#5. Testing contracts

Writing automated tests when building smart contracts is of crucial importance, as your user's money is what's at stake.

To test our contract, we are going to use Hardhat Network, a local Ethereum network designed for development. It comes built-in with Hardhat, and it's used as the default network. You don't need to setup anything to use it.

In our tests we're going to use ethers.js to interact with the Ethereum contract we built in the previous section, and we'll use Mocha as our test runner.

# Writing tests

Create a new directory called test inside our project root directory and create a new file in there called Token.js.

Let's start with the code below. We'll explain it next, but for now paste this into Token.js:

const { expect } = require("chai");

describe("Token contract", function () {
  it("Deployment should assign the total supply of tokens to the owner", async function () {
    const [owner] = await ethers.getSigners();

    const hardhatToken = await ethers.deployContract("Token");

    const ownerBalance = await hardhatToken.balanceOf(owner.address);
    expect(await hardhatToken.totalSupply()).to.equal(ownerBalance);
  });
});

In your terminal run npx hardhat test. You should see the following output:

$ npx hardhat test

  Token contract
    ✓ Deployment should assign the total supply of tokens to the owner (654ms)


  1 passing (663ms)

This means the test passed. Let's now explain each line:

const [owner] = await ethers.getSigners();

A Signer in ethers.js is an object that represents an Ethereum account. It's used to send transactions to contracts and other accounts. Here we're getting a list of the accounts in the node we're connected to, which in this case is Hardhat Network, and we're only keeping the first one.

The ethers variable is available in the global scope. If you like your code always being explicit, you can add this line at the top:

const { ethers } = require("hardhat");

TIP

To learn more about Signer, you can look at the Signers documentation.

const hardhatToken = await ethers.deployContract("Token");

Calling ethers.deployContract("Token") will start the deployment of our token contract, and return a Promise that resolves to a Contract. This is the object that has a method for each of your smart contract functions.

const ownerBalance = await hardhatToken.balanceOf(owner.address);

Once the contract is deployed, we can call our contract methods on hardhatToken. Here we get the balance of the owner account by calling the contract's balanceOf() method.

Recall that the account that deploys the token gets its entire supply. By default, Contract instances are connected to the first signer. This means that the account in the owner variable executed the deployment, and balanceOf() should return the entire supply amount.

expect(await hardhatToken.totalSupply()).to.equal(ownerBalance);

Here we're again using our Contract instance to call a smart contract function in our Solidity code. totalSupply() returns the token's supply amount and we're checking that it's equal to ownerBalance, as it should be.

To do this we're using Chai which is a popular JavaScript assertion library. These asserting functions are called "matchers", and the ones we're using here come from the @nomicfoundation/hardhat-chai-matchers plugin, which extends Chai with many matchers useful to test smart contracts.

#Using a different account

If you need to test your code by sending a transaction from an account (or Signer in ethers.js terminology) other than the default one, you can use the connect() method on your ethers.js Contract object to connect it to a different account, like this:

const { expect } = require("chai");

describe("Token contract", function () {
  // ...previous test...

  it("Should transfer tokens between accounts", async function() {
    const [owner, addr1, addr2] = await ethers.getSigners();

    const hardhatToken = await ethers.deployContract("Token");

    // Transfer 50 tokens from owner to addr1
    await hardhatToken.transfer(addr1.address, 50);
    expect(await hardhatToken.balanceOf(addr1.address)).to.equal(50);

    // Transfer 50 tokens from addr1 to addr2
    await hardhatToken.connect(addr1).transfer(addr2.address, 50);
    expect(await hardhatToken.balanceOf(addr2.address)).to.equal(50);
  });
});

#Reusing common test setups with fixtures

The two tests that we wrote begin with their setup, which in this case means deploying the token contract. In more complex projects, this setup could involve multiple deployments and other transactions. Doing that in every test means a lot of code duplication. Plus, executing many transactions at the beginning of each test can make the test suite much slower.

You can avoid code duplication and improve the performance of your test suite by using fixtures. A fixture is a setup function that is run only the first time it's invoked. On subsequent invocations, instead of re-running it, Hardhat will reset the state of the network to what it was at the point after the fixture was initially executed.

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

describe("Token contract", function () {
  async function deployTokenFixture() {
    const [owner, addr1, addr2] = await ethers.getSigners();

    const hardhatToken = await ethers.deployContract("Token");

    // Fixtures can return anything you consider useful for your tests
    return { hardhatToken, owner, addr1, addr2 };
  }

  it("Should assign the total supply of tokens to the owner", async function () {
    const { hardhatToken, owner } = await loadFixture(deployTokenFixture);

    const ownerBalance = await hardhatToken.balanceOf(owner.address);
    expect(await hardhatToken.totalSupply()).to.equal(ownerBalance);
  });

  it("Should transfer tokens between accounts", async function () {
    const { hardhatToken, owner, addr1, addr2 } = await loadFixture(
      deployTokenFixture
    );

    // Transfer 50 tokens from owner to addr1
    await expect(
      hardhatToken.transfer(addr1.address, 50)
    ).to.changeTokenBalances(hardhatToken, [owner, addr1], [-50, 50]);

    // Transfer 50 tokens from addr1 to addr2
    // We use .connect(signer) to send a transaction from another account
    await expect(
      hardhatToken.connect(addr1).transfer(addr2.address, 50)
    ).to.changeTokenBalances(hardhatToken, [addr1, addr2], [-50, 50]);
  });
});

Here we wrote a deployTokenFixture function that does the necessary setup and returns every value we use later in the tests. Then in each test, we use loadFixture to run the fixture and get those values. loadFixture will run the setup the first time, and quickly return to that state in the other tests.

