README.md

Intersection types

In TypeScript, you can combine types using intersection types.

We do this using the & operator.

type NamedEntity = { name: string };
type AgedEntity = { age: number };

type Person = NamedEntity & AgedEntity;
// ^ { name: string, age: number }

You may read & as "and". So, a Person is a NamedEntity and an AgedEntity.

We have seen union types in a previous chapter, where a variable could be one of several types. The | could be read as "or".

type StringOrNumber = string | number;

Intersection types are the opposite of union types. A variable of an intersection type has all the properties of all the types it is composed of.

Use cases

Use case 1: Deriving new types from existing types

Consider User and UserWithRole types:

type User = {
  id: string;
  name: string;
}

type UserWithRole = {
  id: string;
  name: string;
  role: string;
}

This works, but every time we update the User type, we have to remember to update the UserWithRole type as well. This can lead to bugs if we forget to update both types together.

Instead, we can use intersection types to define UserWithRole with User:

type User = {
  id: string;
  name: string;
}

type UserWithRole = User & {
  role: string;
}

This way, whenever we update fields in User, UserWithRole will automatically update as well.

Use case 2: Constraining existing types

Consider a function that accepts a User type:

type User = {
  id: string;
  name: string;
  isAdmin: boolean;
}

We can use intersection types to create AdminUser, which is a User with isAdmin set to true:

type User = {
  id: string;
  name: string;
  isAdmin: boolean;
}

type AdminUser = User & {
  isAdmin: true;
}

This works because AdminUser has to satisfy both User and { isAdmin: true }.

P.S. Trying intersecting types which are not compatible

.