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KAMMYSLIWIEC DIVING INTO TYPESCRIPT 
 DESIGN METADATA 
 WITH NESTJS

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KAMMYSLIWIEC KAMIL MYSLIWIEC CREATOR OF NESTJS | CO-FOUNDER OF TRILON.IO
 GOOGLE DEVELOPER EXPERT @KAMMYSLIWIEC


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KAMIL MYŚLIWIEC SOFTWARE ENGINEER
 @KAMMYSLIWIEC


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KAMMYSLIWIEC Adonis Express Koa Nest Fastify

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NESTJS WWW.NESTJS.COM
 @NESTFRAMEWORK
 KAMMYSLIWIEC

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DESIGN-TIME METADATA

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KAMMYSLIWIEC PARAMETER TYPE 
 METADATA

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DEPENDENCY INJECTION

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KAMMYSLIWIEC @Injectable() class CatsService { private httpService = new HttpService(); private logger = new Logger(); }

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KAMMYSLIWIEC @Injectable() class CatsService { private httpService = new HttpService(); private logger = new Logger(); }

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KAMMYSLIWIEC @Injectable() class CatsService { constructor( private httpService: HttpService, private logger: Logger, ) {} }

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KAMMYSLIWIEC @Injectable() class CatsService { constructor( private httpService: HttpService, private logger: Logger, ) {} }

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KAMMYSLIWIEC @Injectable() class CatsService { constructor( private httpService: HttpService, private logger: Logger, ) {} }

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KAMMYSLIWIEC @Injectable() class CatsService { constructor( private httpService: HttpService, private logger: Logger, ) {} }

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KAMMYSLIWIEC const Injectable: ClassDecorator = (target: Object) => {};

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KAMMYSLIWIEC COMPILER 
 METADATA

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KAMMYSLIWIEC @Injectable() class CatsService { constructor( private httpService: HttpService, private logger: Logger, ) {} }

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KAMMYSLIWIEC CatsService = __decorate( [__metadata('design:paramtypes', [HttpService, Logger])], CatsService, );

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KAMMYSLIWIEC CatsService = __decorate( [__metadata('design:paramtypes', [HttpService, Logger])], CatsService, );

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KAMMYSLIWIEC CatsService = __decorate( [__metadata('design:paramtypes', [HttpService, Logger])], CatsService, );

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KAMMYSLIWIEC @Injectable() class CatsService { constructor( private httpService: HttpService, private logger: Logger, ) {} }

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KAMMYSLIWIEC @Injectable() class CatsService { constructor( private httpService: HttpService, private logger: Logger, ) {} }

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REFLECTION
 API

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KAMMYSLIWIEC Reflect.getMetadata('design:paramtypes', CatsService);

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KAMMYSLIWIEC Reflect.getMetadata('design:paramtypes', CatsService);

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KAMMYSLIWIEC Reflect.getMetadata('design:paramtypes', CatsService);

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KAMMYSLIWIEC const metadata = Reflect.getMetadata( 'design:paramtypes', CatsService, ); // metadata = [HttpService, Logger]

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PIPES

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KAMMYSLIWIEC @Injectable() class ExponentialStrengthPipe implements PipeTransform { transform( value: number, metadata: ArgumentMetadata, ): number {} }

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KAMMYSLIWIEC @Injectable() class ExponentialStrengthPipe implements PipeTransform { transform( value: number, metadata: ArgumentMetadata, ): number {} }

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KAMMYSLIWIEC @Injectable() class ExponentialStrengthPipe implements PipeTransform { transform( value: number, metadata: ArgumentMetadata, ): number {} }

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KAMMYSLIWIEC @Injectable() class ExponentialStrengthPipe implements PipeTransform { transform( value: number, metadata: ArgumentMetadata, ): number {} }

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KAMMYSLIWIEC interface ArgumentMetadata { type: 'body' | 'query' | 'param' | 'custom'; metatype?: Type; data?: string; }

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KAMMYSLIWIEC @Post() async create( @Body() createDto: CreateRecipeDto, ): Promise { return 'This action adds a new recipe'; } TYPE METATYPE DATA

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KAMMYSLIWIEC @Post() async create( @Body() createDto: CreateRecipeDto, ): Promise { return 'This action adds a new recipe'; } TYPE METATYPE DATA

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KAMMYSLIWIEC const metadata = Reflect.getMetadata( 'design:paramtypes', new HostClass(),
 'create' ); // metadata = [CreateRecipeDto]

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KAMMYSLIWIEC REFLECTION API
 CHALLENGES

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KAMMYSLIWIEC INTERFACES

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KAMMYSLIWIEC @Injectable() class CatsService { constructor( private httpService: HttpService, private logger: Logger, ) {} }

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KAMMYSLIWIEC @Injectable() class CatsService { constructor( private httpService: IHttpService, private logger: Logger, ) {} }

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KAMMYSLIWIEC const metadata = Reflect.getMetadata( 'design:paramtypes', CatsService, ); // metadata = [Object, Logger]
 // IHttpService === Object

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KAMMYSLIWIEC POTENTIAL 
 SOLUTION

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KAMMYSLIWIEC GENERICS

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KAMMYSLIWIEC @Injectable() class CatsService { constructor( private httpService: HttpService, private logger: Logger, ) {} }

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KAMMYSLIWIEC @Injectable() class CatsService { constructor( private httpService: HttpService, private catsRepository: Repository, ) {} }

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KAMMYSLIWIEC const metadata = Reflect.getMetadata( 'design:paramtypes', CatsService, ); // metadata = [HttpService, Repository] // what about Cat class?

