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Level 7 - Create An Ethereum Dapp with Ethers.js

(Forked from BlockDevsUnited/BasicFrontEndTutorial)

This is a step-by-step tutorial on how to create a front end, deploy a Solidity smart contract, and connect them together. We will use Metamask, Remix IDE and Ethers.js.

By the end of this tutorial you will be able to create a simple HTML front end with buttons that can interact with smart contract functions. The tutorial takes place in 3 stages

  • Create a basic HTML web page
  • Create a basic Solidity smart contract
  • Connect the web page with the smart contracts using Ethers.js.

Prefer a Video?

If you would rather learn from a video, we have a recording available of this tutorial on our YouTube. Watch the video by clicking on the screenshot below, or go ahead and read the tutorial!

Cryptocurrency Tutorial

Preparation

  1. Download and Install MetaMask

    • Never used Metamask? Watch this explainer video

      The important bits for us are: 1:06 to 4:14

    • Click Ethereum Mainnet in the top. Change to the Ropsten Tesnet and get a copy of the account's public address on your Metamask Wallet.

  2. Request some Ropsten Tesnet Ether from a faucet loaded into your Metamask Wallet.

  3. Install a http server. Use any you like, but we recommend lite-server for beginners:

    npm install -g lite-server #install lite-server globally

Create and Serve a Simple Webpage

The first step is to create a basic HTML page.

  1. Create a new folder (directory) in your terminal using mkdir <directory name>
  2. In a code editor (e.g. Atom, or Visual Studio Code), open the folder
  3. Create a new file called index.html
  4. Open index.html
  5. Create HTML boilerplate
<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>My First dApp</title>
  </head>
  <body></body>
</html>

We will create an app that simply reads and writes a value to the blockchain. We will need a label, an input, and buttons.

  1. Inside the body tag, add some text, a label and input.
<body>
  <div>
    <h1>This is my dApp!</h1>
    <p>Here we can set or get the mood:</p>
    <label for="mood">Input Mood:</label> <br />
    <input type="text" id="mood" />
  </div>
</body>
  1. Inside the div tag add some buttons.
<button onclick="getMood()">Get Mood</button>
<button onclick="setMood()">Set Mood</button>

OPTIONAL: Inside the <head> tag, add some styles to make it look nicer

<style>
  body {
    text-align: center;
    font-family: Arial, Helvetica, sans-serif;
  }

  div {
    width: 20%;
    margin: 0 auto;
    display: flex;
    flex-direction: column;
  }

  button {
    width: 100%;
    margin: 10px 0px 5px 0px;
  }
</style>

  1. Serve the webpage via terminal/command prompt from the directory that has index.html in it and run:

    lite-server

  2. Go to http://127.0.0.1:3000/ in your browser to see your page!

  3. Your front end is now complete!

Create a Basic Smart Contract

Now it's time to create a Solidity smart contract.

  1. You can use any editor you like to make the contract. For this tutorial we recommend the online IDE Remix

  2. Go to Remix

  3. Check out the "Solidity Compiler", and "Deploy and Run Transactions" tabs. If they are not present, enable them in the plugin manager

  4. Create a new solidity file in remix, named mood.sol

  5. Write the contract

    • Specify the solidity version and add a license
    // SPDX-License-Identifier: MIT
 pragma solidity ^0.8.1;
    • Define the contract
     contract MoodDiary{

 }
    • Inside the contract create a variable called mood
     string mood;
    • Next, create Read and Write functions
     //create a function that writes a mood to the smart contract
 function setMood(string memory _mood) public{
     mood = _mood;
 }

 //create a function the reads the mood from the smart contract
 function getMood() public view returns(string memory){
     return mood;
 }
    • And that's it! Your code should look like this

  6. Deploy the contract on the Ropsten Testnet.

    • Make sure your Metamask is connected to the Ropsten Testnet.
    • Make sure you select the right compiler version to match the solidity contract. (In the compile tab)
    • Compile the code using the "Solidity Compiler" tab. Note that it may take a moment to load the compiler
    • Deploy the contract under the "Deploy and Run Transactions" tab
    • Under the Deployed Contracts section, you can test out your functions on the Remix Run tab to make sure your contract works as expected!

