Using Alchemy

🌎 Hello World Smart Contract on Polygon

Estimated time to complete this guide: ~15 minutes

If you are new to blockchain development and don’t know where to start, or if you just want to understand how to deploy and interact with smart contracts, this guide is for you. We will walk through creating and deploying a simple smart contract on the Polygon Mumbai test network using a virtual wallet (Metamask), Solidity, Hardhat, and Alchemy (don’t worry if you don’t understand what any of this means yet, we will explain it!).

If you have questions at any point feel free to reach out in the Alchemy Discord!

Create and Deploy your Smart Contract using Hardhat

Step 1: Connect to the Polygon network

There are many ways to make requests to the Polygon chain. For simplicity, we’ll use a free account on Alchemy, a blockchain developer platform and API that allows us to communicate with the Polygon chain without having to run our own nodes. The platform also has developer tools for monitoring and analytics that we’ll take advantage of in this tutorial to understand what’s going on under the hood in our smart contract deployment. If you don’t already have an Alchemy account, you can sign up for free here.

Step 2: Create your app (and API key)

Once you’ve created an Alchemy account, you can generate an API key by creating an app. This will allow us to make requests to the Polygon Mumbai test network. If you’re not familiar with testnets, check out this guide.

Navigate to the “Create App” page in your Alchemy Dashboard by hovering over “Apps” in the nav bar and clicking “Create App”.

Name your app “Hello World”, offer a short description, select “Staging” for the Environment (used for your app bookkeeping), click "Polygon" for the Chain, and choose “Polygon Mumbai” for your network.

Click “Create app” and that’s it! Your app should appear in the table below.

Step 3: Create an wallet address

Since Polygon is a Layer-2 scaling solution for Ethereum, we need to get an Ethereum wallet and add a custom Polygon URL to send and receive transactions on the Polygon network. For this tutorial, we’ll use Metamask, a virtual wallet in the browser used to manage your wallet address. If you want to understand more about how transactions on Ethereum work, check out this page from the Ethereum foundation.

To get your customer Polygon RPC URL from Alchemy, go to your "Hello World" app in your Alchemy dashboard and click "View Key" in the top right corner. Then go ahead and copy your Alchemy HTTP API key!

You can download and create a Metamask account for free here. Once you've created an account, follow these steps to set up the Polygon network on your wallet.

  1. Select “Settings” from the drop down menu on the top right corner of your Metamask wallet.

  2. Select “Networks” from the menu to the left.

  3. Connect your wallet to the Mumbai Testnet using the following parameters.

    Network Name: Polygon Mumbai Testnet

    New RPC URL: https://polygon-mumbai.g.alchemy.com/v2/your-api-key

    ChainID: 80001

    Symbol: MATIC

    Block Explorer URL: https://mumbai.polygonscan.com/

Step 4: Add Polygon Mumbai Test MATIC from a Faucet

In order to deploy our smart contract to the test network, we’ll need some fake MATIC. To get MATIC, you can go to the Polygon Mumbai Faucet, select "Mumbai", choose "MATIC Token", and enter your Polygon wallet address, then click “Submit.” It may take some time to receive your fake Eth due to network traffic. (At the time of writing this, it took around 30 minutes.) You should see Eth in your Metamask account soon after!

Step 5: Check your Balance

To double check our balance is there, let’s make an eth_getBalance request using Alchemy’s composer tool. Select "Polygon" as the chain, "Polygon Mumbai" as the network, "eth_getBalance" as the method, and input your address. This will return the amount of MATIC in our wallet. Check out this video for instructions on how to use the composer tool!

After you input your Metamask account address and click “Send Request”, you should see a response that looks like this:

{ "jsonrpc": "2.0", "id": 0, "result": "0xde0b6b3a7640000" }

NOTE: This result is in wei not eth. Wei is used as the smallest denomination of ether. The conversion from wei to eth is: 1 eth = 10^18 wei. So if we convert 0xde0b6b3a7640000 to decimal we get 1*10^18 which equals 1 eth, which can be mapped to 1 MATIC based on denomination.

Step 6: Initialize our project

First, we’ll need to create a folder for our project. Navigate to your command line and type:

mkdir hello-world
cd hello-world

Now that we’re inside our project folder, we’ll use npm init to initialize the project. If you don’t already have npm installed, follow these instructions (we’ll also need Node.js so download that too!).

npm init # (or npm init --yes)

It doesn’t really matter how you answer the installation questions, here is how we did it for reference:

package name: (hello-world)
version: (1.0.0)
description: hello world smart contract
entry point: (index.js)
test command:
git repository:
keywords:
author:
license: (ISC)
About to write to /Users/.../.../.../hello-world/package.json:
{
"name": "hello-world",
"version": "1.0.0",
"description": "hello world smart contract",
"main": "index.js",
"scripts": {
"test": "echo \"Error: no test specified\" && exit 1"
},
"author": "",
"license": "ISC"
}

Approve the package.json and we’re good to go!

