#Validation

It is best practice to validate the correctness of any data sent into a web application. To automatically validate incoming requests, Nest provides several pipes available right out-of-the-box:

• ValidationPipe
• ParseIntPipe
• ParseBoolPipe
• ParseArrayPipe
• ParseUUIDPipe

The ValidationPipe makes use of the powerful class-validator package and its declarative validation decorators. The ValidationPipe provides a convenient approach to enforce validation rules for all incoming client payloads, where the specific rules are declared with simple annotations in local class/DTO declarations in each module.

#Overview

In the Pipes chapter, we went through the process of building simple pipes and binding them to controllers, methods or to the global app to demonstrate how the process works. Be sure to review that chapter to best understand the topics of this chapter. Here, we'll focus on various real world use cases of the ValidationPipe, and show how to use some of its advanced customization features.

#Using the built-in ValidationPipe

To begin using it, we first install the required dependency.

$ npm i --save class-validator class-transformer

info Hint The ValidationPipe is exported from the @nestjs/common package.

Because this pipe uses the class-validator and class-transformer libraries, there are many options available. You configure these settings via a configuration object passed to the pipe. Following are the built-in options:

export interface ValidationPipeOptions extends ValidatorOptions {
  transform?: boolean;
  disableErrorMessages?: boolean;
  exceptionFactory?: (errors: ValidationError[]) => any;
}

In addition to these, all class-validator options (inherited from the ValidatorOptions interface) are available:

info Notice Find more information about the class-validator package in its repository.

#Auto-validation

We'll start by binding ValidationPipe at the application level, thus ensuring all endpoints are protected from receiving incorrect data.

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  app.useGlobalPipes(new ValidationPipe());
  await app.listen(process.env.PORT ?? 3000);
}
bootstrap();

To test our pipe, let's create a basic endpoint.

@Post()
create(@Body() createUserDto: CreateUserDto) {
  return 'This action adds a new user';
}

info Hint Since TypeScript does not store metadata about generics or interfaces, when you use them in your DTOs, ValidationPipe may not be able to properly validate incoming data. For this reason, consider using concrete classes in your DTOs.

info Hint When importing your DTOs, you can't use a type-only import as that would be erased at runtime, i.e. remember to import { CreateUserDto } instead of import type { CreateUserDto }.

Now we can add a few validation rules in our CreateUserDto. We do this using decorators provided by the class-validator package, described in detail here. In this fashion, any route that uses the CreateUserDto will automatically enforce these validation rules.

import { IsEmail, IsNotEmpty } from 'class-validator';

export class CreateUserDto {
  @IsEmail()
  email: string;

  @IsNotEmpty()
  password: string;
}

With these rules in place, if a request hits our endpoint with an invalid email property in the request body, the application will automatically respond with a 400 Bad Request code, along with the following response body:

{
  "statusCode": 400,
  "error": "Bad Request",
  "message": ["email must be an email"]
}

In addition to validating request bodies, the ValidationPipe can be used with other request object properties as well. Imagine that we would like to accept :id in the endpoint path. To ensure that only numbers are accepted for this request parameter, we can use the following construct:

@Get(':id')
findOne(@Param() params: FindOneParams) {
  return 'This action returns a user';
}

FindOneParams, like a DTO, is simply a class that defines validation rules using class-validator. It would look like this:

import { IsNumberString } from 'class-validator';

export class FindOneParams {
  @IsNumberString()
  id: string;
}

#Disable detailed errors

Error messages can be helpful to explain what was incorrect in a request. However, some production environments prefer to disable detailed errors. Do this by passing an options object to the ValidationPipe:

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

As a result, detailed error messages won't be displayed in the response body.

#Stripping properties

Our ValidationPipe can also filter out properties that should not be received by the method handler. In this case, we can whitelist the acceptable properties, and any property not included in the whitelist is automatically stripped from the resulting object. For example, if our handler expects email and password properties, but a request also includes an age property, this property can be automatically removed from the resulting DTO. To enable such behavior, set whitelist to true.

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

When set to true, this will automatically remove non-whitelisted properties (those without any decorator in the validation class).

Alternatively, you can stop the request from processing when non-whitelisted properties are present, and return an error response to the user. To enable this, set the forbidNonWhitelisted option property to true, in combination with setting whitelist to true.

#Transform payload objects

Payloads coming in over the network are plain JavaScript objects. The ValidationPipe can automatically transform payloads to be objects typed according to their DTO classes. To enable auto-transformation, set transform to true. This can be done at a method level:

@Post()
@UsePipes(new ValidationPipe({ transform: true }))
async create(@Body() createCatDto: CreateCatDto) {
  this.catsService.create(createCatDto);
}

To enable this behavior globally, set the option on a global pipe:

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

With the auto-transformation option enabled, the ValidationPipe will also perform conversion of primitive types. In the following example, the findOne() method takes one argument which represents an extracted id path parameter:

@Get(':id')
findOne(@Param('id') id: number) {
  console.log(typeof id === 'number'); // true
  return 'This action returns a user';
}

By default, every path parameter and query parameter comes over the network as a string. In the above example, we specified the id type as a number (in the method signature). Therefore, the ValidationPipe will try to automatically convert a string identifier to a number.

