type-fest/source/readonly-keys-of.d.ts

import type {IsEqual} from './is-equal';

/**
Extract all readonly keys from the given type.

This is useful when you want to create a new type that contains readonly keys only.

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

interface User {
	name: string;
	surname: string;
	readonly id: number;
}

type UpdateResponse<Entity extends object> = Pick<Entity, ReadonlyKeysOf<Entity>>;

const update1: UpdateResponse<User> = {
    id: 123,
};
```

@category Utilities
*/
export type ReadonlyKeysOf<T> = NonNullable<{
	[P in keyof T]: IsEqual<{[Q in P]: T[P]}, {readonly [Q in P]: T[P]}> extends true ? P : never
}[keyof T]>;