feat: add UnionKeys

src/types/objects.ts

@@ -102,6 +102,15 @@ export type DiffKeys<T, U> = Diff<Keys<T>, Keys<U>>;
  */
 export type HasKey<T, K extends keyof any> = K extends Keys<T> ? True : False;
 
+/**
+ * @param T the union to get the keys of
+ * @returns a union containing all the keys of members of `T`
+ */
+export type UnionKeys<T>
+    // Using a conditional here, so that it distributes over members of the union
+    // See https://www.typescriptlang.org/docs/handbook/advanced-types.html#distributive-conditional-types
+    = T extends any ? keyof T : never;
+
 // -------------
 // Manipulations
 // -------------

test/objects/UnionKeys.test.ts

@@ -0,0 +1,16 @@
+import test from 'ava';
+import { assert } from '../helpers/assert';
+
+import { AllKeys, UnionKeys } from '../../src';
+
+test('Can get all keys between objects in a union', t => {
+    type a = { w: number, x: string };
+    type b = { x: number, z: boolean };
+    type c = { y: boolean, z: string };
+
+    type got = UnionKeys<a | b | c>;
+    type expected = 'w' | 'x' | 'y' | 'z';
+
+    assert<got, expected>(t);
+    assert<expected, got>(t);
+});