x64644f3B6B95e81A385c8114DF81663C39084C6a: 0
0xBB68FF2935080c807D5A534b1fc481Aa3fafF1C0: 0
0x38d4A3d635B451Cb006d63ce542950C067D47F58: 0
0x7878bA9138361A08522418BD1c8376Af7220a506: 0
0xf400c0e749Fe02E7073E08d713E0A207dc91FBeb: 0
0x7070d1712a25eb7FCf78A549F17705AA66B0aD47: 0

This code:

  • Deploys our smart contract to the blockchain.
  • Transfers £10 from account 1 to account 2.
  • Decodes that transaction and shows the output.
  • Shows the balances of all the dummy accounts.

The full example is available in my ethereum-nursery GitHub repository. Thomas also has a follow-up post that shows how to deploy a contract on a remote node where client-side signatures become necessary.

Find out how Waratek’s award-winning application security platform can improve the security of your new and legacy applications and platforms with no false positives, code changes or slowing your application.

Topics:
smart contracts ,ethereum ,node.js ,security ,blockchain development

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