Testing for emitted events in Hardhat

This example shows how to work with events in Hardhat in general, and it includes a test that illustrates some scenarios.

The tl;dr is that, given a contract like this one:

contract EventEmitter {
  event MyEventWithData(uint, string);

  function emitMyEventWithData(uint x, string calldata s) public {
    emit MyEventWithData(x, s);
  }
}

You can check it in a test like this (notice that this relies on hardhat-waffle):

  it("Should emit MyEventWithData", async function () {
    const EventEmitter = await ethers.getContractFactory("EventEmitter");
    const eventEmitter = await EventEmitter.deploy();
    await eventEmitter.deployed();

    await expect(eventEmitter.emitMyEventWithData(42, "foo"))
      .to.emit(eventEmitter, "MyEventWithData")
      .withArgs(42, "foo");
  });

My program tester

await expect(mycontract.connect(addr1).mint(1, {value: 1000000000000000}))
                .to.emit(mycontract, "Transfer")
                .withArgs("0x0000000000000000000000000000000000000000", addr1.getAddress, 9);