TypeScript Intersection Types

Summary: In this tutorial, you will learn about TypeScript intersection types to create a new type by combining multiple existing types.

Introduction to TypeScript Intersection Types #

An intersection type creates a new type by combining multiple existing types. The new type has all features of the existing types.

To combine types, you use the & operator as follows:

type typeAB = typeA & typeB;

The typeAB will have all properties of typeA and typeB.

Note that the union type uses the | operator that defines a variable that can hold a value of either typeA or typeB

let varName = typeA | typeB; // union type

Suppose that you have three interfaces: BusinessPartner, Identity, and Contact:

interface BusinessPartner {
    name: string;
    credit: number;
}

interface Identity {
    id: number;
    name: string;
}

interface Contact {
    email: string;
    phone: string;
}

The following defines two intersection types:

type Employee = Identity & Contact;
type Customer = BusinessPartner & Contact;

The Employee type contains all properties of the Identity and Contact type:

type Employee = Identity & Contact;

let e: Employee = {
    id: 100,
    name: 'John Doe',
    email: '[email protected]',
    phone: '(408)-897-5684'
};

And the Customer type contains all properties of the BusinessPartner and Contact type:

type Customer = BusinessPartner & Contact;

let c: Customer = {
    name: 'ABC Inc.',
    credit: 1000000,
    email: '[email protected]',
    phone: '(408)-897-5735'
};

Later, if you want to implement employee sales, you can create a new intersection type that contains all properties of Identity, Contact, and BusinessPartner types:

type Employee = Identity & BusinessPartner & Contact;

let e: Employee = {
    id: 100,
    name: 'John Doe',
    email: '[email protected]',
    phone: '(408)-897-5684',
    credit: 1000
};

Notice both BusinessPartner and Identity have the property name with the same type. If they do not, then you will have an error.

Type Order #

When you intersect types, the order of the types doesn’t matter. For example:

type typeAB = typeA & typeB;
type typeBA = typeB & typeA;

In this example, typeAB and typeBA have the same properties.

Summary #

An intersection type combines two or more types to create a new type that has all properties of the existing types.
The type order is not important when you combine types.

Quiz #

Quiz

TypeScript Intersection Types Quiz

5 questions

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