Step 7: Download Hardhat

Hardhat is a development environment to compile, deploy, test, and debug your Ethereum software. It helps developers when building smart contracts and dApps locally before deploying to the live chain.

Inside our hello-world project run:

npm install --save-dev hardhat

Check out this page for more details on installation instructions.

Step 8: Create Hardhat project

Inside our hello-world project folder, run:

npx hardhat

You should then see a welcome message and option to select what you want to do. Select “create an empty hardhat.config.js”:

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8888888888 8888b. 888d888 .d88888 88888b. 8888b. 888888
888 888 "88b 888P" d88" 888 888 "88b "88b 888
888 888 .d888888 888 888 888 888 888 .d888888 888
888 888 888 888 888 Y88b 888 888 888 888 888 Y88b.
888 888 "Y888888 888 "Y88888 888 888 "Y888888 "Y888
👷 Welcome to Hardhat v2.0.11 👷‍
What do you want to do? …
Create a sample project
❯ Create an empty hardhat.config.js
Quit

This will generate a hardhat.config.js file for us, which is where we’ll specify all of the set up for our project (on step 13).

Step 9: Add project folders

To keep our project organized we’ll create two new folders. Navigate to the root directory of your hello-world project in your command line and type:

mkdir contracts
mkdir scripts
  • contracts/ is where we’ll keep our hello world smart contract code file
  • scripts/ is where we’ll keep scripts to deploy and interact with our contract

Step 10: Write our contract

You might be asking yourself, when the heck are we going to write code?? Well, here we are, on Step 10 😄

Open up the hello-world project in your favorite editor (we like VSCode). Smart contracts are written in a language called Solidity which is what we will use to write our HelloWorld.sol smart contract.‌

  1. Navigate to the “contracts” folder and create a new file called HelloWorld.sol
  2. Below is a sample Hello World smart contract from the Ethereum Foundation that we will be using for this tutorial. Copy and paste in the contents below into your HelloWorld.sol file, and be sure to read the comments to understand what this contract does:
// SPDX-License-Identifier: None
// Specifies the version of Solidity, using semantic versioning.
// Learn more: https://solidity.readthedocs.io/en/v0.5.10/layout-of-source-files.html#pragma
pragma solidity >=0.8.9;
// Defines a contract named `HelloWorld`.
// A contract is a collection of functions and data (its state). Once deployed, a contract resides at a specific address on the Ethereum blockchain. Learn more: https://solidity.readthedocs.io/en/v0.5.10/structure-of-a-contract.html
contract HelloWorld {
//Emitted when update function is called
//Smart contract events are a way for your contract to communicate that something happened on the blockchain to your app front-end, which can be 'listening' for certain events and take action when they happen.
event UpdatedMessages(string oldStr, string newStr);
// Declares a state variable `message` of type `string`.
// State variables are variables whose values are permanently stored in contract storage. The keyword `public` makes variables accessible from outside a contract and creates a function that other contracts or clients can call to access the value.
string public message;
// Similar to many class-based object-oriented languages, a constructor is a special function that is only executed upon contract creation.
// Constructors are used to initialize the contract's data. Learn more:https://solidity.readthedocs.io/en/v0.5.10/contracts.html#constructors
constructor(string memory initMessage) {
// Accepts a string argument `initMessage` and sets the value into the contract's `message` storage variable).
message = initMessage;
}
// A public function that accepts a string argument and updates the `message` storage variable.
function update(string memory newMessage) public {
string memory oldMsg = message;
message = newMessage;
emit UpdatedMessages(oldMsg, newMessage);
}
}

This is a super simple smart contract that stores a message upon creation and can be updated by calling the update function.

Step 11: Connect Metamask & Alchemy to your project

We’ve created a Metamask wallet, Alchemy account, and written our smart contract, now it’s time to connect the three.

Every transaction sent from your virtual wallet requires a signature using your unique private key. To provide our program with this permission, we can safely store our private key (and Alchemy API key) in an environment file.

To learn more about sending transactions, check out this tutorial on sending transactions using web3.

First, install the dotenv package in your project directory:

npm install dotenv --save

Then, create a .env file in the root directory of our project, and add your Metamask private key and HTTP Alchemy API URL to it.

Your environment file must be named .env or it won't be recognized as an environment file.

Do not name it process.env or .env-custom or anything else.

