How to use the qtumjs-wallet.generateMnemonic function in qtumjs-wallet

const login = (_privateKey, app) => {
  SwapApp.required(app)

  const storageKey = `${app.network}:qtum:privateKey`
  const privateKey = _privateKey || app.env.storage.getItem(storageKey)
  let account

  const network = app.isMainNet()
    ? networks.mainnet
    : networks.testnet

  // TODO check format private key
  if (privateKey) {
    account = network.fromWIF(privateKey)
  } else {
    const mnemonic  = generateMnemonic()
    account = network.fromMnemonic(mnemonic)
    app.env.storage.setItem(storageKey, account.toWIF())
    console.log('account', account.address)
  }

  return account
}

const createWallet = () => {
  const mnemonic  = generateMnemonic()
  const password  = 'superstrong'

  wallet = network.fromMnemonic(mnemonic, password)

  return _getData()
}