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Turbo Panumarch
Turbo Panumarch

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Hardhat: How to Test ETH Balance Change.

#solidity #testing #web3 #hardhat

UPDATED: May 23, 2022

We can use this quick way to do it. Read more here: https://ethereum-waffle.readthedocs.io/en/latest/matchers.html

expect(() => contract.connect(owner).withdrawAllETH()) .to.changeEtherBalance(owner, parseEther(10));
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Let's say you are implementing the NFT selling contract. And the owner can withdraw the ETH to herself.

Something like this.

// solidity function withdrawAllETH() onlyOwner { uint256 balance = address(this).balance; Address.sendValue(payable(owner()), balance); }
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What do we want to test? 🧑‍🔧

One of them is to check if the owner receive the ETH correctly! 🤑

Let's do it together! 🔥

Note: I am using Hardhat here as a sample of testing.

// javascript it('owner can withdraw all ETH', async () => { // let's say contract has 5 ETH, owner has 10 ETH // 1. let's do a withdrawal await contract.connect(owner).withdrawAllETH() // 2. Now owner should have 15 ETH, right? expect(await owner.getBalance()).to.eq(parseEther(15)) })
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And when we run it.

❌ AssertionError: Expected "14999957824852287212" to be equal 15000000000000000000
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What the bug!? 🐞🐞🐞

The test says the actual balance is 14.999.. ETH not 15 ETH. Well... I forgot to check about the gas spent. But how do we do it?

Thanks to our beloved StackOverflow thread (as always 😂), we got a solution.

Gas Spent = Gas Used x Effective Gas Price
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And we can get the data from the transaction receipt!

Note: we can use cumulativeGasUsed instead of gasUsed if you want to get the gas spent from the whole block.

The Solution 🌟

Ok, now I know it, let's do this instead.

// 2nd attempt it('owner can withdraw all ETH', async () => { // let's say contract has 5 ETH, owner has 10 ETH // 1. let's do a withdrawal const tx = await contract.connect(owner).withdrawAllETH() // 2. Let's calculate the gas spent const receipt = await tx.wait() const gasSpent = receipt.gasUsed.mul(receipt.effectiveGasPrice) // 3. Now we know, it is 15 ETH minus by gasSpent!  expect(await owner.getBalance()).to.eq(parseEther(15).sub(gasSpent)) })
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And when we run it.

✔ owner can withdraw all ETH
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Yey! And now it works! 🥳🥳🥳

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FernBryant •

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