SetFieldType: Add option to preserve property modifiers (#1017)

source/set-field-type.d.ts

@@ -1,6 +1,16 @@
-import type {Except} from './except';
 import type {Simplify} from './simplify';
 
+type SetFieldTypeOptions = {
+	/**
+	Preserve optional and readonly modifiers for properties being updated.
+
+	NOTE: Property modifiers will always be preserved for properties that are not being updated.
+
+	@default true
+	*/
+	preservePropertyModifiers?: boolean;
+};
+
 /**
 Create a type that changes the type of the given keys.
 
@@ -15,23 +25,33 @@ Use-cases:
 import type {SetFieldType} from 'type-fest';
 
 type MyModel = {
-	id: number;
-	createdAt: Date;
-	updatedAt: Date;
+	readonly id: number;
+	readonly createdAt: Date;
+	updatedAt?: Date;
 };
 
 type MyModelApi = SetFieldType<MyModel, 'createdAt' | 'updatedAt', string>;
 // {
-// 	id: number;
-// 	createdAt: string;
-// 	updatedAt: string;
+// 	readonly id: number;
+// 	readonly createdAt: string;
+// 	updatedAt?: string;
+// }
+
+// `preservePropertyModifiers` option can be set to `false` if you want to remove property modifiers for properties being updated
+type MyModelApi = SetFieldType<MyModel, 'createdAt' | 'updatedAt', string, {preservePropertyModifiers: false}>;
+// {
+// 	readonly id: number;
+// 	createdAt: string; // no longer readonly
+// 	updatedAt: string; // no longer optional
 // }
 ```
 
 @category Object
 */
-export type SetFieldType<BaseType, Keys extends keyof BaseType, NewType> =
-	Simplify<
-	Except<BaseType, Keys> &
-	Record<Keys, NewType>
-	>;
+export type SetFieldType<BaseType, Keys extends keyof BaseType, NewType, Options extends SetFieldTypeOptions = {preservePropertyModifiers: true}> =
+	Simplify<{
+		[P in keyof BaseType]: P extends Keys ? NewType : BaseType[P];
+	} & (
+		// `Record` is used to remove property modifiers
+		Options['preservePropertyModifiers'] extends false ? Record<Keys, NewType> : unknown
+	)>;

test-d/set-field-type.ts

@@ -12,3 +12,29 @@ expectType<{a: string; b: number; c: number}>(variation3);
 
 declare const variation4: SetFieldType<{a: string; b: string; c: string}, 'b', number>;
 expectNotAssignable<{a: string; b: string; c: string}>(variation4);
+
+// Works with union types
+declare const variation5: SetFieldType<{a: string; b: string} | {a: number; c: number}, 'a', boolean>;
+expectType<{a: boolean; b: string} | {a: boolean; c: number}>(variation5);
+
+declare const variation6: SetFieldType<{a: string; b: string} | {a: number; c: number}, 'a', boolean, {preservePropertyModifiers: false}>;
+expectType<{a: boolean; b: string} | {a: boolean; c: number}>(variation6);
+
+// Property modifiers are always preserved for properties that are not being updated
+declare const variation7: SetFieldType<{a?: string; readonly b: string; c: string}, 'c', number>;
+expectType<{a?: string; readonly b: string; c: number}>(variation7);
+
+declare const variation8: SetFieldType<{a?: string; readonly b: string; c: string}, 'c', number, {preservePropertyModifiers: false}>;
+expectType<{a?: string; readonly b: string; c: number}>(variation8);
+
+// Preserves property modifiers
+declare const variation9: SetFieldType<{a?: string; readonly b: string; readonly c?: string}, 'a' | 'c', number>;
+expectType<{a?: number; readonly b: string; readonly c?: number}>(variation9);
+
+// Doesn't preserve property modifiers when `preservePropertyModifiers` is `false`
+declare const variation10: SetFieldType<{a?: string; readonly b: string; readonly c?: string}, 'a' | 'c', number, {preservePropertyModifiers: false}>;
+expectType<{a: number; readonly b: string; c: number}>(variation10);
+
+// Falls back to default of `true`, if `preservePropertyModifiers` is set to `boolean`
+declare const variation11: SetFieldType<{a?: string; readonly b: string; readonly c?: string}, 'a' | 'c', number, {preservePropertyModifiers: boolean}>;
+expectType<{a?: number; readonly b: string; readonly c?: number}>(variation11);