🏗 scaffold-eth | 🏰 BuidlGuidl

🚩 Challenge 2: 🏵 Token Vendor 🤖

🤖 Smart contracts are kind of like "always on" vending machines that anyone can access. Let's make a decentralized, digital currency. Then, let's build an unstoppable vending machine that will buy and sell the currency. We'll learn about the "approve" pattern for ERC20s and how contract to contract interactions work.

🏵 Create YourToken.sol smart contract that inherits the ERC20 token standard from OpenZeppelin. Set your token to _mint() 1000 (* 10 ** 18) tokens to the msg.sender. Then create a Vendor.sol contract that sells your token using a payable buyTokens() function.

🎛 Edit the frontend that invites the user to <input\> an amount of tokens they want to buy. We'll display a preview of the amount of ETH (or USD) it will cost with a confirm button.

🔍 It will be important to verify your token's source code in the block explorer after you deploy. Supporters will want to be sure that it has a fixed supply and you can't just mint more.

🌟 The final deliverable is an app that lets users purchase your ERC20 token, transfer it, and sell it back to the vendor. Deploy your contracts on your public chain of choice and then yarn build and yarn surge your app to a public web server. Submit the url on SpeedRunEthereum.com!

💬 Meet other builders working on this challenge and get help in the Challenge 2 telegram!

🧫 Everything starts by ✏️ Editing YourToken.sol in packages/hardhat/contracts

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Checkpoint 0: 📦 Install 📚

Want a fresh cloud environment? Click this to open a gitpod workspace, then skip to Checkpoint 1 after the tasks are complete.

git clone https://github.com/scaffold-eth/scaffold-eth-challenges challenge-2-token-vendor
cd challenge-2-token-vendor
git checkout challenge-2-token-vendor
yarn install

🔏 Edit your smart contract YourToken.sol in packages/hardhat/contracts

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Checkpoint 1: 🔭 Environment 📺

You'll have three terminals up for:

yarn chain   (hardhat backend)
yarn start   (react app frontend)
yarn deploy  (to compile, deploy, and publish your contracts to the frontend)

👀 Visit your frontend at http://localhost:3000

👩‍💻 Rerun yarn deploy --reset whenever you want to deploy new contracts to the frontend.

ignore any warnings, we'll get to that...

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Checkpoint 2: 🏵Your Token 💵

👩‍💻 Edit YourToken.sol to inherit the ERC20 token standard from OpenZeppelin

Mint 1000 (* 10 ** 18) to your frontend address using the constructor().

(Your frontend address is the address in the top right of http://localhost:3000)

You can yarn deploy --reset to deploy your contract until you get it right.

🥅 Goals

• Can you check the balanceOf() your frontend address in the YourToken of the Debug Contracts tab?
• Can you transfer() your token to another account and check that account's balanceOf?

(Use an incognito window to create a new address and try sending to that new address. Use the transfer() function in the Debug Contracts tab.)

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Checkpoint 3: ⚖️ Vendor 🤖

👩‍💻 Edit the Vendor.sol contract with a payable buyTokens() function

Use a price variable named tokensPerEth set to 100:

uint256 public constant tokensPerEth = 100;

📝 The buyTokens() function in Vendor.sol should use msg.value and tokensPerEth to calculate an amount of tokens to yourToken.transfer() to msg.sender.

📟 Emit event BuyTokens(address buyer, uint256 amountOfETH, uint256 amountOfTokens) when tokens are purchased.

Edit deploy/01_deploy_vendor.js to deploy the Vendor (uncomment Vendor deploy lines).

🥅 Goals

• When you try to buy tokens from the vendor, you should get an error: 'ERC20: transfer amount exceeds balance'

⚠️ this is because the Vendor contract doesn't have any YourTokens yet!

⚔️ Side Quest: send tokens from your frontend address to the Vendor contract address and then try to buy them.

✏️ We can't hard code the vendor address like we did above when deploying to the network because we won't know the vendor address at the time we create the token contract.

✏️ So instead, edit YourToken.sol to mint the tokens to the msg.sender (deployer) in the constructor().

✏️ Then, edit deploy/01_deploy_vendor.js to transfer 1000 tokens to vendor.address.

await yourToken.transfer( vendor.address, ethers.utils.parseEther("1000") );

You can yarn deploy --reset to deploy your contract until you get it right.

