Writing and testing a Solidity contract

Now that your project is set up, you’ll write, compile, and test a contract.

Writing a smart contract

Create a new file at contracts/Counter.sol and add the following code:

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.28;

contract Counter {
  uint public x;

  event Increment(uint by);

  function inc() public {
    x += 2; // 🐞 bug!
    emit Increment(1);
  }

  function incBy(uint by) public {
    require(by > 0, "incBy: increment should be positive");
    x += by;
    emit Increment(by);
  }
}

// Random number to make this contract file unique: 000000000

This contract has an intentional bug that you’ll fix in the next section when you test it. You can ignore the comment at the end for now, just don’t delete it. You’ll learn about it when you get to contract verification.

Now compile the contract:

npx hardhat build

pnpm hardhat build

yarn hardhat build

If everything went well, you should see an output saying that one Solidity file was compiled.

Writing unit tests in Solidity

With your contract compiled, you’ll add some tests. In Hardhat 3, you can run Solidity-based tests without any dependency.

Create a new file at contracts/Counter.t.sol with the following content:

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.28;

import { Counter } from "./Counter.sol";

contract CounterTest {
  Counter counter;

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

  function test_InitialValueIsZero() public view {
    require(counter.x() == 0, "x should start at 0");
  }

  function test_IncIncreasesByOne() public {
    counter.inc();
    require(counter.x() == 1, "inc should increase x by 1");
  }

  function test_IncByIncreasesByGivenAmount() public {
    counter.incBy(3);
    require(counter.x() == 3, "incBy should increase x by the given amount");
  }
}

Solidity tests are regular Solidity files. Hardhat treats any file ending with .t.sol, or any Solidity file inside the test directory, as a test file. Each public function starting with test runs as a test. If it completes without reverting, the test passes. If it reverts, it fails.

Run all the tests in your project:

npx hardhat test

pnpm hardhat test

yarn hardhat test

One of your tests fails, as expected.

Fixing the bug we just found

Fix the problem in contracts/Counter.sol:

6 collapsed lines
// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.28;

contract Counter {
  uint public x;

  event Increment(uint by);

  function inc() public {
    x += 2; // 🐞 bug!
    x += 1;
    emit Increment(1);
  }
6 collapsed lines

  function incBy(uint by) public {
    require(by > 0, "incBy: increment should be positive");
    x += by;
    emit Increment(by);
  }
}

Run the tests again. This time, you’ll use a different command that only runs Solidity tests:

npx hardhat test solidity

pnpm hardhat test solidity

yarn hardhat test solidity

All tests should pass this time.

Great! You’ve successfully written, compiled, and tested your first smart contract with Hardhat 3.