Test smart contracts comprehensively using Hardhat

Web3 Smart Contract Testing

Master comprehensive testing strategies for smart contracts using Hardhat, Foundry, and advanced testing patterns.

When to Use This Skill

• Writing unit tests for smart contracts
• Setting up integration test suites
• Performing gas optimization testing
• Fuzzing for edge cases
• Forking mainnet for realistic testing
• Automating test coverage reporting
• Verifying contracts on Etherscan

Hardhat Testing Setup

// hardhat.config.js
require("@nomicfoundation/hardhat-toolbox");
require("@nomiclabs/hardhat-etherscan");
require("hardhat-gas-reporter");
require("solidity-coverage");

module.exports = {
  solidity: {
    version: "0.8.19",
    settings: {
      optimizer: {
        enabled: true,
        runs: 200,
      },
    },
  },
  networks: {
    hardhat: {
      forking: {
        url: process.env.MAINNET_RPC_URL,
        blockNumber: 15000000,
      },
    },
    goerli: {
      url: process.env.GOERLI_RPC_URL,
      accounts: [process.env.PRIVATE_KEY],
    },
  },
  gasReporter: {
    enabled: true,
    currency: "USD",
    coinmarketcap: process.env.COINMARKETCAP_API_KEY,
  },
  etherscan: {
    apiKey: process.env.ETHERSCAN_API_KEY,
  },
};

Unit Testing Patterns

const { expect } = require("chai");
const { ethers } = require("hardhat");
const {
  loadFixture,
  time,
} = require("@nomicfoundation/hardhat-network-helpers");

describe("Token Contract", function () {
  // Fixture for test setup
  async function deployTokenFixture() {
    const [owner, addr1, addr2] = await ethers.getSigners();

    const Token = await ethers.getContractFactory("Token");
    const token = await Token.deploy();

    return { token, owner, addr1, addr2 };
  }

  describe("Deployment", function () {
    it("Should set the right owner", async function () {
      const { token, owner } = await loadFixture(deployTokenFixture);
      expect(await token.owner()).to.equal(owner.address);
    });

    it("Should assign total supply to owner", async function () {
      const { token, owner } = await loadFixture(deployTokenFixture);
      const ownerBalance = await token.balanceOf(owner.address);
      expect(await token.totalSupply()).to.equal(ownerBalance);
    });
  });

  describe("Transactions", function () {
    it("Should transfer tokens between accounts", async function () {
      const { token, owner, addr1 } = await loadFixture(deployTokenFixture);

      await expect(token.transfer(addr1.address, 50)).to.changeTokenBalances(
        token,
        [owner, addr1],
        [-50, 50],
      );
    });

    it("Should fail if sender doesn't have enough tokens", async function () {
      const { token, addr1 } = await loadFixture(deployTokenFixture);
      const initialBalance = await token.balanceOf(addr1.address);

      await expect(
        token.connect(addr1).transfer(owner.address, 1),
      ).to.be.revertedWith("Insufficient balance");
    });

    it("Should emit Transfer event", async function () {
      const { token, owner, addr1 } = await loadFixture(deployTokenFixture);

      await expect(token.transfer(addr1.address, 50))
        .to.emit(token, "Transfer")
        .withArgs(owner.address, addr1.address, 50);
    });
  });

  describe("Time-based tests", function () {
    it("Should handle time-locked operations", async function () {
      const { token } = await loadFixture(deployTokenFixture);

      // Increase time by 1 day
      await time.increase(86400);

      // Test time-dependent functionality
    });
  });

  describe("Gas optimization", function () {
    it("Should use gas efficiently", async function () {
      const { token } = await loadFixture(deployTokenFixture);

      const tx = await token.transfer(addr1.address, 100);
      const receipt = await tx.wait();

      expect(receipt.gasUsed).to.be.lessThan(50000);
    });
  });
});

Foundry Testing (Forge)

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "forge-std/Test.sol";
import "../src/Token.sol";

contract TokenTest is Test {
    Token token;
    address owner = address(1);
    address user1 = address(2);
    address user2 = address(3);

    function setUp() public {
        vm.prank(owner);
        token = new Token();
    }

    function testInitialSupply() public {
        assertEq(token.totalSupply(), 1000000 * 10**18);
    }

    function testTransfer() public {
        vm.prank(owner);
        token.transfer(user1, 100);

        assertEq(token.balanceOf(user1), 100);
        assertEq(token.balanceOf(owner), token.totalSupply() - 100);
    }

    function testFailTransferInsufficientBalance() public {
        vm.prank(user1);
        token.transfer(user2, 100); // Should fail
    }

    function testCannotTransferToZeroAddress() public {
        vm.prank(owner);
        vm.expectRevert("Invalid recipient");
        token.transfer(address(0), 100);
    }

    // Fuzzing test
    function testFuzzTransfer(uint256 amount) public {
        vm.assume(amount > 0 && amount <= token.totalSupply());

        vm.prank(owner);
        token.transfer(user1, amount);

        assertEq(token.balanceOf(user1), amount);
    }

    // Test with cheatcodes
    function testDealAndPrank() public {
        // Give ETH to address
        vm.deal(user1, 10 ether);

        // Impersonate address
        vm.prank(user1);

        // Test functionality
        assertEq(user1.balance, 10 ether);
    }

    // Mainnet fork test
    function testForkMainnet() public {
        vm.createSelectFork("https://eth-mainnet.alchemyapi.io/v2/...");

        // Interact with mainnet contracts
        address dai = 0x6B175474E89094C44Da98b954EedeAC495271d0F;
        assertEq(IERC20(dai).symbol(), "DAI");
    }
}

Advanced Testing Patterns

Snapshot and Revert

describe("Complex State Changes", function () {
  let snapshotId;

  beforeEach(async function () {
    snapshotId = await network.provider.send("evm_snapshot");
  });

  afterEach(async function () {
    await network.provider.send("evm_revert", [snapshotId]);
  });

  it("Test 1", async function () {
    // Make state changes
  });

  it("Test 2", async function () {
    // State reverted, clean slate
  });
});

Mainnet Forking

describe("Mainnet Fork Tests", function () {
  let uniswapRouter, dai, usdc;

  before(async function () {
    await network.provider.request({
      method: "hardhat_reset",
      params: [
        {
          forking: {
            jsonRpcUrl: process.env.MAINNET_RPC_URL,
            blockNumber: 15000000,
          },
        },
      ],
    });

    // Connect to existing mainnet contracts
    uniswapRouter = await ethers.getContractAt(
      "IUniswapV2Router",
      "0x7a250d5630B4cF539739dF2C5dAcb4c659F2488D",
    );

    dai = await ethers.getContractAt(
      "IERC20",
      "0x6B175474E89094C44Da98b954EedeAC495271d0F",
    );
  });

  it("Should swap on Uniswap", async function () {
    // Test with real Uniswap contracts
  });
});

Impersonating Accounts

it("Should impersonate whale account", async function () {
  const whaleAddress = "0x...";

  await network.provider.request({
    method: "hardhat_impersonateAccount",
    params: [whaleAddress],
  });

  const whale = await ethers.getSigner(whaleAddress);

  // Use whale's tokens
  await dai
    .connect(whale)
    .transfer(addr1.address, ethers.utils.parseEther("1000"));
});

Additional patterns and templates

More detailed templates and worked examples live in references/details.md. Read that file for the full pattern library.