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How to interact with EVM Smart Contracts in Javascript

#smartcontract #pancakeswap #bnbsmartchain #web3

This tutorial is an introduction to the fundamentals of interacting with EVM compatible Smart Contracts in Javascript. We will demonstrates how to read from or write to Pancakeswap Prediction Smart Contract using web3.js. Pancakeswap Prediction Smart contract allows you to place bets on BNBUSD or CAKEUSD price moves.

Step 1: connect to web3

Pancakeswap runs on BNB Smart Chain. To interact with it we will use web3.js package which contains a collection of libraries that allows interaction with a blockchain node using HTTP, RPC or WebSocket providers. We will use HTTP provider https://bsc-dataseed1.binance.org:443

const Web3 = require("web3"); const provider = "https://bsc-dataseed1.binance.org:443"; const web3 = new Web3(provider);
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Step 2: connect to Smart Contract

To connect to Smart Contract we would need its address and ABI. Address of the contract could be found in Pancakeswap docs . Once we know the address, we can retrieve its ABI from BscScan by using its API. To make a network call we will use axios library

const { default: axios } = require("axios"); const contractAddress = "0x18B2A687610328590Bc8F2e5fEdDe3b582A49cdA"; const execute = async() => { const response = await axios.get(`https://api.bscscan.com/api?module=contract&action=getabi&address=${contractAddress}&apikey=YOUR_API_KEY`) const contract = new web3.eth.Contract(JSON.parse(response.data.result), contractAddress); } execute()
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Step 3: interact with Smart Contract

In EVM world Smart Contract functions could be of 2 types: read and write. Read functions allow to read Smart Contract data and they do not change Smart Contract state. Write functions instead do alter the state and we would require to pay for it to be executed (calling address should have enough blockchain native currency balance)

const epoch = await contract.methods.currentEpoch().call(); // read function const roundData = await contract.methods.rounds(epoch).call(); // read function const gasPrice = await web3.eth.getGasPrice(); // blockchain utility function const betBullFunctionABI = contract.methods.betBull(epoch).encodeABI(); // prepare write function const betBearFunctionABI = contract.methods.betBear(epoch).encodeABI(); // prepare write function const custodialAddress = "your_address"; const custodialAddressKey = "your_address_key"; const account = web3.eth.accounts.privateKeyToAccount(custodialAddressKey); // used to sign transaction const nonce = await web3.eth.getTransactionCount(custodialAddress); // get your account nonce const transaction = { nonce, from: custodialAddress, to: contractAddress, gasPrice, gas: 5000000, data: betBullFunctionABI, value: "12008140256521106" } // compose transaction object const signedTransaction = await account.signTransaction(transaction); // sign transaction const sentTransaction = await web3.eth.sendSignedTransaction(signedTransaction.rawTransaction); // send transaction to blockchain console.log(sentTransaction) const isClaimable = await contract.methods.claimable(epoch, custodialAddress).call(); // read function if(isClaimable){ // recompose transaction to claim wins const claimFunctionABI = contract.methods.claim([epoch]).encodeABI(); // prepare write function const nonce = await web3.eth.getTransactionCount(custodialAddress); // get your account nonce const claimTransaction = { nonce, from: custodialAddress, to: bnbPredictionContractAddress, gasPrice, gas: 5000000, data: claimFunctionABI, }; // compose transaction object const signedClaimTransaction = await account.signTransaction(claimTransaction); // sign transaction const sentClaimTransaction = await web3.eth.sendSignedTransaction(signedClaimTransaction.rawTransaction); // send transaction to blockchain console.log(sentClaimTransaction) }
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Our final code should look like this

const { default: axios } = require("axios"); const Web3 = require("web3"); const provider = "https://bsc-dataseed1.binance.org:443"; const web3 = new Web3(provider); const contractAddress = "0x18B2A687610328590Bc8F2e5fEdDe3b582A49cdA"; const execute = async() => { const response = await axios.get(`https://api.bscscan.com/api?module=contract&action=getabi&address=${contractAddress}&apikey=YOUR_API_KEY`) const contract = new web3.eth.Contract(JSON.parse(response.data.result), contractAddress); const epoch = await contract.methods.currentEpoch().call(); // read function const roundData = await contract.methods.rounds(epoch).call(); // read function const gasPrice = await web3.eth.getGasPrice(); // blockchain utility function const betBullFunctionABI = contract.methods.betBull(epoch).encodeABI(); // prepare write function const betBearFunctionABI = contract.methods.betBear(epoch).encodeABI(); // prepare write function const custodialAddress = "your_address"; const custodialAddressKey = "your_address_key"; const account = web3.eth.accounts.privateKeyToAccount(custodialAddressKey); // used to sign transaction const nonce = await web3.eth.getTransactionCount(custodialAddress); // get your account nonce const transaction = { nonce, from: custodialAddress, to: contractAddress, gasPrice, gas: 5000000, data: betBullFunctionABI, value: "12008140256521106" } // compose transaction object const signedTransaction = await account.signTransaction(transaction); // sign transaction const sentTransaction = await web3.eth.sendSignedTransaction(signedTransaction.rawTransaction); // send transaction to blockchain console.log(sentTransaction) const isClaimable = await contract.methods.claimable(epoch, custodialAddress).call(); // read function if(isClaimable){ // recompose transaction to claim wins const claimFunctionABI = contract.methods.claim([epoch]).encodeABI(); // prepare write function const nonce = await web3.eth.getTransactionCount(custodialAddress); // get your account nonce const claimTransaction = { nonce, from: custodialAddress, to: bnbPredictionContractAddress, gasPrice, gas: 5000000, data: claimFunctionABI, }; // compose transaction object const signedClaimTransaction = await account.signTransaction(claimTransaction); // sign transaction const sentClaimTransaction = await web3.eth.sendSignedTransaction(signedClaimTransaction.rawTransaction); // send transaction to blockchain console.log(sentClaimTransaction) } } execute()
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Link to gist here

P.S Keep in mind that executing the above script might lead to money loss and you would require a good prediction strategy that will help to minimize it.

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