Mapped Types

Mapped Types

Mapped types allows you to generate a different version of base types.

This article describes three types.

• PartialType
• PickType
• OmitType

PartialType

PartialType returns a type (class) with all the properties of the input type (base type) to set to optional (not required).

For example, let’s assume that there is a InputType Class CreateUserInput with three properties: email, password and firstName. There is also an another class UpdateUserInput, whose properties come in part from the class. When a user will update his information, the user might not need firstName (or the user could need the property firstName. It’s just not sure). Then the developer has an option to set the property as optional. It is illustrated again in the figure below. The class UpdateUserInput extends then the class CreateUserInput with the option of PartialType.

PickType

PickType consturcts a new type (class) by picking a set of properties from an input type (base type).

We will stay in the upper example. There is now an another Class UpdateUsersEmailInput. User might only need one property in user information: email. email comes from createUserInput (base type), so that it can be picked up in the class CreateUserInput. It is illustrated again in the figure below. The class UpdateUsersEmailInput extends then the class CreateUserInput with the option of PickType. In this case, it should be mentioned, which properties will be selected (picked up): PickType(CreateUserInput, ['email'] as const).

OmitType

OmitType constructs a type by picking all properties from an input type and then removing a particular set of keys. This means that OmitType excludes certain properties from the base type that are identified when defining a class. If the class UpdateUserInput is sure for not using the property email of the base type CreateUserInput, then it could be defined as follows: