What is a type alias?
- TypeScript course
- Beginner
- Lesson 55/89
- 4 min
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.
Practice 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.
Starter code
// Kimlik adında bir type alias tanımla (number)
// Metin adında bir type alias tanımla (string)
// id değişkenini Kimlik türünde tanımla, değeri 42
// id değerini yazdırSample solution
type Kimlik = number;
type Metin = string;
let id: Kimlik = 42;
console.log(id);Expected output
42
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does <code>type ID = number</code> do?
- Names the number type Correct answer
- Creates a variable named ID
- Creates an interface
- Creates a function
Explanation: 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.
2 Complete the type alias
type Kimlik = ____;
let id: Kimlik = 100;type Kimlik = number;
let id: Kimlik = 100;Explanation: An identity number is a number; <code>type Kimlik = number</code> is the correct definition. <code>string</code> is wrong for an ID, and <code>boolean</code> and <code>void</code> make no sense as an identity number.
3 Type alias concepts
| Term | Meaning |
|---|---|
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 |