#Reference

This is the reference for the Hardhat Chai Matchers plugin. This is a chai plugin that adds new assertion capabilities for testing smart contracts.

# Numbers

When @nomicfoundation/hardhat-chai-matchers is used, equality comparisons of numbers will work even if the numbers are represented by different types. This means that assertions like this:

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

will work. These assertions don't normally work because the value returned by totalSupply() is an ethers BigNumber, and an instance of a BigNumber will always be different than a plain number.

The supported types are:

• Plain javascript numbers
• BigInts
• Ethers BigNumbers
• bn.js instances
• bignumber.js instances

This also works when deep-equal comparisons of arrays or objects are performed:

expect(await contract.getRatio()).to.deep.equal([100, 55]);

# Reverted transactions

Several matchers are included to assert that a transaction reverted, and the reason of the revert.

#.reverted

Assert that a transaction reverted for any reason, without checking the cause of the revert:

await expect(token.transfer(address, 0)).to.be.reverted;

#.revertedWith

Assert that a transaction reverted with a specific reason string:

await expect(token.transfer(address, 0)).to.be.revertedWith(
  "transfer value must be positive"
);

#.revertedWithCustomError

Assert that a transaction reverted with a specific custom error:

await expect(token.transfer(address, 0)).to.be.revertedWithCustomError(
  token,
  "InvalidTransferValue"
);

The first argument must be the contract that defines the error.

If the error has arguments, the .withArgs matcher can be added:

await expect(token.transfer(address, 0))
  .to.be.revertedWithCustomError(token, "InvalidTransferValue")
  .withArgs(0);

See the .withArgs matcher entry to learn more.

#.revertedWithPanic

Assert that a transaction reverted with a panic code:

await expect(token.transfer(address, 0)).to.be.revertedWithPanic();

An optional argument can be passed to assert that a specific panic code was thrown:

await expect(token.transfer(address, 0)).to.be.revertedWithPanic(0x12);

You can also import and use the PANIC_CODES dictionary:

import { PANIC_CODES } from "@nomicfoundation/hardhat-chai-matchers/panic";

await expect(token.transfer(address, 0)).to.be.revertedWithPanic(
  PANIC_CODES.DIVISION_BY_ZERO
);

#.revertedWithoutReason

Assert that a transaction reverted without returning a reason:

await expect(token.transfer(address, 0)).to.be.revertedWithoutReason();

This matcher differs from .reverted in that it will fail if the transaction reverts with a reason string, custom error or panic code. Examples of Solidity expressions that revert without a reason are require(false) (without the reason string) and assert(false) before Solidity v0.8.0. This also happens for out-of-gas errors.

# Events

#.emit

Assert that a transaction emits a specific event:

await expect(token.transfer(address, 100)).to.emit(token, "Transfer");

The first argument must be the contract that emits the event.

If the event has arguments, the .withArgs matcher can be added:

await expect(token.transfer(address, 0))
  .to.be.revertedWithCustomError(token, "InvalidTransferValue")
  .withArgs(100);

See the .withArgs matcher entry to learn more.

# Balance change

These matchers can be used to assert how a given transaction affects the ether balance, or an ERC20 token balance, of a specific address.

All these matchers assume that the given transaction is the only transaction mined in its block.

#.changeEtherBalance

Assert that the ether balance of an address changed by a specific amount:

await expect(
  sender.sendTransaction({ to: receiver, value: 1000 })
).to.changeEtherBalance(sender, -1000);

This matcher ignores the fees of the transaction, but you can include them with the includeFee option:

await expect(
  sender.sendTransaction({ to: receiver, value: 1000 })
).to.changeEtherBalance(sender, -22000, { includeFee: true });

#.changeTokenBalance

Assert that an ERC20 token balance of an address changed by a specific amount:

await expect(token.transfer(receiver, 1000)).to.changeTokenBalance(
  token,
  sender,
  -1000
);

The first argument must be the contract of the token.

#.changeEtherBalances

Like .changeEtherBalance, but allows checking multiple addresses at the same time:

await expect(
  sender.sendTransaction({ to: receiver, value: 1000 })
).to.changeEtherBalances([sender, receiver], [-1000, 1000]);

#.changeTokenBalances

Like .changeTokenBalance, but allows checking multiple addresses at the same time:

await expect(token.transfer(receiver, 1000)).to.changeTokenBalances(
  token,
  [sender, receiver],
  [-1000, 1000]
);

# Other matchers

#.withArgs

Can be used after a .emit or a .revertedWithCustomError matcher to assert the values of the event/error's arguments:

// events
await expect(token.transfer(address, 0))
  .to.be.revertedWithCustomError(token, "InvalidTransferValue")
  .withArgs(100);

// errors
await expect(token.transfer(address, 0))
  .to.be.revertedWithCustomError(token, "InvalidTransferValue")
  .withArgs(0);

If you don't care about the value of one of the arguments, you can use the anyValue predicate:

import { anyValue } from "@nomicfoundation/hardhat-chai-matchers/withArgs";

await expect(factory.create(9999))
  .to.emit(factory, "Created")
  .withArgs(anyValue, 9999);

Predicates are just function that return true if the value is correct, and return false if it isn't, so you can create your own predicates:

function isEven(x: BigNumber): boolean {
  return x.mod(2).isZero();
}

await expect(token.transfer(100)).to.emit(token, "Transfer").withArgs(isEven);

#.properAddress

Assert that the given string is a proper address:

expect("0xf39fd6e51aad88f6f4ce6ab8827279cfffb92266").to.be.properAddress;

#.properPrivateKey

Assert that the given string is a proper private key:

expect("0xac0974bec39a17e36ba4a6b4d238ff944bacb478cbed5efcae784d7bf4f2ff80").to
  .be.properPrivateKey;

#.properHex

Assert that the given string is a proper hexadecimal string of a specific length:

expect("0x1234").to.be.properHex(4);

#.hexEqual

Assert that the given strings hexadecimal strings correspond to the same numerical value:

expect("0x00012AB").to.hexEqual("0x12ab");