Merge pull request #99 from andnp/UnionToIntersection

Union to intersection

README.md

@@ -21,7 +21,7 @@ npm install --save-dev simplytyped
 
 **[Utils](#utils)**
 
-[NoInfer](#noinfer) - [Nominal](#nominal) - [Nullable](#nullable) - [PromiseOr](#promiseor)
+[NoInfer](#noinfer) - [Nominal](#nominal) - [Nullable](#nullable) - [PromiseOr](#promiseor) - [UnionToIntersection](#uniontointersection)
 
 **[Functions](#functions)**
 
@@ -581,6 +581,30 @@ test('Will give back a promise containing given type union the type itself', t =
 
 ```
 
+### UnionToIntersection
+Defines an intersection type of all union items.
+```ts
+test('Union of Strings', t => {
+    type got = UnionToIntersection<'hi' | 'there'>;
+    type expected = 'hi' & 'there';
+
+    assert<got, expected>(t);
+});
+
+test('Union of Objects', t => {
+    type got = UnionToIntersection<{ a: 0 } | { b: 1 } | { c: 2 }>;
+
+    type expected = {
+        a: 0,
+        b: 1,
+        c: 2,
+    };
+
+    assert<got, expected>(t);
+});
+
+```
+
 ## Functions
 
 ### AnyFunc

src/types/tuples.ts

@@ -1,3 +1,5 @@
+import {UnionToIntersection} from './utils';
+
 export interface Vector<T> { readonly [x: number]: T; readonly length: number; }
 export type Length<T extends Vector<any>> = T['length'];
 
@@ -9,18 +11,7 @@ export type Length<T extends Vector<any>> = T['length'];
 export type UnionizeTuple<T extends Vector<any>> = T[number];
 /**
  * Gives an intersection of all values contained in a tuple.
- * @param T a tuple of items up to 10
+ * @param T a tuple of items
  * @returns an intersection of all items in the tuple
  */
-export type IntersectTuple<T extends Vector<any>> =
-    Length<T> extends 1 ? T[0] :
-    Length<T> extends 2 ? T[0] & T[1] :
-    Length<T> extends 3 ? T[0] & T[1] & T[2] :
-    Length<T> extends 4 ? T[0] & T[1] & T[2] & T[3] :
-    Length<T> extends 5 ? T[0] & T[1] & T[2] & T[3] & T[4] :
-    Length<T> extends 6 ? T[0] & T[1] & T[2] & T[3] & T[4] & T[5] :
-    Length<T> extends 7 ? T[0] & T[1] & T[2] & T[3] & T[4] & T[5] & T[6] :
-    Length<T> extends 8 ? T[0] & T[1] & T[2] & T[3] & T[4] & T[5] & T[6] & T[7] :
-    Length<T> extends 9 ? T[0] & T[1] & T[2] & T[3] & T[4] & T[5] & T[6] & T[7] & T[8] :
-    Length<T> extends 10 ? T[0] & T[1] & T[2] & T[3] & T[4] & T[5] & T[6] & T[7] & T[8] & T[9] :
-        any;
+export type IntersectTuple<T extends Vector<any>>= UnionToIntersection<T[number]>;

src/types/utils.ts

@@ -34,3 +34,13 @@ export type Nominal<T, N extends string> = T & Tagged<N>;
  * @returns a the type union with a promise containing the given type
  */
 export type PromiseOr<T> = Promise<T> | T;
+
+
+/**
+ * Defines an intersection type of all union items.
+ * @param U Union of any types that will be intersected.
+ * @returns U items intersected
+ * @see https://stackoverflow.com/a/50375286/9259330
+ */
+export type UnionToIntersection<U> =
+    (U extends unknown ? (k: U) => void : never) extends ((k: infer I) => void) ? I : never;

test/utils/UnionToIntersection.test.ts

@@ -0,0 +1,23 @@
+import test from 'ava';
+import { assert } from '../helpers/assert';
+
+import { UnionToIntersection} from '../../src';
+
+test('Union of Strings', t => {
+    type got = UnionToIntersection<'hi' | 'there'>;
+    type expected = 'hi' & 'there';
+
+    assert<got, expected>(t);
+});
+
+test('Union of Objects', t => {
+    type got = UnionToIntersection<{ a: 0 } | { b: 1 } | { c: 2 }>;
+
+    type expected = {
+        a: 0,
+        b: 1,
+        c: 2,
+    };
+
+    assert<got, expected>(t);
+});