Add Mutable type

base.d.ts

@@ -18,6 +18,7 @@ export {Promisable} from './source/promisable';
 export {Opaque} from './source/opaque';
 export {SetOptional} from './source/set-optional';
 export {SetRequired} from './source/set-required';
+export {SetMutable} from './source/set-mutable';
 export {ValueOf} from './source/value-of';
 export {PromiseValue} from './source/promise-value';
 export {AsyncReturnType} from './source/async-return-type';

readme.md

@@ -73,6 +73,7 @@ Click the type names for complete docs.
 - [`Opaque`](source/opaque.d.ts) - Create an [opaque type](https://codemix.com/opaque-types-in-javascript/).
 - [`SetOptional`](source/set-optional.d.ts) - Create a type that makes the given keys optional.
 - [`SetRequired`](source/set-required.d.ts) - Create a type that makes the given keys required.
+- [`SetMutable`](source/set-mutable.d.ts) - Create a type that converts the given keys from `readonly` to mutable.
 - [`ValueOf`](source/value-of.d.ts) - Create a union of the given object's values, and optionally specify which keys to get the values from.
 - [`PromiseValue`](source/promise-value.d.ts) - Returns the type that is wrapped inside a `Promise`.
 - [`AsyncReturnType`](source/async-return-type.d.ts) - Unwrap the return type of a function that returns a `Promise`.

source/mutable.d.ts

@@ -3,17 +3,21 @@ Convert an object with `readonly` keys into a mutable object. Inverse of `Readon
 
 This can be used to [store and mutate options within a class](https://github.com/sindresorhus/pageres/blob/4a5d05fca19a5fbd2f53842cbf3eb7b1b63bddd2/source/index.ts#L72), [edit `readonly` objects within tests](https://stackoverflow.com/questions/50703834), and [construct a `readonly` object within a function](https://github.com/Microsoft/TypeScript/issues/24509).
 
+@see [`SetMutable`](source/set-mutable.d.ts) – a type that can selectively set some keys as mutable.
+
 @example
 ```
 import {Mutable} from 'type-fest';
 
 type Foo = {
 	readonly a: number;
-	readonly b: string;
+	readonly b: readonly string[];
 };
 
-const mutableFoo: Mutable<Foo> = {a: 1, b: '2'};
+const mutableFoo: Mutable<Foo> = {a: 1, b: ['2']};
 mutableFoo.a = 3;
+mutableFoo.b[0] = '3'; //=> Index signature in type 'readonly string[]' only permits reading.ts(2542).
+mutableFoo.b = ['3'];
 ```
 */
 export type Mutable<ObjectType> = {

source/set-mutable.d.ts

@@ -0,0 +1,37 @@
+import {Except} from './except';
+import {Mutable} from './mutable';
+
+/**
+Create a type that converts the given keys from `readonly` to mutable. 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 mutable.
+
+@see [`Mutable`](source/mutable.d.ts) - a type that sets all (not selectively) keys as mutable.
+
+@example
+```
+import {SetMutable} from 'type-fest';
+
+type Foo = {
+	readonly a: number;
+	b: readonly string[];
+	readonly c: boolean;
+}
+
+type SomeMutable = SetMutable<Foo, 'b' | 'c'>;
+// type SomeMutable = {
+// 	readonly a: number;
+// 	b: readonly string[]; // Was already mutable and still is, type of the property remains unaffected.
+// 	c: boolean; // Is now mutable.
+// }
+```
+*/
+export type SetMutable<BaseType, Keys extends keyof BaseType> =
+	// Pick just the keys that are not mutable from the base type.
+	Except<BaseType, Keys> &
+	// Pick the keys that should be mutable from the base type and make them mutable.
+	Mutable<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;

source/set-optional.d.ts

@@ -23,7 +23,7 @@ type SomeOptional = SetOptional<Foo, 'b' | 'c'>;
 // }
 ```
 */
-export type SetOptional<BaseType, Keys extends keyof BaseType = keyof BaseType> =
+export type SetOptional<BaseType, Keys extends 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.

source/set-required.d.ts

@@ -23,7 +23,7 @@ type SomeRequired = SetRequired<Foo, 'b' | 'c'>;
 // }
 ```
 */
-export type SetRequired<BaseType, Keys extends keyof BaseType = keyof BaseType> =
+export type SetRequired<BaseType, Keys extends keyof BaseType> =
 	// Pick just the keys that are not required from the base type.
 	Except<BaseType, Keys> &
 	// Pick the keys that should be required from the base type and make them required.

test-d/set-mutable.ts

@@ -0,0 +1,18 @@
+import {expectType, expectError} from 'tsd';
+import {SetMutable} from '..';
+
+// Update one required and one optional to required.
+declare const variation1: SetMutable<{readonly a: number; b: string; readonly c: boolean}, 'b' | 'c'>;
+expectType<{readonly a: number; b: string; c: boolean}>(variation1);
+
+// Update two optional to required.
+declare const variation2: SetMutable<{readonly a: number; readonly b: string; readonly c: boolean}, 'a' | 'b'>;
+expectType<{a: number; b: string; readonly c: boolean}>(variation2);
+
+// Three required remain required.
+declare const variation3: SetMutable<{a: number; b: string; c: boolean}, 'a' | 'b' | 'c'>;
+expectType<{a: number; b: string; c: boolean}>(variation3);
+
+// Fail if type changes even if optional is right.
+declare const variation4: SetMutable<{readonly a: number; b: string; readonly c: boolean}, 'b' | 'c'>;
+expectError<{readonly a: boolean; b: string; c: boolean}>(variation4);