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KAMMYSLIWIEC POTENTIAL 
 SOLUTION

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KAMMYSLIWIEC CIRCULAR
 DEPENDENCIES

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KAMMYSLIWIEC @Injectable() class CatsService { constructor( private dogsService: DogsService, ) {} }

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KAMMYSLIWIEC @Injectable() class DogsService { constructor( private catsService: CatsService, ) {} }

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KAMMYSLIWIEC const metadata = Reflect.getMetadata( 'design:paramtypes', CatsService, ); // metadata = [undefined] // DogsService === undefined

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KAMMYSLIWIEC POTENTIAL 
 SOLUTION

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KAMMYSLIWIEC @Injectable() class CatsService { constructor( @Inject(forwardRef(() => DogsService)) private dogsService: DogsService, ) {} }

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KAMMYSLIWIEC function forwardRef(forwardRefFn: ForwardRefFn): Type { (forwardRefFn).__forward_ref__ = forwardRef; ...
 }

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KAMMYSLIWIEC function forwardRef(forwardRefFn: ForwardRefFn): Type { (forwardRefFn).__forward_ref__ = forwardRef; ...
 }

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KAMMYSLIWIEC TYPE METADATA

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KAMMYSLIWIEC export class User { email: string; password: string; }

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KAMMYSLIWIEC export class User { email: string; password: string; }

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KAMMYSLIWIEC const metadata = Reflect.getMetadata( 'design:type', new User(),
 'email' ); // metadata = undefined

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KAMMYSLIWIEC const metadata = Reflect.getMetadata( 'design:type', X ‘design:paramtype’ new User(),
 'email' ); // metadata = undefined

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KAMMYSLIWIEC const metadata = Reflect.getMetadata( 'design:type', new User(),
 'email' ); // metadata = undefined

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KAMMYSLIWIEC export class User { email: string; password: string; }

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KAMMYSLIWIEC export class User { @Property email: string; @Property password: string; }

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KAMMYSLIWIEC const metadata = Reflect.getMetadata( 'design:type', new User(),
 'email' ); // metadata = String

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KAMMYSLIWIEC WHAT’S THE POINT?

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No content

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KAMMYSLIWIEC export class User { @Property email: string; @Property password: string; }

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KAMMYSLIWIEC User: type: object properties: email: type: string password: type: string

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KAMMYSLIWIEC LIMITATIONS

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KAMMYSLIWIEC export class User { @Property email: string; @Property password: string; }

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KAMMYSLIWIEC export class User { @Property email: string; @Property password: string; @Property tags: string[]; }

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KAMMYSLIWIEC const metadata = Reflect.getMetadata( 'design:type', new User(),
 'tags' ); // metadata = Array

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KAMMYSLIWIEC export class User { @Property email: string; @Property password: string; @Property tags: string[]; }

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KAMMYSLIWIEC export class User { @Property email: string; @Property password: string; @Property tags: Array; }

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KAMMYSLIWIEC POTENTIAL 
 SOLUTION

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KAMMYSLIWIEC TYPESCRIPT
 PLUGINS

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KAMMYSLIWIEC const emitResult = program.emit( undefined, undefined, undefined, undefined, { before: [genericsMetadataPlugin], after: [], },
 );

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KAMMYSLIWIEC const emitResult = program.emit( undefined, undefined, undefined, undefined, { before: [genericsMetadataPlugin], after: [], },
 );

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KAMMYSLIWIEC RETURN TYPE METADATA

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KAMMYSLIWIEC @Post() create(): string { return 'This action adds a new recipe'; }

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KAMMYSLIWIEC const metadata = Reflect.getMetadata( ‘design:returntype', new HostClass(),
 'create' ); // metadata = String

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KAMMYSLIWIEC const metadata = Reflect.getMetadata( ‘design:returntype', X ‘design:type’ new HostClass(),
 'create' ); // metadata = String

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KAMMYSLIWIEC const metadata = Reflect.getMetadata( 'design:returntype', new HostClass(),
 'create' ); // metadata = String

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KAMMYSLIWIEC WHY IS IT USEFUL?

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KAMMYSLIWIEC LIMITATIONS

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KAMMYSLIWIEC @Post() async create(): Promise { return 'This action adds a new recipe'; }

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KAMMYSLIWIEC const metadata = Reflect.getMetadata( 'design:returntype', new HostClass(),
 'create' ); // metadata = Promise // what about “String”?

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RECAP

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KAMMYSLIWIEC OPEN SOURCE

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KAMMYSLIWIEC KAMMYSLIWIEC WWW.TRILON.IO

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KAMMYSLIWIEC OPEN SOURCE

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KAMMYSLIWIEC KAMMYSLIWIEC THANK
 YOU @KAMMYSLIWIEC