Add SetFieldType type (#721)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

index.d.ts

@@ -101,6 +101,7 @@ export type {IsUnknown} from './source/is-unknown';
 export type {IfUnknown} from './source/if-unknown';
 export type {ArrayIndices} from './source/array-indices';
 export type {ArrayValues} from './source/array-values';
+export type {SetFieldType} from './source/set-field-type';
 
 // Template literal types
 export type {CamelCase} from './source/camel-case';

readme.md

@@ -175,6 +175,7 @@ Click the type names for complete docs.
 - [`IntRange`](source/int-range.d.ts) - Generate a union of numbers.
 - [`ArrayIndices`](source/array-indices.d.ts) - Provides valid indices for a constant array or tuple.
 - [`ArrayValues`](source/array-values.d.ts) - Provides all values for a constant array or tuple.
+- [`SetFieldType`](source/set-field-type.d.ts) - Create a type that changes the type of the given keys.
 
 ### Type Guard
 

source/set-field-type.d.ts

@@ -0,0 +1,37 @@
+import type {Except} from './except';
+import type {Simplify} from './simplify';
+
+/**
+Create a type that changes the type of the given keys.
+
+Use-cases:
+- Creating variations of a base model.
+- Fixing incorrect external types.
+
+@see `Merge` if you need to change multiple properties to different types.
+
+@example
+```
+import type {SetFieldType} from 'type-fest';
+
+type MyModel = {
+	id: number;
+	createdAt: Date;
+	updatedAt: Date;
+};
+
+type MyModelApi = SetFieldType<MyModel, 'createdAt' | 'updatedAt', string>;
+// {
+// 	id: number;
+// 	createdAt: string;
+// 	updatedAt: string;
+// }
+```
+
+@category Object
+*/
+export type SetFieldType<BaseType, Keys extends keyof BaseType, NewType> =
+	Simplify<
+	Except<BaseType, Keys> &
+	Record<Keys, NewType>
+	>;

test-d/set-field-type.ts

@@ -0,0 +1,14 @@
+import {expectNotAssignable, expectType} from 'tsd';
+import type {SetFieldType} from '../index';
+
+declare const variation1: SetFieldType<{a: number}, 'a', string>;
+expectType<{a: string}>(variation1);
+
+declare const variation2: SetFieldType<{a: number; b: boolean; c: Date}, 'a' | 'b', string>;
+expectType<{a: string; b: string; c: Date}>(variation2);
+
+declare const variation3: SetFieldType<{a: string; b: boolean; c: Date}, 'b' | 'c', number>;
+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);