CODE EXAMPLE FOR JAVASCRIPT

check balance of a wallet in js

const testnet = 'https://ropsten.infura.io/';
const walletAddress = '0x8690F1feff62008A396B31c2C3f380bD0Ca6d8b8';

const web3 = new Web3(new Web3.providers.HttpProvider(testnet));
var balance = web3.eth.getBalance(walletAddress); //Will give value in.
balance = web3.toDecimal(balance);