Readonly properties

Readonly properties

Some properties should only be modifiable when an object is first created. You can specify this by putting readonly before the name of the property:

interface Point {
  readonly x: number;
  readonly y: number;
}

You can construct a Point by assigning an object literal. After the assignment, x and y can’t be changed.

let p1: Point = { x: 10, y: 20 };
p1.x = 5; // error!

TypeScript comes with a ReadonlyArray<T> type that is the same as Array<T> with all mutating methods removed, so you can make sure you don’t change your arrays after creation:

let a: number[] = [1, 2, 3, 4];
let ro: ReadonlyArray<number> = a;
ro[0] = 12; // error!
ro.push(5); // error!
ro.length = 100; // error!
a = ro; // error!

On the last line of the snippet you can see that even assigning the entire ReadonlyArray back to a normal array is illegal. You can still override it with a type assertion, though:

a = ro as number[];