Testing Error thrown with hardhat

Question

How would you test the following function with solidity v0.7 and hardhat v2.3.3?

function myFunction(int8 _num) public {
        require(_num > 5, "Num should be bigger than 5");
        ...
}

I've tried:

try {
         await contract.myFunction(3);
} catch(error) {
         expect(error).to.equals("Num should be bigger than 5");
};

And also:

expect.fail(await contract.myFunction(3));

But I always get:

Error: VM Exception while processing transaction: revert Num should be bigger than 5

Which seems to me the Virtual Machine breaking off with the test failing in the spot, without even having the chance to catch the error.

possible values for that var are 1, 2 and 3 @MajdTL. That's why I'm using int8. Not sure it would safe gas, thou. I did not research that. — ntonnelier, Jun 23 '21 at 11:56
no it will cost you more gas, because the default is int and you changing it to int8 — Majd TL, Jun 23 '21 at 13:11
https://ethereum.stackexchange.com/questions/3067/why-does-uint8-cost-more-gas-than-uint256 — Majd TL, Jun 23 '21 at 13:35

score 16 · Accepted Answer · edited Jun 23 '21 at 14:48

16

When I test a function for revert I do the following in my javascript test file (e.g. myContractTest.js):

const { expect } = require('chai');
const { ethers } = require('hardhat');
const { BigNumber } = require('ethers');
describe('Test contract', () => {
    it('deploy the smart contract and reverts', async () => {
        const MyContract = await ethers.getContractFactory('MyContract');
        const contractInstance = await MyContract.deploy(<add something if you have parameters in the constructor>);
        await expect(contractInstance.myFunction(BigNumber.from('6')))
        .to.be.revertedWith('Num should be bigger than 5');
    });
});

Then Run

npx hardhat test --network hardhat ./test/myContractTest.js

Documentation here: https://ethereum-waffle.readthedocs.io/en/latest/matchers.html#revert-with-message

edited Jun 23 '21 at 14:48

ntonnelier

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answered Jun 23 '21 at 11:35

Majd TL

3,217
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thanks @Majd TL, but that still crashes the VM. Same error. Btw, not sure I mentioned it, but it's an async function. – ntonnelier Jun 23 '21 at 11:54
@ntonnelier which solidity version are you using , and which hardhat version. are you changing a state inside that function or it is just a call data function? – Majd TL Jun 23 '21 at 13:14
it's a payable function and it changes data in storage (although after the require that throws the error here). I'm using solidity 0.7 and hardhat 2.3.3 @MajdTL – ntonnelier Jun 23 '21 at 13:23
and you dont get any error when testing that in remix? – Majd TL Jun 23 '21 at 13:30
I updated my answer there was an error in the message idk if you noticed it. @ntonnelier – Majd TL Jun 23 '21 at 13:36
this made it @Majd TL. Just needed to correct to.revertedWith to to.be.revertedWith. Thanks! – ntonnelier Jun 23 '21 at 14:03
okay it works for me without "be", but anyway great accept it as answer please:) – Majd TL Jun 23 '21 at 14:49
to.be.reverted also works if you don't know the thrown error message – goonerify Oct 11 '21 at 09:43

Testing Error thrown with hardhat

1 Answers1