# Handling events

To handle an event, add the fully-qualified name of the event to the when object and provide a function to handle the event. Like commands, this function takes three parameters: the list itself, the event, and a mark object.

Inside of the function, add code that updates the list according to the event. Once you are done, call the mark.asDone function.

E.g., to handle the issued event of an invoice, use the following code:

const when = {
  'accounting.invoice.issued' (invoices, event, mark) {
    // ...
    mark.asDone();
  }
};

# Adding items

To add an item to a list, call the list's add function and provide the item you want to add. You have access to all of the event's data, including its metadata such as the id of the aggregate the event refers to. If you do not provide an id explicitly, wolkenkit will automatically use the id of the aggregate the event refers to.

E.g., to add an invoice to the list of invoices once an issued event is received, use the following code:

const when = {
  'accounting.invoice.issued' (invoices, event, mark) {
    invoices.add({
      amount: event.data.amount,
      participant: event.data.participant
    });
    mark.asDone();
  }
};

# Updating items

To update an item, call the list's update function and provide a where clause as well as an update expression. Use the where clause to specify which items to update, and the update expression to specify how these items will be updated.

E.g., to update an invoice once a sentAsLetterPost event is received, use the following code:

const when = {
  'accounting.invoice.sentAsLetterPost' (invoices, event, mark) {
    invoices.update({
      where: { id: event.aggregate.id },
      set: {
        sentAsLetterPost: true
      }
    });
    mark.asDone();
  }
};

# Removing items

Finally, to remove an item, call the list's remove function and provide a where clause to describe one or more items to be removed.

E.g., to remove an invoice from the list of invoices once a paid event is received, use the following code:

const when = {
  'accounting.invoice.paid' (invoices, event, mark) {
    invoices.remove({
      where: { id: event.aggregate.id }
    });
    mark.asDone();
  }
};