Mapped types

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

Partial#

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 you have a create type as follows:

import { ApiProperty } from '@nestjs/swagger';

export class CreateCatDto {
  @ApiProperty()
  name: string;

  @ApiProperty()
  age: number;

  @ApiProperty()
  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() and pass the class reference (CreateCatDto) as an argument:

export class UpdateCatDto extends PartialType(CreateCatDto) {}

Hint The PartialType() function is imported from the @nestjs/swagger package.

Pick#

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

import { ApiProperty } from '@nestjs/swagger';

export class CreateCatDto {
  @ApiProperty()
  name: string;

  @ApiProperty()
  age: number;

  @ApiProperty()
  breed: string;
}

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

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

Hint The PickType() function is imported from the @nestjs/swagger package.

Omit#

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 you start with a type like:

import { ApiProperty } from '@nestjs/swagger';

export class CreateCatDto {
  @ApiProperty()
  name: string;

  @ApiProperty()
  age: number;

  @ApiProperty()
  breed: string;
}

You can generate a derived type that has every property exceptname, 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) {}

Hint The OmitType() function is imported from the @nestjs/swagger package.

Intersection#

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

import { ApiProperty } from '@nestjs/swagger';

export class CreateCatDto {
  @ApiProperty()
  name: string;

  @ApiProperty()
  breed: string;
}

export class AdditionalCatInfo {
  @ApiProperty()
  color: string;
}

You can generate a new type that combines all properties of both types:

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

Hint The IntersectionType() function is imported from the @nestjs/swagger package.

Composition#

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

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