Testing Contracts

Foundry includes a powerful testing framework that lets you write tests in Solidity. Tests run extremely fast compared to JavaScript-based testing frameworks.

Write Tests

Create Test File

Create a test file in the test/ directory:

test/Counter.t.sol

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.13;

import {Test, console} from "forge-std/Test.sol";
import {Counter} from "../src/Counter.sol";

contract CounterTest is Test {
    Counter public counter;

    function setUp() public {
        counter = new Counter();
    }

    function test_InitialValue() public view {
        assertEq(counter.number(), 0);
    }

    function test_Increment() public {
        counter.increment();
        assertEq(counter.number(), 1);
    }

    function test_SetNumber() public {
        counter.setNumber(42);
        assertEq(counter.number(), 42);
    }

    function testFuzz_SetNumber(uint256 x) public {
        counter.setNumber(x);
        assertEq(counter.number(), x);
    }
}

Run Tests

Execute your tests:

forge test

Output:

Running 4 tests for test/Counter.t.sol:CounterTest
[PASS] test_InitialValue() (gas: 5453)
[PASS] test_Increment() (gas: 28334)
[PASS] test_SetNumber() (gas: 28312)
[PASS] testFuzz_SetNumber(uint256) (runs: 256, μ: 27564, ~: 28387)
Test result: ok. 4 passed; 0 failed; finished in 10.23ms

Verbose Output

See detailed test output with -v flags:

forge test -vv    # Show logs
forge test -vvv   # Show execution traces
forge test -vvvv  # Show full traces including setup

Gas Reports

Generate gas usage reports:

forge test --gas-report

Run Specific Tests

Run a single test file:

forge test --match-path test/Counter.t.sol

Run tests matching a pattern:

forge test --match-test test_Increment

Run tests in a specific contract:

forge test --match-contract CounterTest

Fuzz Testing

Foundry automatically runs fuzz tests on functions prefixed with testFuzz_:

function testFuzz_SetNumber(uint256 x) public {
    counter.setNumber(x);
    assertEq(counter.number(), x);
}

Configure fuzz runs in foundry.toml:

[fuzz]
runs = 1000

Cheatcodes

Foundry provides cheatcodes for testing advanced scenarios:

// Set block timestamp
vm.warp(1641070800);

// Set msg.sender
vm.prank(address(0x1234));

// Expect a revert
vm.expectRevert("Error message");

// Deal ETH to an address
vm.deal(address(0x1234), 1 ether);

Testing Contracts

Write Tests

Create Test File

Run Tests

Verbose Output

Gas Reports

Run Specific Tests

Fuzz Testing

Cheatcodes

Next Steps

Deploying Contracts

Verifying Contracts

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