Web3 & DApp Development Fundamentals · Lesson

Testing Reverts and Events

Assert behavior.

Testing Reverts and Events is a free Web3 & DApp Development Fundamentals lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Web3 & DApp Development Fundamentals learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Reverts and Events

Beyond return values, contracts communicate through reverts (transactions that fail and undo state) and events (logs emitted on success).

Good tests assert both: that valid actions emit the right events, and that invalid actions revert with the right reason.

Hardhat Chai Matchers

The Toolbox adds special Chai matchers for Ethereum, available after importing them (the Toolbox does this automatically):

.to.be.revertedWith(...)
.to.be.revertedWithCustomError(...)
.to.emit(...)
.to.changeEtherBalance(...)

Asserting a Revert

To check that a transaction fails, wrap the call (without awaiting it first) and use revertedWith:

await expect(
  token.connect(alice).withdraw()
).to.be.revertedWith("Not the owner");

The string must match the contract's require message exactly.

await expect(
  token.connect(alice).withdraw()
).to.be.revertedWith("Not the owner");

Custom Error Reverts

Modern Solidity uses gas-efficient custom errors. Assert them with revertedWithCustomError:

await expect(
  vault.connect(alice).withdraw()
).to.be.revertedWithCustomError(vault, "Unauthorized");

You pass the contract instance so the matcher can decode the error from the ABI.

await expect(
  vault.connect(alice).withdraw()
).to.be.revertedWithCustomError(vault, "Unauthorized");

Checking Error Arguments

Custom errors can carry data. Chain withArgs to assert the values:

await expect(
  vault.withdraw(amount)
).to.be.revertedWithCustomError(vault, "InsufficientBalance")
  .withArgs(amount, balance);

This verifies the contract reverted for the exact reason you expect.

await expect(
  vault.withdraw(amount)
).to.be.revertedWithCustomError(vault, "InsufficientBalance")
  .withArgs(amount, balance);

Asserting an Event

Use .to.emit to assert that a transaction emitted an event:

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

You pass the contract and the event name. This confirms the log was produced.

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

Checking Event Arguments

Combine emit with withArgs to verify the event payload:

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

This ensures the from, to, and amount fields are exactly correct.

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

Checking Balance Changes

For ETH transfers, changeEtherBalance asserts the net change for an account:

await expect(
  () => vault.connect(alice).withdraw()
).to.changeEtherBalance(alice, ethers.parseEther("1"));

There is also changeTokenBalance for ERC-20 transfers.

await expect(
  () => vault.connect(alice).withdraw()
).to.changeEtherBalance(alice, ethers.parseEther("1"));

Reverts Without a Reason

Some failures have no message — for example, arithmetic underflow panics or plain revert(). Assert these generically:

await expect(token.transfer(bob.address, 999999))
  .to.be.reverted;

For panic codes specifically, use revertedWithPanic.

await expect(token.transfer(bob.address, 999999))
  .to.be.reverted;

Why This Matters

Testing reverts and events covers the negative and observable paths of your contract:

Reverts prove your guards (access control, balance checks) actually block bad input.
Events prove off-chain systems will receive the right notifications.

Skipping these leaves dangerous gaps in coverage.

Asserting State Did Not Change

After a revert, it is good practice to confirm state was untouched, since reverts roll back all changes:

await expect(
  vault.connect(alice).withdraw()
).to.be.revertedWithCustomError(vault, "Unauthorized");

expect(await vault.balance()).to.equal(initial);

This double-checks the guard truly protected the state.

await expect(
  vault.connect(alice).withdraw()
).to.be.revertedWithCustomError(vault, "Unauthorized");

expect(await vault.balance()).to.equal(initial);

Quick Check

Test your understanding of revert and event assertions.

Recap

You learned to assert behavior beyond return values.

revertedWith checks require messages; revertedWithCustomError checks custom errors.
withArgs verifies error or event arguments precisely.
.to.emit confirms events fired.
changeEtherBalance / changeTokenBalance assert net balance changes.
Use .to.be.reverted for failures without a reason string.

Frequently asked questions

Is the “Testing Reverts and Events” lesson free?

Yes — the full text of “Testing Reverts and Events” is free to read here on the web, and the Web3 & DApp Development Fundamentals course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Web3 & DApp Development Fundamentals course, upgrade to CoddyKit PRO.

What will I learn in “Testing Reverts and Events”?

Assert behavior. You practise Web3 & DApp Development Fundamentals with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Web3 & DApp Development Fundamentals?

No prior experience is required. Web3 & DApp Development Fundamentals on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Testing Reverts and Events” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Web3 & DApp Development Fundamentals lesson?

Yes. Every Web3 & DApp Development Fundamentals lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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