[Nest.js] Decorator for validating headers with ValidationPipe, class-validator and class-transformer

example-of-using-headers-with-validation-decorator.ts

// get-books-headers.dto.ts
import { Contains } from 'class-validator';

class GetBooksHeadersDto {
  @Contains('en')
  'accept-language': string;
}

// books.controller.ts
import { HeadersWithValidation } from './headers-with-validation.decorator.ts'
import { GetBooksHeadersDto } from './get-books-headers.dto.ts';

@Controller()
class BooksController {
  @Get()
  getBooks (
    @HeadersWithValidation(() => GetBooksHeadersDto) headers: GetBooksHeadersDto,
  ) {
    console.log(headers['accept-language']); // contains 'en'
    return [];
  }
}

headers-with-validation.decorator.ts

import { createParamDecorator, ExecutionContext, ValidationPipe } from '@nestjs/common';
import { ClassConstructor } from 'class-transformer';

/**
 * Decorator for validating headers with ValidationPipe, class-transformer and class-validator.
 *
 * @example
 * // require user agent to accept english
 *
 * class GetBooksHeadersDto {
 *  (a)Contains('en')
 *  'accept-language': string
 * }
 *
 * (a)Get()
 * getBooks(
 *  (a)HeadersWithValidation(() => GetBooksHeadersDto) headers: GetBooksHeadersDto,
 * ) {...}
 */
export const HeadersWithValidation = createParamDecorator(
  async (typeResolver: () => ClassConstructor<unknown>, ctx: ExecutionContext) => {
    // extract headers
    const headers = ctx.switchToHttp().getRequest().headers;

    const validationPipe = new ValidationPipe({
      expectedType: typeResolver(),
      transform: true,
      whitelist: true,
      validateCustomDecorators: true,
    });

    const result = await validationPipe.transform(headers, {
      type: 'custom',
    });

    return result;
  },
);