How to use lifecycle hooks in TypeORM?

TypeORM uses Subscribers and Listeners to implement lifecycle hooks:

1. Entity Listeners: Define listeners in the entity class using decorators:

   typescript
   @Entity()
   export class User {
     @BeforeInsert()
     beforeInsert() {
       this.createdAt = new Date();
     }
     
     @BeforeUpdate()
     beforeUpdate() {
       this.updatedAt = new Date();
     }
     
     @AfterLoad()
     afterLoad() {
       // Execute after loading
     }
   }

2. Subscribers: Independent classes that can listen to lifecycle events of multiple entities:

   typescript
   @EventSubscriber()
   export class UserSubscriber implements EntitySubscriberInterface<User> {
     listenTo() {
       return User;
     }
     
     beforeInsert(event: InsertEvent<User>) {
       console.log('Before insert:', event.entity);
     }
   }

3. Available events:

   • BeforeInsert / AfterInsert
   • BeforeUpdate / AfterUpdate
   • BeforeRemove / AfterRemove
   • BeforeSoftRemove / AfterSoftRemove
   • BeforeRecover / AfterRecover
   • AfterLoad

4. Register subscribers: Add the subscribers option in the DataSource configuration.

Lifecycle hooks are suitable for scenarios such as data validation, auto-filling fields, logging, etc.