Introduction

My default way of connecting to a local instance of the Ethereum blockchain using Truffle Ganache is via the browser using Metamask. Using ethers to connect via node.js is however a little different. As the Ethers documentation, at time of writing, contains few specific mentions of Truffle (primarily I suspect as the lead developer doesn't use it), I got a couple of call revert exception errors before I figured out what I was doing wrong.

Setup

Just in case you are completely new to this, some initial set up steps at the terminal:

mkdir justsayhi
cd justsayhi
truffle init
npm init -y
npm install ethers

I am assuming you have already installed Node, Truffle and Ganache, if not see my earlier post PreciousChickenToken: A guided example of OpenZeppelin's ERC20 using Ethers, Truffle and React.

Now would also be a good time to fire up Ganache, selecting the Quickstart Ethereum option when offered.

The JavaScript

Create a file node_server.js and copy and paste:

var ethers = require('ethers');
var JustSayHi = require('./build/contracts/JustSayHi.json');

// This is the localhost port Ganache operates on
const url = "http://127.0.0.1:7545";
const provider = ethers.providers.getDefaultProvider(url);

const contractAddress ='0x02e68a4a2B539451F7d02b166B3376DBc7473F75';

// Connect to the network
// We connect to the Contract using a Provider, so we will only
// have read-only access to the Contract
let contract = new ethers.Contract(contractAddress, JustSayHi.abi, provider);

try {
    contract.sayHi().then(msg => console.log(msg));
} catch (e) {
    console.log(e);
}

The SmartContract

Create a file contracts/JustSayHi.sol and copy and paste:

// SPDX-License-Identifier: Unlicencse
pragma solidity ^0.5.1;

contract JustSayHi {
    function sayHi() public pure returns (string memory) {
        return "Hi";
    }
}

Create a file migrations/2_deploy_contract.js and copy and paste:

var JustSayHi = artifacts.require("JustSayHi");

module.exports = function(deployer) {
  // Arguments are: contract
  deployer.deploy(JustSayHi);
};

Deploy the contract

From the terminal run:

truffle deploy

If all good we should get a screen that looks like:

Copy the highlighted contract address above and paste it into the contractAddress variable line in node_server.js, which is this line in my example above:

const contractAddress ='0x02e68a4a2B539451F7d02b166B3376DBc7473F75';

Run using Node.js

From the terminal:

node node_server.js

And if all is well you should see a nice, friendly Hi in response.

Configuration

At time of writing I'm using: Truffle v5.1.34 (core: 5.1.34), Solidity v0.5.16 (solc-js), Node v14.4.0, Web3.js v1.2.1, ethers v5.07 and Ubuntu 20.04 LTS.