Object type

Learn to explicitly write object types.

Object type

An object type describes the shape of an object: which properties it has and what their types are. Property names and types are written inside curly braces.

let kisi: { ad: string; yas: number } = {
  ad: "Ali",
  yas: 25
};

The object type checks the value's structure at compile time.

Object type matching

The object value must match the type exactly: all required properties must be present with the correct types. A missing property or wrong type causes a compile error. Extra properties are also not accepted.

let p: { x: number; y: number } = {
  x: 3,
  y: 4
};  // valid

The object type must match the value's shape exactly.

Exercise Define an object with a type annotation

Declare a variable named kisi with type { ad: string; yas: number } and value { ad: "Ayşe", yas: 28 }. Print the value of kisi.ad with console.log.

Show example solution

let kisi: { ad: string; yas: number } = { ad: "Ayşe", yas: 28 };
console.log(kisi.ad);

Test yourself

What does <code>{ ad: string; yas: number }</code> describe?
- An object type with ad (string) and yas (number) properties
- An array with two elements
- A function with two parameters
- A tuple of string and number
Show answer

An object type with ad (string) and yas (number) properties — Property names and types inside curly braces describe an object's shape; <code>ad: string; yas: number</code> is an object type with two properties.

Complete the object property type

let nokta: { x: number; y: ____ } = { x: 3, y: 4 };

Show answer

let nokta: { x: number; y: number } = { x: 3, y: 4 };

Object type concepts

{ ad: string }

ad property of type string
{ x: number; y: number }

x and y number properties
Object type

Describing an object's shape
{ yas: number }

yas property of type number

Next lesson: Optional properties

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