In TypeScript the interfaces which describe objects, can have read-only properties. The modifier readonly is used for that purpose.

Example

interface Person {
   readonly name: string;
   readonly age: number;
}

let p: Person = {name: "Ashlee", age: 29};
console.log(p);

Output

Person { _name: 'Ashalee', _age: 28 }

If we try to modify the readonly property a compile time error will occur:

interface Person {
    readonly name: string;
    readonly age: number;
}

let p: Person = {name: "Ashlee", age: 29};
p.age = 30;
console.log(p);

Output

readonly-properties2.ts(7,3): error TS2540: Cannot assign to 'age' because it is a constant or a read-only property.

Example Project

Dependencies and Technologies Used:

• TypeScript 3.0.1