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Listen to balance changes

This example shows how to instantiate a Polkadot API object and use it to connect to a node and retrieve balance updates.

// Import the API
const { ApiPromise } = require('@polkadot/api');

// Known account we want to use (available on dev chain, with funds)
const Alice = '5GrwvaEF5zXb26Fz9rcQpDWS57CtERHpNehXCPcNoHGKutQY';

async function main () {
// Create an await for the API
const api = await ApiPromise.create();

// Retrieve the initial balance. Since the call has no callback, it is simply a promise
// that resolves to the current on-chain value
let { data: { free: previousFree }, nonce: previousNonce } = await api.query.system.account(Alice);

console.log(`${Alice} has a balance of ${previousFree}, nonce ${previousNonce}`);
console.log(`You may leave this example running and start example 06 or transfer any value to ${Alice}`);

// Here we subscribe to any balance changes and update the on-screen value
api.query.system.account(Alice, ({ data: { free: currentFree }, nonce: currentNonce }) => {
// Calculate the delta
const change = currentFree.sub(previousFree);

// Only display positive value changes (Since we are pulling `previous` above already,
// the initial balance change will also be zero)
if (!change.isZero()) {
console.log(`New balance change of ${change}, nonce ${currentNonce}`);

previousFree = currentFree;
previousNonce = currentNonce;
}
});
}

main().catch(console.error);