Unit Testing using Hardhat

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

describe("Token contract", function () {
  let Token;
  let hardhatToken;
  let owner;
  let addr1;
  let addr2;
  let addrs;

  beforeEach(async function () {
    // Get the ContractFactory and Signers here.
    Token = await ethers.getContractFactory("Token");
    [owner, addr1, addr2, ...addrs] = await ethers.getSigners();

    // To deploy our contract, we just have to call Token.deploy() and await
    // for it to be deployed(), which happens onces its transaction has been
    // mined.
    hardhatToken = await Token.deploy();

    // We can interact with the contract by calling `hardhatToken.method()`
    await hardhatToken.deployed();
  });

  describe("Transactions", function () {
    it("Should transfer tokens between accounts", async function () {
      // Transfer 50 tokens from owner to addr1
      await hardhatToken.transfer(addr1.address, 50);
      const addr1Balance = await hardhatToken.balanceOf(
        addr1.address
      );
      expect(addr1Balance).to.equal(50);

      // Transfer 50 tokens from addr1 to addr2
      // We use .connect(signer) to send a transaction from another account
      await hardhatToken.connect(addr1).transfer(addr2.address, 50);
      const addr2Balance = await hardhatToken.balanceOf(
        addr2.address
      );
      expect(addr2Balance).to.equal(50);
    });

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

      // Try to send 1 token from addr1 (0 tokens) to owner (1000 tokens).
      // `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
      );
    });
  });
});

Ethereum development environment for professionals by Nomic Labs

Debugging using console.sol

pragma solidity ^0.6.0;

import "hardhat/console.sol";

contract Token {
  ...
  function transfer(address to, uint256 amount) external {
    console.log("Sender balance is %s tokens", balances[msg.sender]);
    console.log("Trying to send %s tokens to %s", amount, to);

    require(balances[msg.sender] >= amount, "Not enough tokens");

    balances[msg.sender] -= amount;
    balances[to] += amount;
	}
}