Add SetNonNullable type (#431)

index.d.ts

@@ -23,6 +23,7 @@ export {Opaque, UnwrapOpaque} from './source/opaque';
 export {InvariantOf} from './source/invariant-of';
 export {SetOptional} from './source/set-optional';
 export {SetRequired} from './source/set-required';
+export {SetNonNullable} from './source/set-non-nullable';
 export {ValueOf} from './source/value-of';
 export {PromiseValue} from './source/promise-value';
 export {AsyncReturnType} from './source/async-return-type';

readme.md

@@ -142,6 +142,7 @@ Click the type names for complete docs.
 - [`InvariantOf`](source/invariant-of.d.ts) - Create an [invariant type](https://basarat.gitbook.io/typescript/type-system/type-compatibility#footnote-invariance), which is a type that does not accept supertypes and subtypes.
 - [`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.
+- [`SetNonNullable`](source/set-non-nullable.d.ts) - Create a type that makes the given keys non-nullable.
 - [`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.
 - [`ConditionalKeys`](source/conditional-keys.d.ts) - Extract keys from a shape where values extend the given `Condition` type.
 - [`ConditionalPick`](source/conditional-pick.d.ts) - Like `Pick` except it selects properties from a shape where the values extend the given `Condition` type.

source/set-non-nullable.d.ts

@@ -0,0 +1,35 @@
+import type {Except} from './except';
+import type {Simplify} from './simplify';
+
+/**
+Create a type that makes the given keys non-nullable. 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 non-nullable.
+
+@example
+```
+import type {SetNonNullable} from 'type-fest';
+
+type Foo = {
+	a: number;
+	b: string | undefined;
+	c?: boolean | null;
+}
+
+type SomeNonNullable = SetNonNullable<Foo, 'b' | 'c'>;
+// type SomeNonNullable = {
+// 	a: number;
+// 	b: string; // Can no longer be undefined.
+// 	c?: boolean; // Can no longer be null, but is still optional.
+// }
+```
+
+@category Object
+*/
+export type SetNonNullable<BaseType, Keys extends keyof BaseType> =
+	Simplify<
+		// Pick just the keys that are readonly from the base type.
+		Except<BaseType, Keys> &
+		// Pick the keys that should be non-nullable from the base type and make them non-nullable.
+		{[Key in Keys]: NonNullable<BaseType[Key]>}
+	>;

test-d/set-non-nullable.ts

@@ -0,0 +1,18 @@
+import {expectType, expectError} from 'tsd';
+import type {SetNonNullable} from '../index';
+
+// Update one possibly undefined key and one possibly null key to non-nullable.
+declare const variation1: SetNonNullable<{a: number; b: string | undefined; c: boolean | null}, 'b' | 'c'>;
+expectType<{a: number; b: string; c: boolean}>(variation1);
+
+// Update a key that is possibly null or undefined.
+declare const variation2: SetNonNullable<{a: number; b: string | null | undefined}, 'b'>;
+expectType<{a: number; b: string}>(variation2);
+
+// Update an optional key.
+declare const variation3: SetNonNullable<{a: number; b?: string | undefined}, 'b'>;
+expectType<{a: number; b?: string}>(variation3);
+
+// Fail if type changes even if non-nullable is right.
+declare const variation4: SetNonNullable<{a: number; b: string | undefined}, 'b'>;
+expectError<{a: string; b: string}>(variation4);