Web3 & DApp Development Fundamentals · Lesson

forge Testing

Solidity tests.

forge Testing 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.

Tests Are Contracts

In Foundry a test file is a Solidity contract that inherits from Test in the forge-std library. Each public function whose name starts with test is run as an individual test case.

Test files live in test/ and conventionally use the .t.sol suffix.

import {Test} from "forge-std/Test.sol";

contract CounterTest is Test {
    function testInitialZero() public {
        // ...
    }
}

The setUp Function

The special setUp() function runs before every test, giving each test a fresh, isolated state. Deploy your contracts and seed any fixtures here.

Because state resets between tests, one test can never accidentally depend on another.

Counter counter;

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

Assertions

forge-std provides assertion helpers that fail the test with a clear message when they do not hold:

assertEq(a, b) — equality
assertTrue(cond) / assertFalse(cond)
assertGt, assertLt, assertGe, assertLe

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

Running Tests

Run the whole suite with forge test. Filter by name with --match-test or by contract with --match-contract. Increase verbosity with -v flags to see logs and traces.

$ forge test
$ forge test --match-test testIncrement -vvv

Expecting Reverts

Use vm.expectRevert immediately before a call that should fail. The test passes only if the next call reverts, optionally matching a specific error message or custom error.

function testOnlyOwner() public {
    vm.prank(address(0xBEEF));
    vm.expectRevert("not owner");
    counter.adminReset();
}

Impersonating Accounts

vm.prank(addr) sets msg.sender for the very next call. vm.startPrank(addr) applies it to all calls until vm.stopPrank(). This lets you test access control without deploying from many keys.

vm.startPrank(alice);
token.approve(spender, 100);
token.transfer(bob, 50);
vm.stopPrank();

Manipulating Balances and Time

Cheatcodes let you control the environment:

vm.deal(addr, amount) — set an account's ETH balance
vm.warp(timestamp) — set block.timestamp
vm.roll(blockNum) — set block.number

These make time-dependent and balance-dependent logic easy to test deterministically.

vm.deal(alice, 10 ether);
vm.warp(block.timestamp + 7 days);
vm.roll(block.number + 100);

Testing Events

vm.expectEmit declares which event fields to check, followed by the expected event, then the call that should emit it. The booleans select which topics and data to match.

vm.expectEmit(true, true, false, true);
emit Transfer(alice, bob, 100);
token.transfer(bob, 100);

Labels and Logs

vm.label(addr, name) gives addresses readable names in traces. console.log from forge-std prints debug output during tests when run with sufficient verbosity.

import {console} from "forge-std/console.sol";

vm.label(alice, "Alice");
console.log("balance:", token.balanceOf(alice));

Fork Testing

Foundry can fork a live network so your tests run against real deployed contracts and state. Set an RPC URL and use vm.createSelectFork to pin a block.

This is invaluable for testing integrations with protocols like Uniswap or Aave without redeploying them.

function setUp() public {
    vm.createSelectFork(vm.rpcUrl("mainnet"), 19000000);
}

Test Organization

Good practices for forge tests:

One behavior per test function
Descriptive names like testRevertWhenNotOwner
Shared setup in setUp()
Use prank for access control, warp for time logic
Add fork tests for external integrations

Quick Check

What is the purpose of the setUp() function in a Foundry test contract?

Recap

You learned forge testing:

Tests are Solidity contracts inheriting Test; functions start with test
setUp() gives fresh state per test
Assertions: assertEq, assertTrue, etc.
Cheatcodes: prank, deal, warp, expectRevert, expectEmit
Fork tests run against live network state

Next: fuzzing and invariant testing.

Frequently asked questions

Is the “forge Testing” lesson free?

Yes — the full text of “forge Testing” 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 “forge Testing”?

Solidity tests. 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 “forge Testing” 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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