feat: Create an ArrayUnionHandler which flattens the sources results

test/unit/util/handlers/ArrayUnionHandler.test.ts

@@ -0,0 +1,30 @@
+import { ArrayUnionHandler } from '../../../../src/util/handlers/ArrayUnionHandler';
+import type { AsyncHandler } from '../../../../src/util/handlers/AsyncHandler';
+
+describe('An ArrayUnionHandler', (): void => {
+  let handlers: jest.Mocked<AsyncHandler<string, number[]>>[];
+  let handler: ArrayUnionHandler<AsyncHandler<string, number[]>>;
+
+  beforeEach(async(): Promise<void> => {
+    handlers = [
+      {
+        canHandle: jest.fn(),
+        handle: jest.fn().mockResolvedValue([ 1, 2 ]),
+      } as any,
+      {
+        canHandle: jest.fn(),
+        handle: jest.fn().mockResolvedValue([ 3, 4 ]),
+      } as any,
+    ];
+
+    handler = new ArrayUnionHandler(handlers);
+  });
+
+  it('merges the array results.', async(): Promise<void> => {
+    await expect(handler.handle('input')).resolves.toEqual([ 1, 2, 3, 4 ]);
+    expect(handlers[0].handle).toHaveBeenCalledTimes(1);
+    expect(handlers[0].handle).toHaveBeenLastCalledWith('input');
+    expect(handlers[1].handle).toHaveBeenCalledTimes(1);
+    expect(handlers[1].handle).toHaveBeenLastCalledWith('input');
+  });
+});