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Summary - Interfaces

Write a product catalog model with interfaces.

Summary - Interfaces

Interfaces define object shapes as contracts, give types to properties (including optional and readonly), and are a tool specific to objects. Compared to type, interface is specific to objects and can be extended.

interface Urun {
  readonly id: number;
  ad: string;
  fiyat?: number;
}

These features together create powerful and flexible data models.

Practice Product catalog model

Define an interface named Urun with properties readonly id: number, ad: string, and fiyat?: number. Declare a variable named urun of type Urun with value { id: 1, ad: "defter", fiyat: 20 }. Print the value of urun.ad with console.log.

Starter code

index.ts
// Urun adında bir interface tanımla
// readonly id (number), ad (string), fiyat? (number) özellikleri içersin
// urun değişkenini Urun türünde tanımla
// değer: id 1, ad "defter", fiyat 20
// urun.ad değerini yazdır

Sample solution

index.ts
interface Urun {
    readonly id: number;
    ad: string;
    fiyat?: number;
}
let urun: Urun = { id: 1, ad: "defter", fiyat: 20 };
console.log(urun.ad);

Expected output

defter

Questions and answers

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

1 When should <code>interface</code> be preferred over <code>type</code>?

  • When defining a primitive alias
  • When describing an object shape that may be extended Correct answer
  • When defining an intersection type
  • When defining a union type

Explanation: <code>interface</code> is preferred for object shapes that may be extended (<code>extends</code>); union and intersection types can only be defined with <code>type</code>.

2 Complete the optional property type

Question 
interface Urun {
  readonly id: number;
  ad: string;
  fiyat?: ____;
}
Answer 
interface Urun {
  readonly id: number;
  ad: string;
  fiyat?: number;
}

Explanation: A product price is a number; <code>fiyat?: number</code> is the correct completion. <code>string</code> and <code>boolean</code> are wrong types for a price, and <code>void</code> is not a property type.

3 Interface summary

Term Meaning
interface X { ... } Object shape contract
ad?: string Skippable property
readonly id: number Fixed after first assignment
interface vs type Interface can be extended

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