WARNING: If you are using version control system like git to manage your project, please DO NOT track the .env file. Add .env to your .gitignore file so that you don't accidentally publish your secrets to the world

  • Follow these instructions to export your private key
  • To get your Alchemy HTTP API Key (RPC URL), go to your "Hello World" app in your Alchemy dashboard and click "View Key" in the top right corner. Then go ahead and copy your Alchemy HTTP API key!

Your .env should look like this:

API_URL = "https://polygon-mumbai.g.alchemy.com/v2/your-api-key"
PRIVATE_KEY = "your-metamask-private-key"

To actually connect these to our code, we’ll reference these variables in our hardhat.config.js file on step 13.

Step 12: Install Ethers.js

Ethers.js is a library that makes it easier to interact and make requests to Ethereum by wrapping standard JSON-RPC methods with more user friendly methods.

Hardhat makes it super easy to integrate Plugins for additional tooling and extended functionality. We’ll be taking advantage of the Ethers plugin for contract deployment (Ethers.js has some super clean contract deployment methods).

In your project directory type:

npm install --save-dev @nomiclabs/hardhat-ethers "[email protected]^5.0.0"

We’ll also require ethers in our hardhat.config.js in the next step.

Step 13: Update hardhat.config.js

We’ve added several dependencies and plugins so far, now we need to update hardhat.config.js so that our project knows about all of them.

Update your hardhat.config.js to look like this:

/**
* @type import('hardhat/config').HardhatUserConfig
*/
require('dotenv').config();
require("@nomiclabs/hardhat-ethers");
const { API_URL, PRIVATE_KEY } = process.env;
module.exports = {
solidity: "0.7.3",
defaultNetwork: "polygon_mumbai",
networks: {
hardhat: {},
polygon_mumbai: {
url: API_URL,
accounts: [`0x${PRIVATE_KEY}`]
}
},
}

Step 14: Compile our contract

To make sure everything is working so far, let’s compile our contract. The compile task is one of the built-in hardhat tasks.

From the command line run:

npx hardhat compile

You might get a warning about SPDX license identifier not provided in source file , but no need to worry about that — hopefully everything else looks good! If not, you can always message in the Alchemy discord.

Step 15: Write our deploy script

Now that our contract is written and our configuration file is good to go, it’s time to write our contract deploy script.

Navigate to the scripts/ folder and create a new file called deploy.js , adding the following contents to it:

async function main() {
const HelloWorld = await ethers.getContractFactory("HelloWorld");
// Start deployment, returning a promise that resolves to a contract object
const hello_world = await HelloWorld.deploy("Hello World!");
console.log("Contract deployed to address:", hello_world.address);
}
main()
.then(() => process.exit(0))
.catch(error => {
console.error(error);
process.exit(1);
});

Hardhat does an amazing job of explaining what each of these lines of code does in their Contracts tutorial, we’ve adopted their explanations here.

const HelloWorld = await ethers.getContractFactory("HelloWorld");

A ContractFactory in ethers.js is an abstraction used to deploy new smart contracts, so HelloWorld here is a factory for instances of our hello world contract. When using the hardhat-ethers plugin ContractFactory and Contract, instances are connected to the first signer (owner) by default.

const hello_world = await HelloWorld.deploy();

Calling deploy() on a ContractFactory will start the deployment, and return a Promise that resolves to a Contract object. This is the object that has a method for each of our smart contract functions.

Step 16: Deploy our contract

We’re finally ready to deploy our smart contract! Navigate to the command line and run:

npx hardhat run scripts/deploy.js --network polygon_mumbai

You should then see something like:

Contract deployed to address: 0x3d94af870ED272Cd5370e4135F9B2Bd0e311d65D

Please copy and paste this address to save it somewhere, as we will be using this address for later tutorials, so you don't want to lose it.

If we go to the Polygon Mumbai explorer and search for our contract address we should able to see that it has been deployed successfully.

The From address should match your Metamask account address and the To address will say “Contract Creation”. But if we click into the transaction, we’ll see our contract address in the To field:

Congrats! You just deployed a smart contract to the Polygon chain 🎉

To understand what’s going on under the hood, let’s navigate to the Explorer tab in our Alchemy dashboard. If you have multiple Alchemy apps make sure to filter by app and select “Hello World”.

Here you’ll see a handful of JSON-RPC calls that Hardhat/Ethers made under the hood for us when we called the .deploy() function. Two important ones to call out here are eth_sendRawTransaction, which is the request to actually write our contract onto the Polygon chain, and eth_getTransactionByHash which is a request to read information about our transaction given the hash (a typical pattern when sending transactions).

That’s all for this tutorial! Once you complete this tutorial, let us know how your experience was or if you have any feedback by tagging us on Twitter @alchemyplatform!

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