simple_bridge.js

const ethers = require('ethers');

async function app() {

    const privateKey = '...'; // bridge wallet, has permission to mint and burn

    const abi = [
        'event Transfer(address indexed from, address indexed to, uint256 value)',
        'function mint(address receiver, uint256 amount)',
        'function burn(address account, uint256 amount)'
    ];
    
    const providerA = new ethers.providers.JsonRpcProvider('https://data-seed-prebsc-1-s1.binance.org:8545/'); // BSC TESTNET
    const providerB = new ethers.providers.JsonRpcProvider('https://matic-mumbai.chainstacklabs.com'); // MUMBAI
  
    const walletA = new ethers.Wallet(privateKey, providerA);
    const walletB = new ethers.Wallet(privateKey, providerB);

    const contractA = new ethers.Contract('0x..contract_address_chain_A...', abi, walletA);
    const contractB = new ethers.Contract('0x..contract_address_chain_B...', abi, walletB);

    console.log('Standing by...');
    
    contractA.on('Transfer', async function(from, to, value){
        if(to === walletA.address){
            console.log('[A->B] incoming from ', from, 'amount', ethers.utils.formatUnits(value));
            await contractA.burn(to, value);
            console.log('burn on chain A success!');
            await contractB.mint(from, value);
            console.log('mint on chain B success!');
        }
    });

    contractB.on('Transfer', async function(from, to, value){
        if(to === walletA.address){
            console.log('[B->A] incoming from ', from, 'amount', ethers.utils.formatUnits(value));
            await contractB.burn(to, value);
            console.log('burn on chain B success!');
            await contractA.mint(from, value);
            console.log('mint on chain A success!');
        }
    });
}

app();