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How to Mint an NFT on Polygon with Ethers.js

September 09, 2022

Overview

This guide demonstrates how to mint an NFT on the Polygon blockchain using the Ethers.js library and our Factory ERC-1155 contract we built in an earlier guide. With the information you learn from this guide, you'll better understand how to interact with other smart contracts using Ethers.js.

Prerequisites

  • Node.js installed
  • A text editor
  • Terminal
  • MATIC tokens (Polygon mainnet)

What is Ethers.js?

Ethers.js is a complete Ethereum library and wallet implementation in Javascript. It has recently become the library of choice for dApp developers due to its efficient codebase, ease of use, and well-written documentation.

Ethers.js has several different modules. We will be using the Providers, Signers, Contract Interact, and Utility modules in this guide.

  • Providers - read-only abstraction to access the blockchain data.
  • Signers - an abstraction of an Ethereum Account 
  • Contract Interaction - an abstraction of a smart contract 
  • Utilities - Useful tools for application developers (i.e., convert hex to number) 
  • Other Libraries - Ethers ASM Dialect, Hardware Wallets (i.e., Ledger) 
  • Experimental - experimental features

Smart Contract Recap

As a reminder, the act of "minting an NFT" is calling the mint function on an ERC-721 or ERC-1155 contract.

Let us quickly review the solidity code for the function we will be calling to mint our ERC-1155 Tokens:

smart contract recap

Copy 
function mintERC1155(uint _index, string memory _name, uint256 amount) public {
        uint id = getIdByName(_index, _name);
        tokens[_index].mint(indexToOwner[_index], id, amount);
        emit ERC1155Minted(tokens[_index].owner(), address(tokens[_index]), amount);
}

Line 1: a public function named mintERC1155 that requires three arguments

Line 2: A require statement is meant to validate a statement before executing the remaining body of code. In this example, we're checking to ensure the sending address matches the owner's address from the ERC-1155 contract. If the condition is not met, the transaction will throw an error.

Line 3: The variable id is assigned to the output of a mapping. The mapping gets an ID by it's name. In our example, we'll mint Earth NFTs, which map to ID three in the factory contract. We'll explain how we got this info in a bit.

Line 4: The mint function from the token being indexed is called. There are three required parameters in the mint function we are calling.

Line 5: An Event log is emitted with our mint details, including the contract address of the token, the owner of the token, and the number of tokens minted.

Step 1: Booting your Polygon Node

We will deploy our contract on Polygon main-net. We can get a free trial node from QuickNode, which is easier than investing time in looking at different custom configurations to launch your own node. Copy the HTTP URL, which we will need in the next step.

Screenshot of Quicknode Polygon Endpoint

Step 2: Creating the Project

Time to write the code! Start by opening a terminal window and navigating to the directory you'd like to have this project live in. Then, run the following commands:

mkdir mintWithEthers && cd mintWithEthers && npm init -y

Next, we'll create our project files and install the required dependencies:

touch mint.js && touch abi.json && touch .secret && npm install ethers

Our project directory should now look similar to this:

Screenshot of project directory


We will start by pasting our private key into the .secret. file. To find out how to export your private key, check out this MetaMask Exporting Private Key Guide.

After, we will need to retrieve the ABI (application binary interface) of the contract we want to interact with. The ABI defines the methods and structures used to interact with the contract. It is represented in JSON format. Since the factory smart contract we want to interact with is deployed on Polygon, we will navigate to Polygonscan (a block explorer) and search this address: 0xB0C35A41994b75B98fA148b24Fcf0a84db21751D. Once you locate the page, click on the Contract tab and scroll down to the bottom to find the contract ABI. Click the copy button and paste the contents into your abi.json file.

ABI on Polygonscan


Now it's time to write the script that will interact with our Factory ERC-1155 smart contract. We will go over each section of code piece by piece and tie it all together in the end.

Open your mint.js file and start by adding the dependencies.

const { ethers } = require("ethers")
const fs = require('fs')

The line of code below uses the fs module to retrieve the private key stored in your .secret file contained in your project directory.

const privateKey = fs.readFileSync(".secret").toString().trim()

Ethers.js has several provider classes. We will be using the JsonRpcProvider, a popular method for interacting with Ethereum which is available in all major Ethereum node implementations. Add the following bit of code to your mint.js script:

const QUICKNODE_HTTP_ENDPOINT = "YOUR_QUICKNODE_HTTP_ENDPOINT"
const provider = new ethers.providers.JsonRpcProvider(QUICKNODE_HTTP_ENDPOINT)

Now, it's time to create an abstraction of the smart contract we want to interact with. A contract abstraction can be created like this:

const contractAddress = "0xB0C35A41994b75B98fA148b24Fcf0a84db21751D"
const contractAbi = fs.readFileSync("abi.json").toString()
const contractInstance = new ethers.Contract(contractAddress, contractAbi, provider)

We will also create some helper functions that will help us read and modify data to our needs:

async function getGasPrice() {
    let feeData = await provider.getFeeData()
    return feeData.gasPrice
}

async function getWallet(privateKey) {
    const wallet = await new ethers.Wallet(privateKey, provider)
    return wallet
}

async function getChain(_provider) {
    let chainId = await _provider.getNetwork()
    return chainId.chainId
}

async function getContractInfo(index, id) {
    let contract = await contractInstance.getERC1155byIndexAndId(index, id)
    return contract;
}

async function getNonce(signer) {
    return (await signer).getTransactionCount()
}

For example, the getGasPrice() function calls the getFeeData() method on a provider object which returns its response in hexadecimal format. We will then convert this hex value to gwei for useability in our script. The getWallet() function takes a private key and returns an Ethers Wallet object. The remaining functions utilize retrieving the chainId of the network and state information from one of the factory contract's functions.

