#Testing with ethers.js & Waffle

Writing smart contract tests in Hardhat is done using JavaScript or TypeScript.

In this guide, we'll show you how to use Ethers.js, a JavaScript library to interact with Ethereum, and Waffle a simple smart contract testing library built on top of it. This is our recommended choice for testing.

Let's see how to use it going through Hardhat's sample project.

# Setting up

Install Hardhat on an empty directory. When done, run npx hardhat.

$ npx hardhat
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Welcome to Hardhat v2.0.0

? What do you want to do? …
❯ Create a sample project
  Create an empty hardhat.config.js
  Quit

Select Create a sample project. This will create some files and install the @nomiclabs/hardhat-ethers, @nomiclabs/hardhat-waffle plugins, and other necessary packages.

Look at the hardhat.config.js file and you'll see that the Waffle plugin is enabled:

require("@nomiclabs/hardhat-waffle");

// This is a sample Hardhat task. To learn how to create your own go to
// https://hardhat.org/guides/create-task.html
task("accounts", "Prints the list of accounts", async (taskArgs, hre) => {
  const accounts = await hre.ethers.getSigners();

  for (const account of accounts) {
    console.log(account.address);
  }
});

// You need to export an object to set up your config
// Go to https://hardhat.org/config/ to learn more

/**
 * @type import('hardhat/config').HardhatUserConfig
 */
module.exports = {
  solidity: "0.8.4",
};

# Testing

Tests using Waffle are written with Mocha alongside Chai. If you haven't heard of them, they are super popular JavaScript testing utilities.

Inside the test folder you'll find sample-test.js. Let's take a look at it, and we'll explain it next:

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

describe("Greeter", function () {
  it("Should return the new greeting once it's changed", async function () {
    const Greeter = await ethers.getContractFactory("Greeter");
    const greeter = await Greeter.deploy("Hello, world!");
    await greeter.deployed();

    expect(await greeter.greet()).to.equal("Hello, world!");

    const setGreetingTx = await greeter.setGreeting("Hola, mundo!");

    // wait until the transaction is mined
    await setGreetingTx.wait();

    expect(await greeter.greet()).to.equal("Hola, mundo!");
  });
});

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

$ npx hardhat test

  Contract: Greeter
    ✓ Should return the new greeting once it's changed (762ms)

  1 passing (762ms)

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

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

We are requiring Chai which is an assertions library. These asserting functions are called "matchers", and the ones we're using here actually come from Waffle.

This is why we're using the @nomiclabs/hardhat-waffle plugin, which makes it easier to assert values from Ethereum. Check out this section in Waffle's documentation for the entire list of Ethereum-specific matchers.

describe("Greeter", function () {
  it("Should return the new greeting once it's changed", async function () {
    // ...
  });
});

This wrapper just follows Mocha's proposed structure for tests, but you might have noticed the use of async in it's callback function. Interacting with the Ethereum network and smart contracts are asynchronous operations, hence most APIs and libraries use JavaScript's Promise for returning values. This use of async will allow us to await the calls to our contract and the Hardhat Network node.

const Greeter = await ethers.getContractFactory("Greeter");

A ContractFactory in ethers.js is an abstraction used to deploy new smart contracts, so Greeter here is a factory for instances of our greeter contract.

const greeter = await Greeter.deploy("Hello, world!");

Calling deploy() on a ContractFactory will start the deployment, and return a Promise that resolves to a Contract. This is the object that has a method for each of your smart contract functions. Here we're passing the string Hello, world! to the contract's constructor.

Once the contract is deployed, we can call our contract methods on greeter and use them to get the state of the contract.

expect(await greeter.greet()).to.equal("Hello, world!");

Here we're using our Contract instance to call a smart contract function in our Solidity code. greet() returns the greeter's greeting, and we're checking that it's equal to Hello, world!, as it should be. To do this we're using the Chai matchers expect, to and equal.

await greeter.setGreeting("Hola, mundo!");
expect(await greeter.greet()).to.equal("Hola, mundo!");

We can modify the state of a contract in the same way we read from it. Calling setGreeting will set a new greeting message. After the Promise is resolved, we perform another assertion to verify that the greeting change took effect.

#Testing from a different account

If you need to send a transaction from an account other than the default one, you can use the connect() method provided by Ethers.js.

The first step to do so is to get the Signers object from ethers:

const [owner, addr1] = 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 only keeping the first and second ones.

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");

Finally, to execute a contract's method from another account, all you need to do is connect the Contract with the method being executed:

await greeter.connect(addr1).setGreeting("Hallo, Erde!");

# Migrating an existing Waffle project

If you're starting a project from scratch and looking to use Waffle, you can skip this section. If you're setting up an existing Waffle project to use Hardhat you'll need to migrate the configuration options Waffle offers. The following table maps Waffle configurations to their Hardhat equivalents:

As an example, this Waffle configuration file:

{
  "sourcesPath": "./some_custom/contracts_path",
  "targetPath": "../some_custom/build",
  "solcVersion": "v0.4.24+commit.e67f0147",
  "compilerOptions": {
    "evmVersion": "constantinople",
    "optimizer": {
      "enabled": true,
      "runs": 200
    }
  }
}

Would translate into this Hardhat config:

module.exports = {
  paths: {
    sources: "./some_custom/contracts_path",
    artifacts: "../some_custom/build",
  },
  solidity: {
    version: "0.4.24", // Note that this only has the version number
    settings: {
      evmVersion: "constantinople",
      optimizer: {
        enabled: true,
        runs: 200,
      },
    },
  },
};

If you're migrating an existing Waffle project to Hardhat, then the minimum configuration you'll need is changing Hardhat's compilation output path, since Waffle uses a different one by default:

require("@nomiclabs/hardhat-waffle");

module.exports = {
  paths: {
    artifacts: "./build",
  },
};

#Adapting the tests

Now, when testing using a standalone Waffle setup, you should use the different parts of Waffle from Hardhat.

For example, instead of doing:

const { deployContract } = require("ethereum-waffle");

You should do:

const { waffle } = require("hardhat");
const { deployContract } = waffle;

Also, you don't need to call chai.use. This initialization is already handled by @nomiclabs/hardhat-waffle. Just be sure to include require("@nomiclabs/hardhat-waffle"); in your Hardhat config.

Finally, instead of initializing a MockProvider, just use the plugin's provider like this:

const { waffle } = require("hardhat");
const provider = waffle.provider;

Run your tests with npx hardhat test and you should get stack traces when a transaction fails.