Ethers.js – Understanding the Function of Test Code Lines in Hardhat

Test file:

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

describe("NFT Marketplace", function () {
    let NFTMarket;
    let nftMarket;
    let listingPrice;
    let contractOwner;
    let buyerAddress;
    let nftMarketAddress

    // returns a BigNumber representation of value, parsed with
    // digits (if it's a number) or from the unit specified (if it's a string)
    const auctionPrice = ethers.utils.parseUnits("100", "ether")

    // hooks that perform before each test case
    beforeEach(async () => { // get contract that we're targeting, so we can deploy or call the functions in that Contract
        NFTMarket = await ethers.getContractFactory("NFTMarketplace");

        // create a transaction to deploy the transaction and sends it to the network
        // using the contract Signer, and returning a Promise to resolve to a Contract
        nftMarket = await NFTMarket.deploy();

        // return a Promise which will resolve once the contract is deployed
        // or reject if there was an error during deployment
        await nftMarket.deployed();
        nftMarketAddress = nftMarket.address;
        [contractOwner, buyerAddress] = await ethers.getSigners(); // get public address of the user wallet
        listingPrice = await nftMarket.getListingPrice();
        listingPrice = listingPrice.toString();
    })

    // mint and list NFT
    const mintAndListNFT = async (tokenURI, auctionPrice) => {
        const transaction = await nftMarket.createToken(tokenURI, auctionPrice, {value: listingPrice});
        const receipt = await transaction.wait();
        const tokenId = receipt.events[0].args.tokenId;
        return tokenId;
    }

    describe("Resale of a marketplace item", async () => {
        const tokenURI = "https://dummy-token.url/"; // test with a dummy token URI
        const newNFTToken = await mintAndListNFT(tokenURI, auctionPrice);
        await nftMarket.connect(buyerAddress).createMarketSale(newNFTToken, {value: auctionPrice});
        await expect(nftMarket.resellToken(nftMarket, auctionPrice, {value: listingPrice})).to.be.revertedWith("You are not the owner of the token!");
        await expect(nftMarket.connect(buyerAddress).resellToken(newNFTToken, auctionPrice, {value: 0})).to.be.rejectedWith("The amount sold does not equal the original listing price of the token!");
    })

Contract file:

pragma solidity ^0.8.9;
import "@openzeppelin/contracts/utils/Counters.sol";
import "@openzeppelin/contracts/token/ERC721/extensions/ERC721URIStorage.sol";
import "@openzeppelin/contracts/token/ERC721/ERC721.sol";

contract NFTMarketplace is ERC721URIStorage {
    using Counters for Counters.Counter;
    Counters.Counter private _tokenIDs; // total number of items created
    Counters.Counter private _itemsSold; // total number of items sold

    uint256 listingPrice = 0.001 ether; // price to list NFT on marketplace
    address payable owner; // owner of the smart contract

    constructor() ERC721("Metaverse Tokens", "META") {
        owner == payable(msg.sender);
    }

    struct MarketItem {
        uint256 tokenId;
        address payable seller;
        address payable owner;
        uint256 price;
        bool sold;
    }

    mapping(uint256 => MarketItem) private idToMarketItem;

    event MarketItemCreated (uint256 indexed tokenId, address seller, address owner, uint256 price, bool sold);
        
    // return the listing price of the NFT
    function getListingPrice() public view returns(uint256) {
        return listingPrice;
    }

    // update the listing price
    function updatedListingPrice(uint _listingPrice) public payable{
        require(owner == msg.sender, "You are not the owner!");
        listingPrice = _listingPrice;
    }

    // create a NFT in the market
    function createMarketItem(uint256 tokenId, uint256 price) private {
        require(price > 0, "You must list an item with price more than 0!");
        require(msg.value == listingPrice, "The amount of ether sent in the transaction does not equal the listing price!");
        // seller is the msg.sender and owner is the address(this)
        idToMarketItem[tokenId] = MarketItem(tokenId, payable(msg.sender), payable(address(this)), price, false);
        _transfer(msg.sender, address(this), tokenId);
        emit MarketItemCreated (tokenId, msg.sender, address(this), price, true);
    }

    // mints a token and list it in the market
    function createToken(string memory tokenURI, uint256 price) public payable returns(uint) {
        _tokenIDs.increment();
        uint256 newTokenId = _tokenIDs.current();
        _mint(msg.sender, newTokenId);
        _setTokenURI(newTokenId, tokenURI);
        createMarketItem(newTokenId, price);
        return newTokenId;
    }

    // creating the sale of a marketplace item
    // transfers ownership of the item and funds between parties
    function createMarketSale(uint256 tokenId) public payable {
        uint price = idToMarketItem[tokenId].price;
        address seller = idToMarketItem[tokenId].seller;
        require(msg.value == price, "The amount of ethers sent does not equal to the price of the item!");
        idToMarketItem[tokenId].owner = payable(msg.sender); // transfer ownership 
        idToMarketItem[tokenId].seller = payable(address(0));
        idToMarketItem[tokenId].sold = true;
        _itemsSold.increment();
        _transfer(address(this), msg.sender, tokenId);
        payable(owner).transfer(listingPrice);
        payable(seller).transfer(msg.value);
    }

    // allows users to resell a token they have purchased
    function resellToken(uint256 tokenId, uint256 price) public payable {
        require(idToMarketItem[tokenId].owner == msg.sender, "You are not the owner of the token!");
        require(msg.value == listingPrice, "The amount sold does not equal the original listing price of the token!");
        idToMarketItem[tokenId].sold = false;
        idToMarketItem[tokenId].seller = payable(msg.sender); // msg.sender refers to address where the contract is being called from
        idToMarketItem[tokenId].owner = payable(address(this)); // address(this) refers to the address of the instance where the call is being made
        idToMarketItem[tokenId].price = price;
        _itemsSold.decrement();
        _transfer(msg.sender, address(this), tokenId);
    }
}

What do these lines of code do specifically under the test case for "Resale of a marketplace item"?

• const newNFTToken = await mintAndListNFT(tokenURI, auctionPrice);
• await nftMarket.connect(buyerAddress).createMarketSale(newNFTToken, {value: auctionPrice});
• await expect(nftMarket.resellToken(nftMarket, auctionPrice, {value: listingPrice})).to.be.revertedWith("You are not the owner of the token!");
• await expect(nftMarket.connect(buyerAddress).resellToken(newNFTToken, auctionPrice, {value: 0})).to.be.rejectedWith("The amount sold does not equal the original listing price of the token!");

Any help is appreciated. Thank you.