Chai closeTo on Ethereum Events

Question

I know that we can use closeTo chai matcher like this:

expect(BigNumber.from(100)).to.be.closeTo(BigNumber.from(101), 10);

And we can test whether a transaction emits an event with specific arguments like this:

await expect(contract.addAddress("street", "city")).to.emit(contract, 'AddAddress').withArgs("street", "city");

But how can we use closeTo matcher with to.emit. so that we can check whether transaction emits an event with specific arguments that are close to some value?

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Reference: https://ethereum-waffle.readthedocs.io/en/latest/matchers.html

Answer 1

I don't think there's an easy way to do this, at least right now. As an alternative you can do something like this:

//SPDX-License-Identifier: Unlicense
pragma solidity ^0.8.0;

contract Test {
  event MyEvent(uint x);

  function emitEvent(uint n) public {
    emit MyEvent(n);
  }
}

const { expect } = require("chai");
const { ethers } = require("hardhat");
const { BigNumber } = ethers;

describe("Test", function () {
  it("Emit event close to", async function () {
    const Test = await ethers.getContractFactory("Test");
    const test = await Test.deploy();
    await test.deployed();

    const txPromise = test.emitEvent(100);

    await expect(txPromise).to.emit(test, 'MyEvent')

    const receipt = await (await txPromise).wait();

    const myEvent = receipt.events[0];
    const value = myEvent.args[0];

    expect(value).to.be.closeTo(BigNumber.from(100), 1)
  });
});

It's not pretty, and it assumes that the tx only emits that single event. But you could create a helper function that abstracts this away and gets the proper event by filtering all the events, for example.