(You will use the YourToken UI tab and the frontend for most of your testing. Most of the UI is already built for you for this challenge.)

🥅 Goals

• Does the Vendor address start with a balanceOf 1000 in YourToken on the Debug Contracts tab?
• Can you buy 10 tokens for 0.1 ETH?
• Can you transfer tokens to a different account?

📝 Edit Vendor.sol to inherit Ownable.

In deploy/01_deploy_vendor.js you will need to call transferOwnership() on the Vendor to make your frontend address the owner:

await vendor.transferOwnership("**YOUR FRONTEND ADDRESS**");

🥅 Goals

• Is your frontend address the owner of the Vendor?

📝 Finally, add a withdraw() function in Vendor.sol that lets the owner withdraw ETH from the vendor.

🥅 Goals

• Can only the owner withdraw the ETH from the Vendor?

⚔️ Side Quests

• Can anyone withdraw? Test everything!
• What if you minted 2000 and only sent 1000 to the Vendor?

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Checkpoint 4: 🤔 Vendor Buyback 🤯

👩‍🏫 The hardest part of this challenge is to build your Vendor to buy the tokens back.

🧐 The reason why this is hard is the approve() pattern in ERC20s.

😕 First, the user has to call approve() on the YourToken contract, approving the Vendor contract address to take some amount of tokens.

🤨 Then, the user makes a second transaction to the Vendor contract to sellTokens(uint256 amount).

🤓 The Vendor should call yourToken.transferFrom(msg.sender, address(this), theAmount) and if the user has approved the Vendor correctly, tokens should transfer to the Vendor and ETH should be sent to the user.

📝 Edit Vendor.sol and add a sellTokens(uint256 amount) function!

⚠️ You will need extra UI for calling approve() before calling sellTokens(uint256 amount).

🔨 Use the Debug Contracts tab to call the approve and sellTokens() at first but then...

🔍 Look in the App.jsx for the extra approve/sell UI to uncomment!

🥅 Goal

• Can you sell tokens back to the vendor?
• Do you receive the right amount of ETH for the tokens?

⚔️ Side Quest

• Should we disable the owner withdraw to keep liquidity in the Vendor?
• It would be a good idea to display Sell Token Events. Create the event and emit it in your Vendor.sol and look at buyTokensEvents in your App.jsx for an example of how to update your frontend.

⚠️ Test it!

• Now is a good time to run yarn test to run the automated testing function. It will test that you hit the core checkpoints. You are looking for all green checkmarks and passing tests!

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Checkpoint 5: 💾 Deploy it! 🛰

📡 Edit the defaultNetwork in packages/hardhat/hardhat.config.js, as well as targetNetwork in packages/react-app/src/App.jsx, to your choice of public EVM networks

👩‍🚀 You will want to run yarn account to see if you have a deployer address.

🔐 If you don't have one, run yarn generate to create a mnemonic and save it locally for deploying.

🛰 Use a faucet like faucet.paradigm.xyz to fund your deployer address (run yarn account again to view balances)

🚀 Run yarn deploy to deploy to your public network of choice (😅 wherever you can get ⛽️ gas)

🔬 Inspect the block explorer for the network you deployed to... make sure your contract is there.

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Checkpoint 6: 🚢 Ship it! 🚁

📦 Run yarn build to package up your frontend.

💽 Upload your app to surge with yarn surge (you could also yarn s3 or maybe even yarn ipfs?)

😬 Windows users beware! You may have to change the surge code in packages/react-app/package.json to just "surge": "surge ./build",

⚙ If you get a permissions error yarn surge again until you get a unique URL, or customize it in the command line.

🚔 Traffic to your url might break the Infura rate limit, edit your key: constants.js in packages/react-app/src.

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Checkpoint 7: 📜 Contract Verification

Update the api-key in packages/hardhat/package.json. You can get your key here.

Now you are ready to run the yarn verify --network your_network command to verify your contracts on etherscan 🛰

👀 You may see an address for both YouToken and Vendor. You will want the Vendor address.

👉 This will be the URL you submit to 🏃‍♀️SpeedRunEthereum.com.

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💬 Problems, questions, comments on the stack? Post them to the Challenge 2 telegram channel