lcbp3/.agents/skills/nestjs-best-practices/rules/api-use-pipes.md

---
title: Use Pipes for Input Transformation
impact: MEDIUM
impactDescription: Pipes ensure clean, validated data reaches your handlers
tags: api, pipes, validation, transformation
---

## Use Pipes for Input Transformation

Use built-in pipes like `ParseIntPipe`, `ParseUUIDPipe`, and `DefaultValuePipe` for common transformations. Create custom pipes for business-specific transformations. Pipes separate validation/transformation logic from controllers.

**Incorrect (manual type parsing in handlers):**

```typescript
// Manual type parsing in handlers
@Controller('users')
export class UsersController {
  @Get(':id')
  async findOne(@Param('id') id: string): Promise<User> {
    // Manual validation in every handler
    const uuid = id.trim();
    if (!isUUID(uuid)) {
      throw new BadRequestException('Invalid UUID');
    }
    return this.usersService.findOne(uuid);
  }

  @Get()
  async findAll(
    @Query('page') page: string,
    @Query('limit') limit: string,
  ): Promise<User[]> {
    // Manual parsing and defaults
    const pageNum = parseInt(page) || 1;
    const limitNum = parseInt(limit) || 10;
    return this.usersService.findAll(pageNum, limitNum);
  }
}

// Type coercion without validation
@Get()
async search(@Query('price') price: string): Promise<Product[]> {
  const priceNum = +price; // NaN if invalid, no error
  return this.productsService.findByPrice(priceNum);
}
```

**Correct (use built-in and custom pipes):**

```typescript
// Use built-in pipes for common transformations
@Controller('users')
export class UsersController {
  @Get(':id')
  async findOne(@Param('id', ParseUUIDPipe) id: string): Promise<User> {
    // id is guaranteed to be a valid UUID
    return this.usersService.findOne(id);
  }

  @Get()
  async findAll(
    @Query('page', new DefaultValuePipe(1), ParseIntPipe) page: number,
    @Query('limit', new DefaultValuePipe(10), ParseIntPipe) limit: number,
  ): Promise<User[]> {
    // Automatic defaults and type conversion
    return this.usersService.findAll(page, limit);
  }

  @Get('by-status/:status')
  async findByStatus(
    @Param('status', new ParseEnumPipe(UserStatus)) status: UserStatus,
  ): Promise<User[]> {
    return this.usersService.findByStatus(status);
  }
}

// Custom pipe for business logic
@Injectable()
export class ParseDatePipe implements PipeTransform<string, Date> {
  transform(value: string): Date {
    const date = new Date(value);
    if (isNaN(date.getTime())) {
      throw new BadRequestException('Invalid date format');
    }
    return date;
  }
}

@Get('reports')
async getReports(
  @Query('from', ParseDatePipe) from: Date,
  @Query('to', ParseDatePipe) to: Date,
): Promise<Report[]> {
  return this.reportsService.findBetween(from, to);
}

// Custom transformation pipes
@Injectable()
export class NormalizeEmailPipe implements PipeTransform<string, string> {
  transform(value: string): string {
    if (!value) return value;
    return value.trim().toLowerCase();
  }
}

// Parse comma-separated values
@Injectable()
export class ParseArrayPipe implements PipeTransform<string, string[]> {
  transform(value: string): string[] {
    if (!value) return [];
    return value.split(',').map((v) => v.trim()).filter(Boolean);
  }
}

@Get('products')
async findProducts(
  @Query('ids', ParseArrayPipe) ids: string[],
  @Query('email', NormalizeEmailPipe) email: string,
): Promise<Product[]> {
  // ids is already an array, email is normalized
  return this.productsService.findByIds(ids);
}

// Sanitize HTML input
@Injectable()
export class SanitizeHtmlPipe implements PipeTransform<string, string> {
  transform(value: string): string {
    if (!value) return value;
    return sanitizeHtml(value, { allowedTags: [] });
  }
}

// Global validation pipe with transformation
app.useGlobalPipes(
  new ValidationPipe({
    whitelist: true, // Strip non-DTO properties
    transform: true, // Auto-transform to DTO types
    transformOptions: {
      enableImplicitConversion: true, // Convert query strings to numbers
    },
    forbidNonWhitelisted: true, // Throw on extra properties
  }),
);

// DTO with transformation decorators
export class FindProductsDto {
  @IsOptional()
  @Type(() => Number)
  @IsInt()
  @Min(1)
  page?: number = 1;

  @IsOptional()
  @Type(() => Number)
  @IsInt()
  @Min(1)
  @Max(100)
  limit?: number = 10;

  @IsOptional()
  @Transform(({ value }) => value?.toLowerCase())
  @IsString()
  search?: string;

  @IsOptional()
  @Transform(({ value }) => value?.split(','))
  @IsArray()
  @IsString({ each: true })
  categories?: string[];
}

@Get()
async findAll(@Query() dto: FindProductsDto): Promise<Product[]> {
  // dto is already transformed and validated
  return this.productsService.findAll(dto);
}

// Pipe error customization
@Injectable()
export class CustomParseIntPipe extends ParseIntPipe {
  constructor() {
    super({
      exceptionFactory: (error) =>
        new BadRequestException(`${error} must be a valid integer`),
    });
  }
}

// Or use options on built-in pipes
@Get(':id')
async findOne(
  @Param(
    'id',
    new ParseIntPipe({
      errorHttpStatusCode: HttpStatus.NOT_ACCEPTABLE,
      exceptionFactory: () => new NotAcceptableException('ID must be numeric'),
    }),
  )
  id: number,
): Promise<Item> {
  return this.itemsService.findOne(id);
}
```

Reference: [NestJS Pipes](https://docs.nestjs.com/pipes)