How to make fulfillRandomWords() fail for hardhat unit tests

[RohanNero]

I am trying to cover all of my lines in my smart contract unit tests with hardhat, and I am not sure how to test the Raffle__TransferFailed() error. Is there some way to force this transaction to fail?

(bool success, ) = recentWinner.call{value: address(this).balance}("");
        if (!success) {
            revert Raffle__TransferFailed();
        }

Link to question on StackExchangeETH:

https://ethereum.stackexchange.com/questions/133310/how-to-cause-a-call-to-revert-in-hardhat-test

I have tried implementing a solution I saw to a very similar problem from this stack exchange post.

My solidity TestHelper:

import "../Raffle.sol";

error TestHelper__SaysNoThanks();

contract TestHelper {

    Raffle raffle;

    constructor(address rafAddr) {
        raffle = Raffle(rafAddr);
    }
    receive() external payable{
        revert TestHelper__SaysNoThanks();
    }

    function enterRaffle() public payable {
        raffle.enterRaffle{value: msg.value}();
    }
}

My javascript test:

describe.only("TestHelper for 100% coverage", function () {
              it("should revert if funds aren't transfered to winner", async function () {
                  const contractFactory = await ethers.getContractFactory(
                      "TestHelper",
                      deployer
                  )
                  //console.log(raffle.address)
                  const contract = await contractFactory.deploy(raffle.address)
                  await contract.deployed()
                  //console.log(contract.address)
                  const tx = await contract.enterRaffle({
                      value: raffleEntranceFee,
                  })
                  const player = await raffle.getPlayer(0)
                  console.log(player)
                  network.provider.send("evm_increaseTime", [
                      interval.toNumber() + 1,
                  ])
                  network.provider.send("evm_mine")
                  const txResponse = await raffle.performUpkeep([])
                //   const txReceipt = await txResponse.wait(1)
                //   const requestId = txReceipt.events[1].args.requestId
                //   const players = await raffle.getNumberOfPlayers()
                //   console.log(players.toString())
                  console.log(await raffle.getPlayer(0))
                  console.log(await raffle.getContractBalance())
                  const transactionResponse =
                      await vrfCoordinatorV2Mock.fulfillRandomWords(
                          1,
                          raffle.address
                      )
                  const transactionReceipt = await transactionResponse.wait(1)
                  console.log(await raffle.getContractBalance())
                //   console.log(winnerAddr)
                  //console.log(transactionResponse)
                  const winner = await raffle.getRecentWinner()
                  console.log(winner)
              })
          })

The problem I encounter now is that the "transactionResponse" doesn't fail, but it also doesn't send the funds, revert, or emit the WinnerPicked() event.
I have used console.logs to see that the Raffle contract balance doesn't change after fulfillRandomWords() is called. Also the recentWinenr is the default 0x00000000 address, despite the TestHelper address being entered into the raffle. The address is even returned when I call raffle.getPlayer(0).

Here is what my terminal returns:

[krakxn]

The only causative factor here is address(this).balance -- besides that, there seems to be no other way for this to fail.

[RohanNero]

I saw on here that they were able to make it fail using a helper contract that can't accept funds, but I am having trouble implementing something similar.

[krakxn]

You could hard-code a gas limit.

Or, instead of this:

if (!success) {
            revert Raffle__TransferFailed();
        }

Utilize a conditional statement which checks for the conditions for the txn to fail -- before using the call keyword. This also goes hand-in-hand with the fact that we should never use call before all the checks pass.

Cheers.

[RohanNero]

I'll give it a try now, thank you!

[krakxn]

All good!