Typescript: Mapped type to make every property readonly except chosen properties

Question

I ran into a challenge where I didn't want to keep rewriting multiple interfaces.

I need one interface to be fully writable, and also have a "copy" of that interface where all fields are readonly except those that I select to be writeable.

Typescript has mapped types which may allow this.

Answer 1

export type DeepReadOnly<T> = { readonly [key in keyof T]: DeepReadOnly<T[key]> };
export type DeepMutable<T> = { -readonly [key in keyof T]: DeepMutable<T[key]> };
export type Omit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>;
export type DeepKeepMutable<T, K extends keyof T> = DeepReadOnly<Omit<T, K>> & DeepMutable<Pick<T, K>>;

// use as follows
let o: DeepKeepMutable<Metadata, 'time' | 'payload'>;
// this will keep time and payload writeable while the rest are readonly

// it is also possible to extend and modify these types with index signatures, optional properties and level of depth

Answer 2

You can use TypeScript's Readonly utility type:

type SomeType = {
  a: string;
};

const s: SomeType = {
  a: "can modify",
};

s.a = "new value";
// ^ Can modify

type ReadOnlyType = Readonly<SomeType>;

const r: ReadOnlyType = {
  a: "cannot modify",
};

r.a = "new value";
// ^ ERROR! - Cannot assign to 'a' because it is a read-only property.