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.

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 we 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() passing the class reference (CreateCatDto) as an argument:

export class UpdateCatDto extends PartialType(CreateCatDto) {}

info 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 we start with a type like:

import { ApiProperty } from '@nestjs/swagger';
export class CreateCatDto {
  @ApiProperty()
  name: string;
  @ApiProperty()
  age: number;
  @ApiProperty()
  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/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 we start with a type like:

import { ApiProperty } from '@nestjs/swagger';
export class CreateCatDto {
  @ApiProperty()
  name: string;
  @ApiProperty()
  age: number;
  @ApiProperty()
  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/swagger package.

Intersection

The IntersectionType() function combines two types into one new type (class). For example, suppose we 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;
}

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/swagger package.

Composition

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),
) {}