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Alias vs interface

Learn the distinction between type alias and interface.

Alias vs interface

Both type and interface can define object shapes. However, type is more general: it can define unions, primitives, and tuples. interface is for objects and can be extended with extends.

// union with type
type ID = number | string;
// object with interface
interface Kisi { ad: string }

Type is more general; interface is specific to objects.

When to use alias and interface?

Use type for union types, intersection types, primitives, and tuples. Use interface for object shapes that may be extended or merged. Both work for simple object shapes.

// type: union and tuple
type ID = number | string;
type Nokta = [number, number];
// interface: extensible object
interface Hayvan { ad: string }

Choose type or interface based on the need.

Practice Use type and interface together

Define a type alias named ID (number | string union). Define an interface named Kisi with properties ad: string and id: ID. Declare a variable named kisi of type Kisi with value { ad: "Ali", id: 42 }. Print the value of kisi.ad with console.log.

Starter code

index.ts
// ID adında bir type alias tanımla (number | string birleşimi)
// Kisi adında bir interface tanımla (ad: string, id: ID)
// kisi değişkenini Kisi türünde tanımla, değer: { ad: "Ali", id: 42 }
// kisi.ad değerini yazdır

Sample solution

index.ts
type ID = number | string;
interface Kisi {
    ad: string;
    id: ID;
}
let kisi: Kisi = { ad: "Ali", id: 42 };
console.log(kisi.ad);

Expected output

Ali

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Which can define a union type?

  • const
  • class
  • interface
  • type Correct answer

Explanation: The <code>type</code> keyword can define union types (<code>type X = A | B</code>); <code>interface</code> only defines object shapes and does not support unions.

2 Add the second half of the union type

Question 
type ID = number | ____;
Answer 
type ID = number | string;

Explanation: Since an ID can be both a number and a string, the union must be <code>number | string</code>; <code>boolean</code> makes no sense for an ID, <code>void</code> is not a value type, and <code>number</code> is a duplicate.

3 Alias vs interface concepts

Term Meaning
type X = string | number Union type (type only)
interface X { ... } Object shape (extensible)
type X = string Primitive type alias
Alias vs interface Type is more general

All lessons in Beginner

1 TypeScript intro and setup

  1. What is TypeScript?
  2. Why TypeScript?
  3. tsc compiler
  4. tsconfig basics
  5. Summary - TypeScript basics

2 Primitive types

  1. string type
  2. number type
  3. boolean type
  4. let and const
  5. Summary - Primitive types

3 Arrays

  1. Array type
  2. Array methods
  3. readonly arrays
  4. Multidimensional arrays
  5. Summary - Arrays

4 Tuples

  1. What is a tuple?
  2. Tuple usage
  3. readonly tuple
  4. Summary - Tuple

5 Enums

  1. Numeric enum
  2. String enum
  3. const enum
  4. Enum members
  5. Summary - Enum

6 any, unknown, never

  1. any type
  2. unknown type
  3. never type
  4. void type
  5. Summary - Special types

7 Type annotation

  1. Variable annotation
  2. Parameter annotation
  3. Return type annotation
  4. Summary - Type annotation

8 Type inference

  1. Basic inference
  2. Inference with const
  3. Best common type
  4. Summary - Type inference

9 Objects

  1. Object type
  2. Optional properties
  3. Readonly properties
  4. Summary - Objects

10 Functions

  1. Function declaration
  2. Optional and default params
  3. Rest params
  4. Function overloads
  5. Summary - Functions

11 Arrow functions

  1. Arrow syntax
  2. this with arrow
  3. Arrow type
  4. Summary - Arrow functions

12 Interfaces

  1. What is an interface?
  2. Property types
  3. Interface vs type alias
  4. Summary - Interfaces

13 Type alias

  1. What is a type alias?
  2. Composition with alias
  3. Alias vs interface
  4. Summary - Type alias

14 Union and intersection

  1. Union types
  2. Intersection types
  3. Literal types
  4. Summary - Union & intersection

15 null and undefined

  1. null vs undefined
  2. Optional chaining
  3. Nullish coalescing
  4. Summary - null & undefined

16 Strict mode

  1. strict: true
  2. strictNullChecks
  3. strictFunctionTypes
  4. Summary - Strict mode

17 Project: Shopping cart

  1. Cart design
  2. Add product
  3. Total calculation
  4. Apply discount
  5. Integration

18 Project: Todo CLI

  1. Task model
  2. Add and list
  3. Completion
  4. Filtering
  5. Integration

19 Project: Library

  1. Book model
  2. Add and search
  3. Borrow
  4. Return
  5. Integration

20 Beginner final

  1. Challenge 1: Types
  2. Challenge 2: Functions
  3. Challenge 3: Unions
  4. Challenge 4: Mini app