expectRevert

Signature

function expectRevert() external;

function expectRevert(bytes4 message) external;

function expectRevert(bytes calldata message) external;

function expectPartialRevert(bytes4 message) external;

Description

If the next call does not revert with the expected data message, then expectRevert will.

After calling expectRevert, calls to other cheatcodes before the reverting call are ignored.

This means, for example, we can call prank immediately before the reverting call.

There are 3 signatures for expectRevert:

  • Without parameters: Asserts that the next call reverts, regardless of the message.
  • With bytes4: Asserts that the next call reverts with the specified 4 bytes and exact match of revert data.
  • With bytes: Asserts that the next call reverts with the specified bytes.

and one signature for expectPartialRevert:

  • bytes4: Asserts that the next call reverts and the specified 4 bytes match the first 4 bytes of revert data.

ℹ️ Note:

Custom errors can have arguments that sometimes are difficult to calculate in a testing environment or they may be unrelated to the test at hand (e.g. a value computed in the internal function of a third-party contract). In such cases, expectPartialRevert can be used to ignore arguments and match only on the selector of custom error. For example, testing a function that reverts with WrongNumber(uint256 number) custom error:

function count() public {
    revert WrongNumber(0);
}

should pass when using expectPartialRevert:

vm.expectPartialRevert(Counter.WrongNumber.selector);
counter.count();

but fails if exact match expected:

vm.expectRevert(Counter.WrongNumber.selector);
counter.count();

⚠️ Gotcha: Usage with low-level calls

Normally, a call that succeeds returns a status of true (along with any return data) and a call that reverts returns false.

The Solidity compiler will insert checks that ensures that the call succeeded, and revert if it did not.

On low level calls, the expectRevert cheatcode works by making the status boolean returned by the low level call correspond to whether the expectRevert succeeded or not, NOT whether or not the low-level call succeeds. Therefore, status being false corresponds to the cheatcode failing.

Apart from this, expectRevert also mangles return data on low level calls, and is not usable.

See the following example. For clarity, status has been renamed to revertsAsExpected:

function testLowLevelCallRevert() public {
    vm.expectRevert(bytes("error message"));
    (bool revertsAsExpected, ) = address(myContract).call(myCalldata);
    assertTrue(revertsAsExpected, "expectRevert: call did not revert");
}

Examples

To use expectRevert with a string, pass it as a string literal.

vm.expectRevert("error message");

To use expectRevert with a custom error type without parameters, use its selector.

vm.expectRevert(CustomError.selector);

To use expectRevert with a custom error type with parameters, ABI encode the error type.

vm.expectRevert(
    abi.encodeWithSelector(CustomError.selector, 1, 2)
);

If you need to assert that a function reverts without a message, you can do so with expectRevert(bytes("")).

function testExpectRevertNoReason() public {
    Reverter reverter = new Reverter();
    vm.expectRevert(bytes(""));
    reverter.revertWithoutReason();
}

Message-less reverts happen when there is an EVM error, such as when the transaction consumes more than the block’s gas limit.

If you need to assert that a function reverts a four character message, e.g. AAAA, you can do so with:

function testFourLetterMessage() public {
    vm.expectRevert(bytes("AAAA"));
}

If used expectRevert("AAAA"), the compiler would throw an error because it wouldn’t know which overload to use.

Finally, you can also have multiple expectRevert() checks in a single test.

function testMultipleExpectReverts() public {
    vm.expectRevert("INVALID_AMOUNT");
    vault.send(user, 0);

    vm.expectRevert("INVALID_ADDRESS");
    vault.send(address(0), 200);
}

To use expectPartialRevert with a custom error type, use its selector.

vm.expectRevert(CustomError.selector);

SEE ALSO

Forge Standard Library

Std Errors