Listening to events using ethers.js on a hardhat test network

Question

I am currently migrating a truffle project to hardhat. I am facing a problem with listening to events. I have a simple ERC20 token and want to listen to the Transfer event. I am running the contract on a hardhat test network. Here is my test code, which does not catch the Transferevent. Any hits on what I am doing wrong is highly appreciated:

import { ethers } from "hardhat";
import chai from "chai";
import { solidity } from "ethereum-waffle";
import { FlanToken } from "../typechain/MyToken";
import { BigNumber} from "ethers";

chai.use(solidity);
const { assert } = chai;


describe("MyToken", () => {

    let myToken: MyToken;
    let accounts: any;

    beforeEach(async () => {
        accounts = await ethers.getSigners();
        const tokenFactory = await ethers.getContractFactory("MyToken", accounts[0]);
        myToken = (await myToken.deploy()) as MyToken;
        await myToken.deployed();
    });

    describe("token transfer", async () => {

        it("valid transfer", async () => {
            myToken.on("Transfer", (_from,_to,_value) => {
                console.log(_from,_to,_value);
            });

            await myToken.connect(accounts[0]).transfer(accounts[1].address, 1);

            // balance checks
            let balanceSender: BigNumber = await myToken.balanceOf(accounts[0].address);
            let balanceReceiver: BigNumber = await myToken.balanceOf(accounts[1].address);
            assert.equal(balanceSender.toNumber(), 999999, "Unexpected balance of sender afer token transfer.");
            assert.equal(balanceReceiver.toNumber(), 1, "Unexpected balance of receiver afer token transfer.");
        })
    });

});

Answer 1

While I did not find a solution on why the .on() method is not working, I found a workaround using the transaction's receipt. The following code describes this workaround:

let tx: ContractTransaction = await myToken.connect(accounts[0]).transfer(accounts[1].address, 1);

let receipt: ContractReceipt = await tx.wait();
console.log(receipt.events?.filter((x) => {return x.event == "Transfer"}));

Answer 2

As far as I can tell it isn't possible to actually use the arguments which are emitted, however you can test for them being emitted using the following chai code example:

await expect(token.transfer(walletTo.address, 7))
  .to.emit(token, 'Transfer')
  .withArgs(wallet.address, walletTo.address, 7);

Here is the reference

Update:

You can also use the emitted arguments, for instance:

Contract:

event Transfer(
        address indexed from, 
        address indexed to, 
        uint256 value
    );

function transfer(address _to, uint256 _value) public {
   emit Transfer(msg.sender, _to, _value);
}

Test:

const transferTx = await token.connect(bob.address).transfer(alice.address, 10);
const result = await transferTx.wait();
expect(result.events[0].args._from).to.equal(bob.address);
expect(result.events[0].args._to).to.equal(alice.address);
expect(result.events[0].args._value).to.equal(10);

Answer 3

Check-out this example:

async function main() {
  const EventEmitter = await ethers.getContractFactory("EventEmitter");
  const eventEmitter = await EventEmitter.deploy();
  await eventEmitter.deployed();

  const tx = await eventEmitter.emitBothEvents(42, "foo");

  const receipt = await tx.wait()

  for (const event of receipt.events) {
    console.log(`Event ${event.event} with args ${event.args}`);
  }
}

main()
  .then(() => process.exit(0))
  .catch(error => {
    console.error(error);
    process.exit(1);
  });

from https://github.com/fvictorio/hardhat-examples/blob/master/reading-events/scripts/getEventsFromTx.js.