Transformers

A transformer is a provider class decorated with the @Transformer() decorator. The class, though, must implement the TransformerHandler interface.

@Transformer()
export class TestTransformer implements TransformerHandler {
  transform(value: unknown): unknown | Promise<unknown> {
    return `Adding some text: ${value}`;
  }
}

Transformers can be applied to the following route parameters decorators:

• @Param(key: string, transformer?: TransformerClass)
• @Query(key: string, transformer?: TransformerClass)
• @Header(key: string, transformer?: TransformerClass)
• @Body(transformer?: TransformerClass)

@Controller('/test')
export class TestController {
  
  @Get('/transformer')
  async transformer(@Query('testTransformer', TestTransformer) param: string) {
    return param;
  }
}

Validation

Transformers can also suite the role of validators and errors can be thrown from its.

@Transformer()
export class NumberTransform implements TransformerHandler {
  transform(value: unknown): unknown | Promise<unknown> {
    if (!isNumber(value)) {
      throw new BadRequestException({ error: { }, message: 'Not a number' })
    }
    
    return parseInt(value);
  }
}

Built-in transformers and validators