set-optional.d.ts

import {Except} from './except';

/**
Create a type that makes the given keys optional. The remaining keys are kept as is. The sister of the `SetRequired` type.

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

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

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

type SomeOptional = SetOptional<Foo, 'b' | 'c'>;
// type SomeOptional = {
// 	a: number;
// 	b?: string; // Was already optional and still is.
// 	c?: boolean; // Is now optional.
// }
```
*/
export type SetOptional<BaseType, Keys extends keyof BaseType = keyof BaseType> =
	// Pick just the keys that are not optional from the base type.
	Except<BaseType, Keys> &
	// Pick the keys that should be optional from the base type and make them optional.
	Partial<Pick<BaseType, Keys>> extends
	// If `InferredType` extends the previous, then for each key, use the inferred type key.
	infer InferredType
		? {[KeyType in keyof InferredType]: InferredType[KeyType]}
		: never;