Next, we will create a mint function that will utilize all the helper functions we created above to send and sign a transaction. Let us dive a bit into the explanation of the code below:

This function takes three inputs and is designed with a Try-Catch block for better error handling. When the function runs, it first attempts to run the code in the Try statement. The Try statement checks if the Provider is connected to the Polygon network (ID 137), then if that statement passes, it will create a wallet instance, get the gas price, and continue setting the transaction.

async function mintERC1155(index, name, amount) {
    try {
        if (await getChain(provider) === 137) {
            const wallet = getWallet(privateKey)
            const nonce = await getNonce(wallet)
            const gasFee = await getGasPrice()
            let rawTxn = await contractInstance.populateTransaction.mintERC1155(index, name, amount, {
                gasPrice: gasFee, 
                nonce: nonce
            })
            console.log("...Submitting transaction with gas price of:", ethers.utils.formatUnits(gasFee, "gwei"), " - & nonce:", nonce)
            let signedTxn = (await wallet).sendTransaction(rawTxn)
            let reciept = (await signedTxn).wait()
            if (reciept) {
                console.log("Transaction is successful!!!" + '\n' + "Transaction Hash:", (await signedTxn).hash + '\n' + "Block Number: " + (await reciept).blockNumber + '\n' + "Navigate to https://polygonscan.com/tx/" + (await signedTxn).hash, "to see your transaction")
            } else {
                console.log("Error submitting transaction")
            }
        }
        else {
            console.log("Wrong network - Connect to configured chain ID first!")
        }
    } catch (e) {
        console.log("Error Caught in Catch Statement: ", e)
    }
}

All that's left is to add the function call at the bottom of your script. In our example, we will be minting a Saturn NFT. To do this, we will call the mintERC1155 function with the following parameters:

mintERC1155(0, "Saturn", 1)

The first argument (0) refers to the specific contract we want to interact with within our smart contracts token array. Since we have only deployed one ERC-1155 token via this Factory contract, only the zero index has a contract we can interact with. The second argument specifies which NFT we want to mint, and the third is the amount of Saturn NFTs.

In the end, your complete script should look like:

const { ethers } = require("ethers")
const fs = require('fs')

const privateKey = fs.readFileSync(".secret").toString().trim()

const QUICKNODE_HTTP_ENDPOINT = "YOUR_QUICKNODE_HTTP_ENDPOINT"
const provider = new ethers.providers.JsonRpcProvider(QUICKNODE_HTTP_ENDPOINT)

const contractAddress = "0xB0C35A41994b75B98fA148b24Fcf0a84db21751D"
const contractAbi = fs.readFileSync("abi.json").toString()
const contractInstance = new ethers.Contract(contractAddress, contractAbi, provider)

async function getGasPrice() {
    let feeData = await provider.getFeeData()
    return feeData.gasPrice
}

async function getWallet(privateKey) {
    const wallet = await new ethers.Wallet(privateKey, provider)
    return wallet
}

async function getChain(_provider) {
    let chainId = await _provider.getNetwork()
    return chainId.chainId
}

async function getContractInfo(index, id) {
    let contract = await contractInstance.getERC1155byIndexAndId(index, id)
    return contract;
}

async function getNonce(signer) {
    return (await signer).getTransactionCount()
}

async function mintERC1155(index, name, amount) {
    try {
        if (await getChain(provider) === 137) {
            const wallet = getWallet(privateKey)
            const nonce = await getNonce(wallet)
            const gasFee = await getGasPrice()
            let rawTxn = await contractInstance.populateTransaction.mintERC1155(index, name, amount, {
                gasPrice: gasFee, 
                nonce: nonce
            })
            console.log("...Submitting transaction with gas price of:", ethers.utils.formatUnits(gasFee, "gwei"), " - & nonce:", nonce)
            let signedTxn = (await wallet).sendTransaction(rawTxn)
            let reciept = (await signedTxn).wait()
            if (reciept) {
                console.log("Transaction is successful!!!" + '\n' + "Transaction Hash:", (await signedTxn).hash + '\n' + "Block Number:" + (await reciept).blockNumber + '\n' + "Navigate to https://polygonscan.com/tx/" + (await signedTxn).hash, "to see your transaction")
            } else {
                console.log("Error submitting transaction")
            }
        }
        else {
            console.log("Wrong network - Connect to configured chain ID first!")
        }
    } catch (e) {
        console.log("Error Caught in Catch Statement: ", e)
    }
}


mintERC1155(0, "Saturn", 1)

The last line of code calls the mintERC1155 function with some input data.

Step 3: Minting our NFT

Note, you will need some MATIC on Polygon main-net in order to proceed with this mint transaction

One command away from minting our Saturn NFT! Navigate to your terminal into your project's main directory and run the command `node mint.js`. The output should look like this:

log of mint script

We can verify the NFT was minted by checking a Polygonscan and OpenSea:

Transaction reciept on polygon

Our minted NFT on OpenSea



Conclusion

That’s it! You have minted an NFT using Ethers.js! To learn more about Ethers, you can check out some of our other Web3 SDK guides or take a look at the Ethers documentation.

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