14. What does the readonly modifier do, and how does it differ from const?

The readonly modifier in TypeScript makes a property of an object read-only, meaning its value cannot be changed after initialization. It's useful for creating types of objects that shouldn't be modified after they're created. Example:

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

const person: Person = { name: 'John', age: 25 };
person.name = 'Mike'; // Error! 'name' is read-only.

The const keyword, on the other hand, is used for variables, indicating that the variable itself cannot be reassigned. However, the value assigned to a const variable can still be modified if it is an object or array.

const person = { name: 'John', age: 25 };
person.age = 30; // OK, but person cannot be reassigned.

In summary: readonly is used for object properties, while const is used for variables.