solana-program-examples/basics/counter/native/ts/accounts/counter.ts

import * as BN from 'bn.js';

export type Counter = {
    count: BN
}

export const COUNTER_ACCOUNT_SIZE = 8;

export function deserializeCounterAccount(data: Buffer): Counter {
    if (data.byteLength !== 8) {
        throw Error("Need exactly 8 bytes to deserialize counter")
    }

    return {
        count: new BN(data, 'le')
    }
}