Add UnionToIntersection type (#89)

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

index.d.ts

@@ -20,6 +20,7 @@ export {AsyncReturnType} from './source/async-return-type';
 export {ConditionalExcept} from './source/conditional-except';
 export {ConditionalKeys} from './source/conditional-keys';
 export {ConditionalPick} from './source/conditional-pick';
+export {UnionToIntersection} from './source/union-to-intersection';
 
 // Miscellaneous
 export {PackageJson} from './source/package-json';

readme.md

@@ -79,6 +79,7 @@ Click the type names for complete docs.
 - [`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.
 - [`ConditionalExcept`](source/conditional-except.d.ts) - Like `Omit` except it removes properties from a shape where the values extend the given `Condition` type.
+- [`UnionToIntersection`](source/union-to-intersection.d.ts) - Convert a union type to an intersection type.
 
 ### Miscellaneous
 

source/set-optional.d.ts

@@ -1,3 +1,5 @@
+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.
 
@@ -23,7 +25,7 @@ type SomeOptional = SetOptional<Foo, 'b' | 'c'>;
 */
 export type SetOptional<BaseType, Keys extends keyof BaseType = keyof BaseType> =
 	// Pick just the keys that are not optional from the base type.
-	Pick<BaseType, Exclude<keyof BaseType, Keys>> &
+	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.

source/set-required.d.ts

@@ -1,3 +1,5 @@
+import {Except} from './except';
+
 /**
 Create a type that makes the given keys required. The remaining keys are kept as is. The sister of the `SetOptional` type.
 
@@ -23,7 +25,7 @@ type SomeRequired = SetRequired<Foo, 'b' | 'c'>;
 */
 export type SetRequired<BaseType, Keys extends keyof BaseType = keyof BaseType> =
 	// Pick just the keys that are not required from the base type.
-	Pick<BaseType, Exclude<keyof BaseType, Keys>> &
+	Except<BaseType, Keys> &
 	// Pick the keys that should be required from the base type and make them required.
 	Required<Pick<BaseType, Keys>> extends
 	// If `InferredType` extends the previous, then for each key, use the inferred type key.

source/union-to-intersection.d.ts

@@ -0,0 +1,58 @@
+/**
+Convert a union type to an intersection type using [distributive conditional types](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-8.html#distributive-conditional-types).
+
+Inspired by [this Stack Overflow answer](https://stackoverflow.com/a/50375286/2172153).
+
+@example
+```
+import {UnionToIntersection} from 'type-fest';
+
+type Union = {the(): void} | {great(arg: string): void} | {escape: boolean};
+
+type Intersection = UnionToIntersection<Union>;
+//=> {the(): void; great(arg: string): void; escape: boolean};
+```
+
+A more applicable example which could make its way into your library code follows.
+
+@example
+```
+import {UnionToIntersection} from 'type-fest';
+
+class CommandOne {
+	commands: {
+		a1: () => undefined,
+		b1: () => undefined,
+	}
+}
+
+class CommandTwo {
+	commands: {
+		a2: (argA: string) => undefined,
+		b2: (argB: string) => undefined,
+	}
+}
+
+const union = [new CommandOne(), new CommandTwo()].map(instance => instance.commands);
+type Union = typeof union;
+//=> {a1(): void; b1(): void} | {a2(argA: string): void; b2(argB: string): void}
+
+type Intersection = UnionToIntersection<Union>;
+//=> {a1(): void; b1(): void; a2(argA: string): void; b2(argB: string): void}
+```
+*/
+export type UnionToIntersection<Union> = (
+	// `extends any` is always going to be the case and is used to convert the
+	// `Union` into a [distributive conditional
+	// type](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-8.html#distributive-conditional-types).
+	Union extends unknown
+		// The union type is used as the only argument to a function since the union
+		// of function arguments is an intersection.
+		? (distributedUnion: Union) => void
+		// This won't happen.
+		: never
+		// Infer the `Intersection` type since TypeScript represents the positional
+		// arguments of unions of functions as an intersection of the union.
+	) extends ((mergedIntersection: infer Intersection) => void)
+		? Intersection
+		: never;

test-d/union-to-intersection.ts

@@ -0,0 +1,9 @@
+import {expectType} from 'tsd';
+import {UnionToIntersection} from '..';
+
+declare const intersection1: UnionToIntersection<{a: string} | {b: number}>;
+expectType<{a: string; b: number}>(intersection1);
+
+// Creates a union of matching properties.
+declare const intersection2: UnionToIntersection<{a: string} | {b: number} | {a: () => void}>;
+expectType<{a: string | (() => void); b: number}>(intersection2);