Hardhat Chai Matchers Viem

Note: THIS IS NOT AN OFFICIAL HARDHAT PACKAGE. IT WAS FORKED FROM ONE.

This plugin adds Ethereum-specific capabilities to the Chai assertion library, making your smart contract tests easy to write and read.

It is based on @nomicfoundation/hardhat-chai-matchers which is for ethers. Check its documentation to learn more.

Installation

We recommend using npm 7 or later. If you do that, then you just need to install the plugin itself:

npm install --save-dev hardhat-chai-matchers-viem

If you are using an older version of npm, you'll also need to install all the packages used by the plugin.

npm install --save-dev hardhat-chai-matchers-viem chai@4 @nomicfoundation/hardhat-viem viem

That's also the case if you are using yarn:

yarn add --dev hardhat-chai-matchers-viem chai@4 @nomicfoundation/hardhat-viem viem

Usage

After installing it, add the plugin to your Hardhat config:

require("hardhat-chai-matchers-viem");

Then you'll be able to use the matchers in your tests:

expect(await token.read.totalSupply()).to.equal(1_000_000);

await expect(token.write.transfer([token, 1000n])).to.be.revertedWith(
  "Cannot transfer to the contract itself"
);

await expect(token.write.transfer([recipient, 1000n]))
  .to.emit(token, "Transfer")
  .withArgs(owner, recipient, 1000);

Known issues

Chaining Async Matchers

Currently, the following matchers do not support chaining:

• reverted
• revertedWith
• revertedWithCustomError
• revertedWithoutReason
• revertedWithPanic
• changeEtherBalance
• changeEtherBalances
• changeTokenBalance
• changeTokenBalances
• emit (with the only exception of chaining multiple emit matchers)

Which means you can't do:

await expect(contract.f(...))
  .to.changeEtherBalance(...)
  .and.to.changeTokenBalance(...)

To work around this limitation, write separate assertions for each matcher:

const tx = contract.f(...);
await expect(tx).to.changeEtherBalance(...)
await expect(tx).to.changeTokenBalance(...)

If you are interested in seeing an implementation of chaining for async matchers, please visit the GitHub issue #4235 and leave an upvote or comment.