What is a type alias?

Learn type naming with the type keyword.

What is a type alias?

A type alias gives a name to a type. You define it by writing the type keyword, the name, and the type. This makes complex types reusable and readable.

type ID = number;
let kullaniciId: ID = 42;

An alias gives a type a reusable name.

Reusing type aliases

Once defined, a type alias can be used anywhere a type is expected. It is useful for commonly used types like IDs, codes, or complex object shapes.

type ID = number;
type Isim = string;
function bul(id: ID): Isim { return "Ali"; }

An alias prevents repeated type definitions.

Exercise Define and use a type alias

Define a type alias named Kimlik (number). Define a type alias named Metin (string). Declare a variable named id of type Kimlik with value 42. Print the value of id with console.log.

Show example solution

type Kimlik = number;
type Metin = string;
let id: Kimlik = 42;
console.log(id);

Test yourself

What does <code>type ID = number</code> do?
- Names the number type
- Creates a variable named ID
- Creates an interface
- Creates a function
Show answer

Names the number type — The <code>type</code> keyword gives a name to a type; <code>type ID = number</code> names the <code>number</code> type as <code>ID</code> and makes it usable as a type.

Complete the type alias

type Kimlik = ____;
let id: Kimlik = 100;

Show answer

type Kimlik = number;
let id: Kimlik = 100;

Type alias concepts

type ID = number

Naming a number type
type Isim = string

Naming a string type
type

Type naming keyword
Type alias

Giving a type a reusable name

Next lesson: Composition with alias

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