Mapped types

warning Warning This chapter applies only to the code first approach.

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:

@InputType()
class CreateUserInput {
  @Field()
  email: string;

  @Field()
  password: string;

  @Field()
  firstName: 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 (CreateUserInput) as an argument:

@InputType()
export class UpdateUserInput extends PartialType(CreateUserInput) {}

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

The PartialType() function takes an optional second argument that is a reference to a decorator factory. This argument can be used to change the decorator function applied to the resulting (child) class. If not specified, the child class effectively uses the same decorator as the parent class (the class referenced in the first argument). In the example above, we are extending CreateUserInput which is annotated with the @InputType() decorator. Since we want UpdateUserInput to also be treated as if it were decorated with @InputType(), we didn't need to pass InputType as the second argument. If the parent and child types are different, (e.g., the parent is decorated with @ObjectType), we would pass InputType as the second argument. For example:

@InputType()
export class UpdateUserInput extends PartialType(User, InputType) {}

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:

@InputType()
class CreateUserInput {
  @Field()
  email: string;

  @Field()
  password: string;

  @Field()
  firstName: string;
}

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

@InputType()
export class UpdateEmailInput extends PickType(CreateUserInput, ['email'] as const) {}

info Hint The PickType() function is imported from the @nestjs/graphql 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:

@InputType()
class CreateUserInput {
  @Field()
  email: string;

  @Field()
  password: string;

  @Field()
  firstName: string;
}

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

@InputType()
export class UpdateUserInput extends OmitType(CreateUserInput, ['email'] as const) {}

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

Intersection

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

@InputType()
class CreateUserInput {
  @Field()
  email: string;

  @Field()
  password: string;
}

@ObjectType()
export class AdditionalUserInfo {
  @Field()
  firstName: string;
  
  @Field()
  lastName: string;
}

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

@InputType()
export class UpdateUserInput extends IntersectionType(CreateUserInput, AdditionalUserInfo) {}

info Hint The IntersectionType() function is imported from the @nestjs/graphql 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 CreateUserInput type except for email, and those properties will be set to optional:

@InputType()
export class UpdateUserInput extends PartialType(
  OmitType(CreateUserInput, ['email'] as const),
) {}