# 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 two parameters, the aggregate instance and the command.

Inside of the function, add code that decides whether the command may be run. If you need to reject the command, call the command.reject function and provide a reason.

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

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

    if (!canInvoiceBeIssued) {
      return command.reject('...');
    }

    // ...
  }
};

Some commands require asynchronous code. Therefore, you can use the keywords async and await. To be able do this, define the command using the async keyword:

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

    const result = await validateInvoice();

    // ...
  }
};

For a detailed 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) {
  // ...
}

To get the aggregate ID, access the property id of the aggregate directly, as the ID is not part of the state.

# Publishing events

Typically 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');

# Verifying whether an aggregate exists

Some commands are intended to initialize new aggregates. Other commands, though, are supposed to change the state of existing aggregates. To ensure that commands are only executed if they are to be executed, you may need to determine whether you are dealing with a new or an existing aggregate. That's what the exists function is for. Depending on the state of the aggregate, this function returns either true or false.

E.g., to verify whether an invoice aggregate already exists, use the following code:

if (invoice.exists()) {
  // ...
}