What is the purpose of maintaining a Domain Event array?

[Kevinizevbigie]

In the User domain, inside the create function the UserCreatedDomainEvent is added to the _domainEvents array.

static create(create: CreateUserProps): UserEntity {
    const id = v4();

    const props: UserProps = { ...create, role: UserRoles.guest };
    const user = new UserEntity({ id, props });
    
    user.addEvent(
      new UserCreatedDomainEvent({
        aggregateId: id,
        email: props.email,
        ...props.address.unpack(),
      }),
    );
    return user;
  }

My question is what is the purpose of adding these into an array?

[Sairyss]

When a user is created, you may want other parts of the system to know about it, that's why we are emitting Domain Events.
It is an array because there may be multiple events emitted, for example if you execute multiple methods, each one may add an event. Or if one method emits multiple events, for instance when created user is an admin you may also want to emit a second event like AdminCreatedDomainEvent, to prevent your event listeners that are interested only in admins to subscribe for every UserCreatedDomainEvent you can listen to the admin events only.

[Kevinizevbigie]

Understood.

Say I have a _domainEvents array filled with multiple events that have been emitted. How would other parts of the system actually use this information?

[Sairyss]

_domainEvents is a place where we store events temporarily until we publish. When we write the changes to the aggregate into the database, we call publishEvents method from aggregate-root.base.ts file, this publishes the events and makes sure everything executes as a part of a transaction.

[Kevinizevbigie]

Quick follow up to this question. Maybe it's different with EventEmitter2 but where are the .addlisteners? Or how would you implement them if they are not in the project at the oment?

[Sairyss]

I don't remember much about .addlisteners, I think it was removed from the project long ago. @nestjs/event-emitter provides @OnEvent() decorator that is easier to use, I'd suggest to use it instead.