Discover how you can build your own NFT auction where the highest bid gets an NFT!
Author: Viraz Malhotra Source code: https://github.com/scaffold-eth/scaffold-eth-examples/tree/nft-auction Intended audience: Beginners/Intermediate Topics: Scaffold-eth basics, NFTs
Discover how you can build your own NFT auction where the highest bid gets an NFT
Table of Contents
This branch uses buyer-mints-nft as a starting point. Please refer to its own README for the context.
We will show you how a simple NFT auction can be built and also will demonstrate how you can spin it up locally as a playground.
Let's start our environment for tinkering and exploring how NFT auction would work.
- 1.Clone the repo first
git clone -b nft-auction https://github.com/scaffold-eth/scaffold-eth-examples.git nft-auction
- 1.Install dependencies
- 1.Start local chain
- 1.Start your React frontend
- 1.Deploy your smart contracts to a local blockchain
- 1.The ui currently depends on a json file so to generate that run
Let's navigate to
packages/hardhat/contractsfolder and check out what contracts we have there.
We are mostly interested in
Auction.solsmart contract which contains all the logic for NFT auction.
First of all, note how we are initializing our smart contract using this line.
contract Auction is IERC721Receiver
We inherit from IERC721Receiver which is an interface created by OpenZeppelin. Inheriting from this contract will allow us to receive transfer of NFT from another account to our contract.
Inheriting from this contract also requires us to paste the implementation of
onERC721Receivedwhich you can find at the bottom of the contract.
The logic for creating an auction is in
createTokenAuctionfunction. It takes an address of NFT contract which in our case is an address of
YourCollectible.soldeployed to our local chain, unique token ID which is going to be sold, minimum bid and duration in seconds.
ERC721(_nft).safeTransferFrom(owner, address(this), _tokenId);
tokenToAuction[_nft][_tokenId] = _auction;
As you can see above, creating an auction means temporarily transfer an NFT to the Auction contract and also save information about auction to our Solidity mapping.
Users place bids by calling
bidfunction which basically checks that the bid which is going to be made is currently the highest one. Note that we store the entire history of all bids made to allow us to return funds back to users who did not win an auction.
executeSaleis a function used to complete the auction and identify the winner. It simply checks the last element of all bids placed and transfers NFT to the winner. If no bids were made, NFT is returned back to the initial owner.
cancelAuctionallows to prematurely cancel the auction and lets the initial owner to get back his NFT.
Firstly, let's get us some funds using local faucet and mint any NFT, so we become its owner.
You can now note that we have an option to Start auction because we are an owner of this newly minted NFT. Let's try to start an auction!
The minimal bid that users will be able to place is 0.1 ETH, and the total duration for our auction will be 5 minutes.
Auction is now in progress, and we can complete it or cancel it. No bids were made yet so there is no information about them yet. Let's try to put some bid now.
After you submit your bid, the information about auction will be updated if your bid is the highest at this point of time.
We placed a bid of 0.2 ETH and now we are the highest bidder. Yay!
Now let's try to open an incognito window and place a higher bid by a different user.
We placed 0.5 ETH big as a different user and now it's the highest bid. Now let's get back to our first account to complete an auction.
As you see, after we finished the auction, we are no longer an owner of the NFT. The account which placed 0.5 ETH is now a new owner. This is why we do not have an option to start an auction now.