type-fest/source/set-readonly.d.ts

A collection of essential TypeScript types

import type {Except} from './except.d.ts';
import type {HomomorphicPick} from './internal/index.d.ts';
import type {Simplify} from './simplify.d.ts';

/**
Create a type that makes the given keys readonly. The remaining keys are kept as is.

Use-case: You want to define a single model where the only thing that changes is whether or not some of the keys are readonly.

@example
```
import type {SetReadonly} from 'type-fest';

type Foo = {
	a: number;
	readonly b: string;
	c: boolean;
}

type SomeReadonly = SetReadonly<Foo, 'b' | 'c'>;
// type SomeReadonly = {
// 	a: number;
// 	readonly b: string; // Was already readonly and still is.
// 	readonly c: boolean; // Is now readonly.
// }
```

@category Object
*/
export type SetReadonly<BaseType, Keys extends keyof BaseType> =
	(BaseType extends (...arguments_: never) => any
		? (...arguments_: Parameters<BaseType>) => ReturnType<BaseType>
		: unknown)
	& _SetReadonly<BaseType, Keys>;

export type _SetReadonly<BaseType, Keys extends keyof BaseType> =
	BaseType extends unknown // To distribute `BaseType` when it's a union type.
		? Simplify<
			Except<BaseType, Keys> &
			Readonly<HomomorphicPick<BaseType, Keys>>
		>
		: never;