How to properly test events with indexed parameters in Foundry?

[NekoCrypto]

Hey everyone,

I'm trying to test that my contract emits the correct event with indexed parameters, but I'm confused about how vm.expectEmit works with the boolean flags.

Here's my contract:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

contract Staking {
    event Staked(address indexed user, uint256 indexed poolId, uint256 amount, uint256 timestamp);

    mapping(address => mapping(uint256 => uint256)) public stakes;

    function stake(uint256 poolId) external payable {
        require(msg.value > 0, "Must stake > 0");
        stakes[msg.sender][poolId] += msg.value;
        emit Staked(msg.sender, poolId, msg.value, block.timestamp);
    }
}

And my test:

function testStakeEmitsEvent() public {
    vm.deal(alice, 1 ether);
    vm.prank(alice);

    vm.expectEmit(true, true, false, true);
    emit Staked(alice, 1, 1 ether, block.timestamp);

    staking.stake{value: 1 ether}(1);
}

My questions:

1. What exactly do the 4 booleans in vm.expectEmit(true, true, false, true) mean?
2. Why would I ever set one of them to false?
3. Is there a simpler way to do this without dealing with the boolean flags?

Thanks!

[0xkoiner]

The 4 booleans in vm.expectEmit map directly to the event's topics and data:

vm.expectEmit(checkTopic1, checkTopic2, checkTopic3, checkData);

In Solidity events, indexed parameters become topics (topic1, topic2, topic3) and non-indexed parameters go into the data field. Topic0 is always the event signature hash and is checked automatically.

So for your event:

event Staked(address indexed user, uint256 indexed poolId, uint256 amount, uint256 timestamp);

The mapping is:

Position	What	Your event
topic0	Event signature (auto-checked)	keccak256("Staked(address,uint256,uint256,uint256)")
topic1 → checkTopic1	1st indexed param	user (address)
topic2 → checkTopic2	2nd indexed param	poolId (uint256)
topic3 → checkTopic3	3rd indexed param	(you don't have a 3rd indexed)
data → checkData	All non-indexed params	amount + timestamp

When you set a boolean to true, Foundry asserts that the expected value must match the actual emitted value. When false, it skips that check.

Why set one to false?

Common use case: you don't know block.timestamp at test time (or it's hard to predict). You can skip checking the data field:

// Only verify the indexed params, skip non-indexed data
vm.expectEmit(true, true, false, false);
emit Staked(alice, 1, 0, 0);  // amount and timestamp don't matter here
staking.stake{value: 1 ether}(1);

This is useful when you care about who did what (indexed) but not the exact values in the data payload.

Your test is actually correct

Your original test works perfectly:

vm.expectEmit(true, true, false, true);
emit Staked(alice, 1, 1 ether, block.timestamp);
staking.stake{value: 1 ether}(1);

• true → check topic1 (alice) ✅
• true → check topic2 (poolId = 1) ✅
• false → skip topic3 (doesn't exist for this event, so false is correct)
• true → check data (amount + timestamp) ✅

Simpler syntax

Since Foundry v0.2.0, you can also use the version without booleans that checks everything:

vm.expectEmit(address(staking));
emit Staked(alice, 1, 1 ether, block.timestamp);
staking.stake{value: 1 ether}(1);

This is equivalent to vm.expectEmit(true, true, true, true, address(staking)) — it checks all topics and data. Much cleaner when you want to verify the full event.