Be sure to deploy on Ropsten via Remix under the Injected Web3 environment and confirm the deployment transaction in Metamask

Make a new temporary file to hold:

  • The deployed contract's address
    • Copy it via the copy button next to the deployed contracts pulldown in remix's Run tab
  • The contract ABI (what is that?)
    • Copy it via the copy button under to the contract in remix's Compile tab (also in Details)

Connect Your Webpage to Your Smart Contract

Back in your local text editor in index.html, add the following code to your html page:

  1. Import the Ethers.js source into your index.html page inside a new set of script tags:
<script
  src="https://cdn.ethers.io/lib/ethers-5.2.umd.min.js"
  type="application/javascript"
></script>

<script>
  ////////////////////
  //ADD YOUR CODE HERE
  ////////////////////
</script>
  1. Inside the script tag, import the contract ABI (what is that?) and specify the contract address on our provider's blockchain:
  const MoodContractAddress = "<contract address>";
  const MoodContractABI = <contract ABI>
  let MoodContract;
  let signer;

For the contract ABI, we want to specifically navigate to the JSON Section. We need to describe our smart contract in JSON format.

Since we have two methods, this should start as an array, with 2 objects:

const MoodContractABI = [{}, {}]

From the above page, each object should have the following fields: constant, inputs, name, outputs, payable, stateMutability and type.

For setMood, we describe each field below:

  • name: setMood, self explanatory
  • type: function, self explanatory
  • outputs: should be [] because this does not return anything
  • stateMutability: This is nonpayable because this function does not accept Ether
  • inputs: this is an array of inputs to the function. Each object in the array should have internalType, name and type, and these are string, _mood and string respectively

For getMood, we describe each field below:

  • name: getMood, self explanatory
  • type: function, self explanatory
  • outputs: this has the same type as inputs in setMood. For internalType, name and type, this should be string, "", and string respectively
  • stateMutability: This is view because this is a view function
  • inputs: this has no arguments so this should be []

Your end result should look like this:

const MoodContractABI = [
	{
		"inputs": [],
		"name": "getMood",
		"outputs": [
			{
				"internalType": "string",
				"name": "",
				"type": "string"
			}
		],
		"stateMutability": "view",
		"type": "function"
	},
	{
		"inputs": [
			{
				"internalType": "string",
				"name": "_mood",
				"type": "string"
			}
		],
		"name": "setMood",
		"outputs": [],
		"stateMutability": "nonpayable",
		"type": "function"
	}
]
  1. Next, Define an ethers provider. In our case it is Ropsten:
const provider = new ethers.providers.Web3Provider(window.ethereum, "ropsten");
  1. Request access to the user's wallet and connect the signer to your metamask account (we use [0] as the default), and define the contract object using your contract address, ABI, and signer
provider.send("eth_requestAccounts", []).then(() => {
  provider.listAccounts().then((accounts) => {
    signer = provider.getSigner(accounts[0]);
    MoodContract = new ethers.Contract(
      MoodContractAddress,
      MoodContractABI,
      signer
    );
  });
});
  1. Create asynchronous functions to call your smart contract functions
async function getMood() {
  const getMoodPromise = MoodContract.getMood();
  const Mood = await getMoodPromise;
  console.log(Mood);
}

async function setMood() {
  const mood = document.getElementById("mood").value;
  const setMoodPromise = MoodContract.setMood(mood);
  await setMoodPromise;
}
  1. Connect your functions to your html buttons
<button onclick="getMood()">Get Mood</button>
<button onclick="setMood()">Set Mood</button>

Test Your Work Out!

  1. Got your webserver up? Go to http://127.0.0.1:3000/ in your browser to see your page!
  2. Test your functions and approve the transactions as needed through Metamask. Note block times are ~15 seconds... so wait a bit to read the state of the blockchain
  3. See your contract and transaction info via https://ropsten.etherscan.io/
  4. Open a console (Ctrl + Shift + i) in the browser to see the magic happen as you press those buttons

DONE!

Celebrate! You just made a webpage that interacted with a real live Ethereum testnet on the internet! That is not something many folks can say they have done!

If you had trouble with the tutorial, you can try out the example app provided.

git clone https://github.com/LearnWeb3DAO/BasicFrontEndTutorial.git
cd BasicFrontEndTutorial
lite-server

Try and use the following information to interact with an existing contract we published on the Roptsen testnet:

  • We have a MoodDiary contract instance created at this transaction

  • Here is the contract (on etherscan)

    • We also verified our source code to ropsten.etherscan.io as an added measure for you to verify what the contract is exactly, and also the ABI is available to the world!

  • The ABI is also in this file

This illustrates an important point: you can also build a dApp without needing to write the Ethereum contract yourself! If you want to use an existing contract written and already on Ethereum, you can!

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