Readonly mapped type

Learn to make all properties readonly with a mapped type.

Readonly mapped type

With a mapped type you can make all of T's properties readonly. Writing { readonly [K in keyof T]: T[K] } makes each property immutable; the value type stays the same. This is exactly what TypeScript's built-in Readonly utility type does.

type Sabit<T> = {
  readonly [K in keyof T]: T[K];
};
interface Nokta { x: number; y: number; }
const n: Sabit<Nokta> = { x: 3, y: 5 };
// n.x = 10;  // compile error

The immutability guarantee

A readonly mapped type gives compile-time immutability. If an object is of type Sabit<T>, its properties cannot be reassigned. This prevents accidental modification, especially in configuration or constant data structures. It has no runtime effect; only the compiler enforces it.

Exercise Immutable point

Write the mapped type Sabit<T> as { readonly [K in keyof T]: T[K] }. Define interface Nokta with x: number and y: number. Define type SabitNokta as Sabit<Nokta>. Assign { x: 3, y: 5 } to a variable n (of type SabitNokta). Print n.x with console.log.

Show example solution

type Sabit<T> = {
  readonly [K in keyof T]: T[K];
};
interface Nokta {
  x: number;
  y: number;
}
type SabitNokta = Sabit<Nokta>;
const n: SabitNokta = { x: 3, y: 5 };
console.log(n.x);

Test yourself

What changes for an object produced by the mapped type below?
```
type Sabit<T> = { readonly [K in keyof T]: T[K]; };
```
- The value types change
- The property names change
- The properties become immutable
- The properties become optional
Show answer

The properties become immutable — Adding readonly prevents properties from being modified; value types and names stay the same. Changing types is done via the right side; making optional is done with ?.

Add readonly

type Sabit<T> = {
  ____ [K in keyof T]: T[K];
};

Show answer

type Sabit<T> = {
  readonly [K in keyof T]: T[K];
};

Readonly mapped type parts

readonly [K in keyof T]

Makes all properties immutable
T[K]

Keeps the value type the same
n.x = 10

Gives a compile error
type Sabit<T>

Readonly mapped type alias

Next lesson: Optional mapped type

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