[Feature] Add UnionToIntersection type (#109)

Resolves: #49
piotrwitek · Oct 22, 2019 · 5b7cd0c · 5b7cd0c
1 parent d935928
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diff --git a/README.md b/README.md
@@ -115,6 +115,7 @@ Issues can be funded by anyone and the money will be transparently distributed t
 * [`PromiseType<T>`](#promisetypet)
 * [`Unionize<T>`](#unionizet)
 * [`Brand<T, U>`](#brandt-u)
+* [`UnionToIntersection<U>`](#uniontointersectionu)
 
 ## Flow's Utility Types
 
@@ -832,6 +833,21 @@ gross(eur); // Type '"EUR"' is not assignable to type '"USD"'.
 
 [⇧ back to top](#flows-utility-types)
 
+### `UnionToIntersection<U>`
+
+Get intersection type given union type `U`
+
+**Usage:**
+
+```ts
+import { UnionToIntersection } from 'utility-types';
+
+// Expect: { name: string } & { age: number } & { visible: boolean }
+UnionToIntersection<{ name: string } | { age: number } | { visible: boolean }>
+```
+
+[⇧ back to top](#table-of-contents)
+
 ---
 
 ## Flow's Utility Types

diff --git a/src/__snapshots__/mapped-types.spec.ts.snap b/src/__snapshots__/mapped-types.spec.ts.snap
@@ -195,6 +195,22 @@ exports[`Subtract testType<Subtract<Props, DefaultProps>>() 1`] = `"Pick<Props,
 
 exports[`SymmetricDifference testType<SymmetricDifference<'1' | '2' | '3', '2' | '3' | '4'>>() 1`] = `"\\"1\\" | \\"4\\""`;
 
+exports[`UnionToIntersection testType<
+      UnionToIntersection<
+        { name: string } | { age: number } | { visible: boolean }
+      >
+    >({
+      name: 'Yolo',
+      age: 99,
+      visible: true,
+    }) 1`] = `"{ name: string; } & { age: number; } & { visible: boolean; }"`;
+
+exports[`UnionToIntersection testType<UnionToIntersection<'name' | 'age'>>() 1`] = `"\\"name\\" & \\"age\\""`;
+
+exports[`UnionToIntersection testType<UnionToIntersection<boolean>>() 1`] = `"false & true"`;
+
+exports[`UnionToIntersection testType<UnionToIntersection<true | false>>() 1`] = `"false & true"`;
+
 exports[`Unionize testType<Unionize<Props>>() 1`] = `"{ name: string; } | { age: number; } | { visible: boolean; }"`;
 
 exports[`ValuesType testType<ValuesType<[1, 2]>>() 1`] = `"1 | 2"`;

diff --git a/src/index.ts b/src/index.ts
@@ -47,6 +47,7 @@ export {
   Subtract,
   SymmetricDifference,
   Unionize,
+  UnionToIntersection,
   ValuesType,
   WritableKeys,
 } from './mapped-types';

diff --git a/src/mapped-types.spec.snap.ts b/src/mapped-types.spec.snap.ts
@@ -39,6 +39,7 @@ import {
   Optional,
   ValuesType,
   AugmentedRequired,
+  UnionToIntersection,
 } from './mapped-types';
 
 /**
@@ -540,3 +541,26 @@ type RequiredOptionalProps = {
     visible: true,
   });
 }
+
+// @dts-jest:group UnionToIntersection
+{
+  // @dts-jest:pass:snap -> { name: string; } & { age: number; } & { visible: boolean; }
+  testType<
+    UnionToIntersection<
+      { name: string } | { age: number } | { visible: boolean }
+    >
+  >({
+    name: 'Yolo',
+    age: 99,
+    visible: true,
+  });
+
+  // @dts-jest:pass:snap -> false & true
+  testType<UnionToIntersection<boolean>>();
+
+  // @dts-jest:pass:snap -> false & true
+  testType<UnionToIntersection<true | false>>();
+
+  // @dts-jest:pass:snap -> "name" & "age"
+  testType<UnionToIntersection<'name' | 'age'>>();
+}
diff --git a/src/mapped-types.spec.ts b/src/mapped-types.spec.ts
@@ -39,6 +39,7 @@ import {
   Optional,
   ValuesType,
   AugmentedRequired,
+  UnionToIntersection,
 } from './mapped-types';
 
 /**
@@ -540,3 +541,26 @@ type RequiredOptionalProps = {
     visible: true,
   });
 }
+
+// @dts-jest:group UnionToIntersection
+{
+  // @dts-jest:pass:snap
+  testType<
+    UnionToIntersection<
+      { name: string } | { age: number } | { visible: boolean }
+    >
+  >({
+    name: 'Yolo',
+    age: 99,
+    visible: true,
+  });
+
+  // @dts-jest:pass:snap
+  testType<UnionToIntersection<boolean>>();
+
+  // @dts-jest:pass:snap
+  testType<UnionToIntersection<true | false>>();
+
+  // @dts-jest:pass:snap
+  testType<UnionToIntersection<'name' | 'age'>>();
+}
diff --git a/src/mapped-types.ts b/src/mapped-types.ts
@@ -615,3 +615,18 @@ export type AugmentedRequired<
   T extends object,
   K extends keyof T = keyof T
 > = Omit<T, K> & Required<Pick<T, K>>;
+
+/**
+ * UnionToIntersection
+ * @desc Get intersection type given union type `U`
+ * Credit: jcalz
+ * @see https://stackoverflow.com/a/50375286/7381355
+ * @example
+ *   // Expect: { name: string } & { age: number } & { visible: boolean }
+ *   UnionToIntersection<{ name: string } | { age: number } | { visible: boolean }>
+ */
+export type UnionToIntersection<U> = (U extends any
+  ? (k: U) => void
+  : never) extends (k: infer I) => void
+  ? I
+  : never;