What is property-based injection in NestJS and when should you use it?

Q: What is property-based injection in NestJS and when should you use it?

// Property injection @Injectable() export class UsersService { @Inject(ConfigService) private readonly config: ConfigService; // undefined in constructor! onModuleInit() { // config is available here — after construction console.log(this.config.get('APP_NAME')); } } // Preferred: constructor injection — explicit and testable @Injectable() export class UsersService { constructor(private readonly config: ConfigService) { // config is available immediately console.log(this.config.get('APP_NAME')); } } Base classes in deep inheritance chains — avoids requiring every subclass to pass all base dependencies through super(). Avoid using it as a shortcut to skip constructor argument lists — that is a code smell. Always prefer constructor injection for testability — property injection requires the full DI container to set the value. Property-injected fields are undefined inside the constructor body — never use them there.

Difficulty: 5/10

dependency injection, property injection, module configuration

Property injection uses @Inject() directly on a class property instead of in the constructor. The property is undefined during the constructor body. It is useful for base classes in inheritance hierarchies to avoid passing all dependencies through super(). Constructor injection is preferred for all other cases.

Property injection vs constructor injection

When property injection is appropriate:

1
Base classes in deep inheritance chains — avoids requiring every subclass to pass all base dependencies through super().
2
Avoid using it as a shortcut to skip constructor argument lists — that is a code smell.
3
Always prefer constructor injection for testability — property injection requires the full DI container to set the value.
4
Property-injected fields are undefined inside the constructor body — never use them there.

Scenario Questions

0-2 years experience

Follow-up Questions