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How to write Smart Contracts on Polygon

In this example, we write a Voter contract, we will compile and then deploy it on the Polygon testnet using Web3. And then we interact with the blockchain using Geth and web3js.

Matic provides SDK and APIs for development.

Table of content​

Requirements​

Account​

First, create an Ethereum account if you don't have one. If you already have, you don't need another. You just need to send some credit to your account using Matic faucet. Click on the link below, and request for a fund.
https://faucet.polygon.technology/
now we need to communicate with the Testnet. Create an Datahub account.

Now let's check if your account is funded. 

geth attach https://matic-mumbai--jsonrpc.datahub.figment.io/apikey/YOURKEY/
> eth.getBalance("0xD8f24D419153E5D03d614C5155f900f4B5C8A65C")
1987540120000000000

Installation​

If you are using Arch-based Linux, like Manjaro here are the commands. for other distributions, are also almost the same. 

sudo pacman -S nodejs npm geth
sudo npm install -g solc
solcjs --version
git clone https://github.com/mlibre/blockchain.git
cd blockchain/Polygon/Smart Contracts
npm install

Implementation​

You can find the Voter contract source code here.
In this contract, you can simply add options to vote for, start a voting process, vote the options and get the result.

Deploying​

We use ethereum-smart-contract-deployer npm module to deploy the contract.

node main.js

The output is something like:

Network Name:  private
Network Peers:  118

Solidity Version: 0.8.9
Compiling contract voter.sol

Deploying Contract Voter
Arguments:  [ [ 'mlibre', 'Good' ] ]

Transaction hash: 0x347177fb10ce9028e75bdb98885ac33f98fcd46ac67f0371d8dbac3d8e5ebe6d
Owner: 0xD8f24D419153E5D03d614C5155f900f4B5C8A65C
Contract Address: 0xeaFEAf1e75B89DabfAf4E8C41858AFBe51c291B8

Congratulations. you made your first contract in Polygon Network.
You can find your contract address in the output log. Check it here: https://mumbai.polygonscan.com/address/0xeaFEAf1e75B89DabfAf4E8C41858AFBe51c291B8

Interacting with the blockchain​

The next step is to interact with the blockchain using Geth and web3js.

Geth​

geth attach  https://matic-mumbai--jsonrpc.datahub.figment.io/apikey/a99e72c92474b4eaec9340d7c03f3b81/
eth.getCode("0xeaFEAf1e75B89DabfAf4E8C41858AFBe51c291B8")
# To load a contract using geth just need to put the abi we created, and the address
var abi = [{"inputs":[{"internalType":"string[]","name":"_options","type":"string[]"}],"stateMutability":"nonpayable","type":"constructor"},{"inputs":[{"internalType":"string","name":"option","type":"string"}],"name":"addOption","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[],"name":"getOptions","outputs":[{"internalType":"string[]","name":"","type":"string[]"}],"stateMutability":"view","type":"function"},{"inputs":[],"name":"getVotes","outputs":[{"internalType":"uint256[]","name":"","type":"uint256[]"}],"stateMutability":"view","type":"function"},{"inputs":[{"internalType":"uint256","name":"","type":"uint256"}],"name":"options","outputs":[{"internalType":"string","name":"","type":"string"}],"stateMutability":"view","type":"function"},{"inputs":[],"name":"owner","outputs":[{"internalType":"address","name":"","type":"address"}],"stateMutability":"view","type":"function"},{"inputs":[],"name":"remove","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[],"name":"startVoting","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[{"internalType":"uint256","name":"option","type":"uint256"}],"name":"vote","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[{"internalType":"uint256","name":"","type":"uint256"}],"name":"votes","outputs":[{"internalType":"uint256","name":"","type":"uint256"}],"stateMutability":"view","type":"function"}]

var voter = eth.contract(abi).at("0xeaFEAf1e75B89DabfAf4E8C41858AFBe51c291B8");
voter

Web3​

To interact with the blockchain using web3js, we need to create a web3 instance. And have the contract address and the abi. So you need to keep the abi and the address in a file for later use.
You can also use the deployer contract instance just after the deployment.

const sender = "0xD8f24D419153E5D03d614C5155f900f4B5C8A65C";
  const deployer = await new Deployer({
   contractFilePath: "voter.sol",
   input: [["mlibre" , "Good"]],
   sender,
   privateKey: secrets.privateKey,
   address: secrets.mumbai
  });
  const contract = await deployer.deploy();
  // let abi = deployer.contract.abi
  // let contract = deployer.contractInstance
  await contract.methods.addOption("new option").send({from: sender});
  const options = await contract.methods.getOptions().call();
  await contract.methods.startVoting().send({from: sender});
  await contract.methods.vote(0).send({from: sender});
  const votes = await contract.methods.getVotes().call({
   from: sender,
  });
  console.log(options, votes);

MATIC​

MATIC is the Polygon Network native token.
The MATIC token will be used as the gas fee by default.
Polygon network provides an API to get the recommended gas price.
The javascript code would be something like this:

fetch('https://gasstation-mainnet.matic.network')
  .then(response => response.json())
  .then(json => console.log(json))

and the result is like:

{
    "safeLow": 1,
    "standard": 1,
    "fast": 5,
    "fastest": 7.5,
    "blockTime": 2,
    "blockNumber": 3091956
}