Missing Events Assertion in Test Functions

Overview

• Severity: Medium
• Confidence: High
• Affected Versions: All

What are Missing Event Assertions?

Missing event assertions occur when test functions for smart contracts do not properly verify that expected events are emitted with the correct parameters. This can lead to incomplete testing of contract behavior and potentially miss critical issues.

Why are Missing Event Assertions Problematic?

Missing event assertions in test functions can lead to several issues:

• Failure to detect when events are not emitted as expected
• Inability to verify the correctness of emitted event data
• Incomplete testing of contract behavior and state changes
• Reduced visibility into contract operations for off-chain systems
• Potential for overlooking critical contract behaviors or state changes

Technical Example of Missing Event Assertions

pragma solidity ^0.8.0;
import "./test.sol";

contract TokenTest is Test {
    Token token;

    function setUp() public {
        token = new Token();
    }

    function testTransfer(address to, uint256 amount) public {
        token.mint(address(this), amount);
        token.transfer(to, amount);
        assertEq(token.balanceOf(to), amount);
        // Missing assertion for Transfer event
    }
}

In this example, the test function verifies the balance change but fails to assert that the Transfer event was emitted with the correct parameters.

Technical Example with Proper Event Assertions

pragma solidity ^0.8.0;
import "./test.sol";

contract TokenTest is Test {
    Token token;

    function setUp() public {
        token = new Token();
    }

    function testTransfer(address to, uint256 amount) public {
        token.mint(address(this), amount);
        
        vm.expectEmit(true, true, false, true);
        emit Transfer(address(this), to, amount);
        
        token.transfer(to, amount);
        
        assertEq(token.balanceOf(to), amount);
    }
}

In this improved version, the test function uses `vm.expectEmit` to verify that the Transfer event is emitted with the correct parameters. This ensures that the contract's event emission behavior is properly tested alongside its state changes.