Add TS utils. Closes #345

lib/index.d.ts

@@ -440,3 +440,18 @@ export function wait(timeout?: number): Promise<void>;
  * Returns a Promise that never resolves.
  */
 export function block(): Promise<void>;
+
+
+export namespace ts {
+
+    /**
+     * Defines a type that can must be one of T or U but not both.
+     */
+    type XOR<T, U> = (T | U) extends object ? (internals.Without<T, U> & U) | (internals.Without<U, T> & T) : T | U;
+}
+
+
+declare namespace internals {
+
+    type Without<T, U> = { [P in Exclude<keyof T, keyof U>]?: never };
+}

test/index.ts

@@ -320,3 +320,18 @@ expect.type<void>(await Hoek.block());
 
 expect.error(Hoek.block(123));
 // $lab:types:on$
+
+
+// ts
+
+interface X { a: number; };
+interface Y { b: number; };
+
+function xor(input: Hoek.ts.XOR<X, Y>): number {
+
+    return input.a || input.b || 10;
+}
+
+xor({ a: 1 });
+xor({ b: 2 });
+expect.error(xor({ a: 1, b: 2 }));