NestJS returns buffer instead of JSON in e2e

Question

While e2e testing with jest/supertest the body returned from my app seems to be a buffer rather than just JSON.

The test module:

const testingModule = await Test.createTestingModule({
  imports: [AppModule],
}).compile();

const app = testingModule.createNestApplication();

app.useGlobalPipes(
  new ValidationPipe({
    whitelist: true,
    forbidNonWhitelisted: true,
  }),
);

await app.init();
server = app.getHttpServer();

In the test I expect the ValidationPipe to throw an error about the password being too short:

const res = request(server)
  .post(path)
  .accept('application/json')
  .responseType('application/json')
  .set('Authorization', `Bearer ${token}`)
  .send({
    email: 'abc@abc.abc',
    displayName: 'Abc',
    password: 'abc'
  });

res.expect({error: 'Bad Request'}); // fails with expected { error: 'Bad Request' } response body, got <Buffer 7b 22 73 74...

The endpoint is:

@Post()
@UseGuards(AnonymousGuard)
@ApiBearerAuth('Firebase JWT')
@ApiCreatedResponse({ description: 'Success' })
@ApiUnauthorizedResponse({ description: 'Authorization failed' })
@ApiConflictResponse({ description: 'User exists' })
@ApiBadRequestResponse({ description: 'Malformed request' })
createUser(@Body() body: CreateUserBody) {
  throw new HttpException('not implemented', HttpStatus.NOT_IMPLEMENTED);
}

And the validation in:

export class CreateUserBody {
  @IsEmail()
  @ApiProperty()
  email: string;

  @IsString()
  @MinLength(2)
  @MaxLength(30)
  @ApiProperty()
  displayName: string;

  @IsString()
  @MinLength(8)
  @ApiProperty()
  password: string;
}

The problem is that the test response doesn't contain JSON but rather a buffer:

console.log(res.body);             // <Buffer 7b 22 73 74 61 74 75...
console.log(JSON.parse(res.body)); // { error: "Bad request",
    message: [ 'password must be longer...

I do not use any interceptors but some middleware I've got is async – could this be a problem?

I can get away with parsing the body, but every example on the internet seems to just use req.body (e.g. https://docs.nestjs.com/fundamentals/testing#end-to-end-testing) while I can't seem to be able to and I'm not sure what could cause that.

Answer 1

Removing .responseType('application/json') that I had copied from some website seems to have solved it. That converts the response into a Buffer.