How to Change readonly Properties to be Writable in TypeScript?

You can create a "mutable" utility type in the following way:

// TypeScript 2.8+
type Mutable<T> = {
  -readonly [P in keyof T]: T[P]
};

This would allow you to make properties declared as readonly to be writable by stripping off the readonly modifier from them. For example:

// TypeScript 2.8+
type Mutable<T> = { -readonly [P in keyof T]: T[P] };

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

const person: Mutable<Person> = { name: 'John Doe', age: 24 };

person.age = 37;

The custom Mutable<T> utility type works because of the "-" prefix added to the readonly modifier (i.e. -readonly), which means that the readonly modifier is removed from a mapped type, making all properties mutable. Without the Mutable<T> type, this would throw an error if you try to reassign values of a type that's declared as readonly:

// TypeScript 2.8+
interface Person {
    readonly name: string;
    readonly age: number;
}

const person: Person = { name: 'John Doe', age: 24 };

// Error: Cannot assign to 'age' because it is a read-only property.
person.age = 37;