#Explicit conversion

In the above section, we showed how the ValidationPipe can implicitly transform query and path parameters based on the expected type. However, this feature requires having auto-transformation enabled.

Alternatively (with auto-transformation disabled), you can explicitly cast values using the ParseIntPipe or ParseBoolPipe (note that ParseStringPipe is not needed because, as mentioned earlier, every path parameter and query parameter comes over the network as a string by default).

@Get(':id')
findOne(
  @Param('id', ParseIntPipe) id: number,
  @Query('sort', ParseBoolPipe) sort: boolean,
) {
  console.log(typeof id === 'number'); // true
  console.log(typeof sort === 'boolean'); // true
  return 'This action returns a user';
}

info Hint The ParseIntPipe and ParseBoolPipe are exported from the @nestjs/common package.

#Mapped types

As you build out features like CRUD (Create/Read/Update/Delete) it's often useful to construct variants on a base entity type. Nest provides several utility functions that perform type transformations to make this task more convenient.

Warning If your application uses the @nestjs/swagger package, see this chapter for more information about Mapped Types. Likewise, if you use the @nestjs/graphql package see this chapter. Both packages heavily rely on types and so they require a different import to be used. Therefore, if you used @nestjs/mapped-types (instead of an appropriate one, either @nestjs/swagger or @nestjs/graphql depending on the type of your app), you may face various, undocumented side-effects.

When building input validation types (also called DTOs), it's often useful to build create and update variations on the same type. For example, the create variant may require all fields, while the update variant may make all fields optional.

Nest provides the PartialType() utility function to make this task easier and minimize boilerplate.

The PartialType() function returns a type (class) with all the properties of the input type set to optional. For example, suppose we have a create type as follows:

export class CreateCatDto {
  name: string;
  age: number;
  breed: string;
}

By default, all of these fields are required. To create a type with the same fields, but with each one optional, use PartialType() passing the class reference (CreateCatDto) as an argument:

export class UpdateCatDto extends PartialType(CreateCatDto) {}

info Hint The PartialType() function is imported from the @nestjs/mapped-types package.

The PickType() function constructs a new type (class) by picking a set of properties from an input type. For example, suppose we start with a type like:

export class CreateCatDto {
  name: string;
  age: number;
  breed: string;
}

We can pick a set of properties from this class using the PickType() utility function:

export class UpdateCatAgeDto extends PickType(CreateCatDto, ['age'] as const) {}

info Hint The PickType() function is imported from the @nestjs/mapped-types package.

The OmitType() function constructs a type by picking all properties from an input type and then removing a particular set of keys. For example, suppose we start with a type like:

export class CreateCatDto {
  name: string;
  age: number;
  breed: string;
}

We can generate a derived type that has every property except name as shown below. In this construct, the second argument to OmitType is an array of property names.

export class UpdateCatDto extends OmitType(CreateCatDto, ['name'] as const) {}

info Hint The OmitType() function is imported from the @nestjs/mapped-types package.

The IntersectionType() function combines two types into one new type (class). For example, suppose we start with two types like:

export class CreateCatDto {
  name: string;
  breed: string;
}

export class AdditionalCatInfo {
  color: string;
}

We can generate a new type that combines all properties in both types.

export class UpdateCatDto extends IntersectionType(
  CreateCatDto,
  AdditionalCatInfo,
) {}

info Hint The IntersectionType() function is imported from the @nestjs/mapped-types package.

The type mapping utility functions are composable. For example, the following will produce a type (class) that has all of the properties of the CreateCatDto type except for name, and those properties will be set to optional:

export class UpdateCatDto extends PartialType(
  OmitType(CreateCatDto, ['name'] as const),
) {}

#Parsing and validating arrays

TypeScript does not store metadata about generics or interfaces, so when you use them in your DTOs, ValidationPipe may not be able to properly validate incoming data. For instance, in the following code, createUserDtos won't be correctly validated:

@Post()
createBulk(@Body() createUserDtos: CreateUserDto[]) {
  return 'This action adds new users';
}

To validate the array, create a dedicated class which contains a property that wraps the array, or use the ParseArrayPipe.

@Post()
createBulk(
  @Body(new ParseArrayPipe({ items: CreateUserDto }))
  createUserDtos: CreateUserDto[],
) {
  return 'This action adds new users';
}

In addition, the ParseArrayPipe may come in handy when parsing query parameters. Let's consider a findByIds() method that returns users based on identifiers passed as query parameters.

@Get()
findByIds(
  @Query('ids', new ParseArrayPipe({ items: Number, separator: ',' }))
  ids: number[],
) {
  return 'This action returns users by ids';
}

This construction validates the incoming query parameters from an HTTP GET request like the following:

GET /?ids=1,2,3

#WebSockets and Microservices

While this chapter shows examples using HTTP style applications (e.g., Express or Fastify), the ValidationPipe works the same for WebSockets and microservices, regardless of the transport method that is used.

#Learn more

Read more about custom validators, error messages, and available decorators as provided by the class-validator package here.