# Using services

To use services, add services as second 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 reactions = {
  'accounting.invoice.issued' (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 reactions = {
  'accounting.invoice.issued' (event, { app }) {
    // ...
  }
};

# Sending commands

Sometimes you may need to send commands to the application from an event handler. For that use the app service, access the context and the aggregate, and call the command function.

E.g., if you want to send a check command from the accounting.invoice.issued event handler to the invoice that caused the event, use the following code:

const reactions = {
  'accounting.invoice.issued' (event, { app }) {
    app.accounting.invoice(event.aggregate.id).check();
  }
};

# 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 reactions = {
  'accounting.invoice.issued' (event, { logger }) {
    logger.info('Handling an issued invoice...');

    // ...
  }
};