#Full coverage

Now that we've covered the basics that you'll need for testing your contracts, here's a full test suite for the token with a lot of additional information about Mocha and how to structure your tests. We recommend reading it thoroughly.

// This is an example test file. Hardhat will run every *.js file in `test/`,
// so feel free to add new ones.

// Hardhat tests are normally written with Mocha and Chai.

// We import Chai to use its asserting functions here.
const { expect } = require("chai");

// We use `loadFixture` to share common setups (or fixtures) between tests.
// Using this simplifies your tests and makes them run faster, by taking
// advantage of Hardhat Network's snapshot functionality.
const {
  loadFixture,
} = require("@nomicfoundation/hardhat-toolbox/network-helpers");

// `describe` is a Mocha function that allows you to organize your tests.
// Having your tests organized makes debugging them easier. All Mocha
// functions are available in the global scope.
//
// `describe` receives the name of a section of your test suite, and a
// callback. The callback must define the tests of that section. This callback
// can't be an async function.
describe("Token contract", function () {
  // We define a fixture to reuse the same setup in every test. We use
  // loadFixture to run this setup once, snapshot that state, and reset Hardhat
  // Network to that snapshot in every test.
  async function deployTokenFixture() {
    // Get the Signers here.
    const [owner, addr1, addr2] = await ethers.getSigners();

    // To deploy our contract, we just have to call ethers.deployContract and await
    // its waitForDeployment() method, which happens once its transaction has been
    // mined.
    const hardhatToken = await ethers.deployContract("Token");

    await hardhatToken.waitForDeployment();

    // Fixtures can return anything you consider useful for your tests
    return { hardhatToken, owner, addr1, addr2 };
  }

  // You can nest describe calls to create subsections.
  describe("Deployment", function () {
    // `it` is another Mocha function. This is the one you use to define each
    // of your tests. It receives the test name, and a callback function.
    //
    // If the callback function is async, Mocha will `await` it.
    it("Should set the right owner", async function () {
      // We use loadFixture to setup our environment, and then assert that
      // things went well
      const { hardhatToken, owner } = await loadFixture(deployTokenFixture);

      // `expect` receives a value and wraps it in an assertion object. These
      // objects have a lot of utility methods to assert values.

      // This test expects the owner variable stored in the contract to be
      // equal to our Signer's owner.
      expect(await hardhatToken.owner()).to.equal(owner.address);
    });

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

  describe("Transactions", function () {
    it("Should transfer tokens between accounts", async function () {
      const { hardhatToken, owner, addr1, addr2 } = await loadFixture(
        deployTokenFixture
      );
      // Transfer 50 tokens from owner to addr1
      await expect(
        hardhatToken.transfer(addr1.address, 50)
      ).to.changeTokenBalances(hardhatToken, [owner, addr1], [-50, 50]);

      // Transfer 50 tokens from addr1 to addr2
      // We use .connect(signer) to send a transaction from another account
      await expect(
        hardhatToken.connect(addr1).transfer(addr2.address, 50)
      ).to.changeTokenBalances(hardhatToken, [addr1, addr2], [-50, 50]);
    });

    it("Should emit Transfer events", async function () {
      const { hardhatToken, owner, addr1, addr2 } = await loadFixture(
        deployTokenFixture
      );

      // Transfer 50 tokens from owner to addr1
      await expect(hardhatToken.transfer(addr1.address, 50))
        .to.emit(hardhatToken, "Transfer")
        .withArgs(owner.address, addr1.address, 50);

      // Transfer 50 tokens from addr1 to addr2
      // We use .connect(signer) to send a transaction from another account
      await expect(hardhatToken.connect(addr1).transfer(addr2.address, 50))
        .to.emit(hardhatToken, "Transfer")
        .withArgs(addr1.address, addr2.address, 50);
    });

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

      // Try to send 1 token from addr1 (0 tokens) to owner.
      // `require` will evaluate false and revert the transaction.
      await expect(
        hardhatToken.connect(addr1).transfer(owner.address, 1)
      ).to.be.revertedWith("Not enough tokens");

      // Owner balance shouldn't have changed.
      expect(await hardhatToken.balanceOf(owner.address)).to.equal(
        initialOwnerBalance
      );
    });
  });
});

This is what the output of npx hardhat test should look like against the full test suite:

$ npx hardhat test

 Token contract
    Deployment
      ✔ Should set the right owner (889ms)
      ✔ Should assign the total supply of tokens to the owner
    Transactions
      ✔ Should transfer tokens between accounts (121ms)
      ✔ Should emit Transfer events
      ✔ Should fail if sender doesn't have enough tokens (74ms)


  5 passing (1s)

Keep in mind that when you run npx hardhat test, your contracts will be automatically compiled if they've changed since the last time you ran your tests.