How to use the @polkadot/util.bnToU8a function in @polkadot/util

toU8a (isBare?: boolean): Uint8Array {
    // HACK 15 Oct 2018 For isBare assume that we are dealing with an AccountIndex
    // lookup (it is the only place where AccountIndex is used in such a manner to
    // construct a query). This is needed to get enumSet(AccountIndex) queries to
    // work in the way it was intended
    return isBare
      ? bnToU8a(this.toBn().div(ENUMSET_SIZE), 32, true)
      : super.toU8a();
  }
}

const value = isHex(_value)
      ? hexToU8a(_value)
      : _value;

    try {
      return new MetadataVersioned(registry, value);
    } catch (error) {
      if (error.message.indexOf('MagicNumber mismatch') === -1) {
        throw error;
      }
    }

    return new MetadataVersioned(
      registry,
      u8aConcat(
        bnToU8a(MAGIC_NUMBER), // manually add the magic number
        Uint8Array.from([0]), // add the version for the original
        value // the actual data as retrieved
      )
    );
  }
}

set: (value: BN | number, keyParam: any): void =>
      base.set(createKey(keyParam), value, bnToU8a(value, bitLength, true)),
    onUpdate: (updater: (value: BN | number, raw: Uint8Array) => void): void =>

function makeBlockHeader (prevNumber: BN) {
  const blockNumber = prevNumber.addn(1);

  return {
    digest: {
      logs: []
    },
    extrinsicsRoot: randomAsU8a(),
    number: blockNumber,
    parentHash: prevNumber.eqn(-1)
      ? new Uint8Array(32)
      : bnToU8a(prevNumber, 256, false),
    stateRoot: bnToU8a(blockNumber, 256, false)
  };
}

function setStorageBn (db: MockState$Db, createKey: StorageFunction, value: BN | number, ...keyParams: Array): void {
  const keyValue = u8aToHex(
    createKey(...keyParams)
  );

  db[keyValue] = bnToU8a(value, 64, true);
}

function makeBlockHeader (prevNumber: BN) {
  const blockNumber = prevNumber.addn(1);

  return {
    digest: {
      logs: []
    },
    extrinsicsRoot: randomAsU8a(),
    number: blockNumber,
    parentHash: prevNumber.eqn(-1)
      ? new Uint8Array(32)
      : bnToU8a(prevNumber, 256, false),
    stateRoot: bnToU8a(blockNumber, 256, false)
  };
}

private setStateBn (key: Uint8Array, value: BN | number): void {
    this.db[u8aToHex(key)] = bnToU8a(value, 64, true);
  }