# Using services

To use services, add services as third parameter to your event handler.

E.g., if you want to use services from within the accounting.invoice.issued event handler, use the following code:

const projections = {
  'accounting.invoice.issued' (invoices, event, services) {
    // ...
  }
};

Since you do not usually need all services at the same time, it will make sense to request only the services you need. To do this, use destructuring to specify the services you need, e.g.:

const projections = {
  'accounting.invoice.issued' (invoices, event, { app }) {
    // ...
  }
};

# Reading other lists

Sometimes you need to read another list, e.g. to decide what to update. For that use the app service, access the list, and call the read or readOne function.

E.g., if you want to read another invoice from the accounting.invoice.issued event handler, use the following code:

const projections = {
  async 'accounting.invoice.issued' (invoices, event, { app }) {
    const otherInvoice = await app.lists.invoices.readOne({
      where: { id: '664745ac-808a-4c16-8420-f43d9deeee04' }
    });

    // ...
  }
};

# Writing log output

To write JSON-formatted log output use the logger service. Internally this service uses flaschenpost. For details on how to use flaschenpost see its documentation.

E.g., to log messages from within the accounting.invoice.issued event handler, use the following code:

const projections = {
  'accounting.invoice.issued' (invoices, event, { logger }) {
    logger.info('Handling an issued invoice...');

    // ...
  }
};