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.

Exercise 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.

Show example solution

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

Test yourself

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
- When defining an intersection type
- When defining a union type
Show answer

When describing an object shape that may be extended — <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>.

Complete the optional property type

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

Show answer

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

Interface summary

interface X { ... }

Object shape contract
ad?: string

Skippable property
readonly id: number

Fixed after first assignment
interface vs type

Interface can be extended

Next lesson: What is a type alias?

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