# Defining commands

To define a command add an appropriately named function to the commands object of the aggregate the command refers to. This function receives three parameters: the aggregate instance, the command, and a mark object.

Inside of the function, add code that decides whether the command may be run. If so, call the mark.asDone function. If not, call the mark.asRejected function and provide a reason.

E.g., to issue an invoice, use the following code:

const commands = {
  issue (invoice, command, mark) {
    const canInvoiceBeIssued = // ...

    if (!canInvoiceBeIssued) {
      return mark.asRejected('...');
    }

    mark.asDone();
  }
};

For a detailled list of a command's properties, see the data structure of commands.

# Accessing the command data

To decide whether a command may be run you may need to access the command data. For that, use the command.data property.

E.g., to verify whether the amount that is given in the issue command is positive, use the following code:

if (command.data.amount > 0) {
  // ...
}

# Accessing the aggregate state

To decide whether a command may be run you may need to access the aggregate state. For that, use the state property of the aggregate.

E.g., to verify whether an invoice was already issued, use the following code (assuming that the aggregate state contains a property isIssued set to true):

if (invoice.state.isIssued) {
  // ...
}

# Publishing events

Typically, before calling mark.asDone, you need to publish events to let the world know about the outcome of the command. For that, call the events.publish function on the aggregate and provide the name and the data of the event.

E.g., to publish an issued event for an invoice, use the following code:

invoice.events.publish('issued', {
  amount: command.data.amount
});

If an event does not have any data, you can omit the second parameter:

invoice.events.publish('issued');