Read-Only Properties in Interface in TypeScript

Read-only properties are set only once when an object is created. They cannot be changed later. The readonly keyword is used to declare these properties.

Let's take the IFigure interface created in the previous lesson. But now let's set the width property to read-only:

interface IFigure {
	height: number;
	readonly width: number;
}

Let's create a rectangle object:

let rectangle: IFigure = {
	height: 200,
	width: 300
}

Let's change the height:

rectangle.height = 150;

Now let's change the width and see an error, since the width is declared immutable:

rectangle.width = 400; //

Create an interface IUser in which the property salary will be read-only.