feat: Implement UnsupportedAsyncHandler.

src/index.ts

@@ -206,5 +206,6 @@ export * from './util/RecordObject';
 export * from './util/SequenceHandler';
 export * from './util/StreamUtil';
 export * from './util/TermUtil';
+export * from './util/UnsupportedAsyncHandler';
 export * from './util/Vocabularies';
 export * from './util/WaterfallHandler';

src/util/UnsupportedAsyncHandler.ts

@@ -0,0 +1,22 @@
+import { AsyncHandler } from './AsyncHandler';
+import { NotImplementedHttpError } from './errors/NotImplementedHttpError';
+
+/**
+ * Handler that does not support any input and will always throw an error.
+ */
+export class UnsupportedAsyncHandler extends AsyncHandler<any, never> {
+  private readonly errorMessage?: string;
+
+  public constructor(errorMessage?: string) {
+    super();
+    this.errorMessage = errorMessage;
+  }
+
+  public async canHandle(): Promise<never> {
+    throw new NotImplementedHttpError(this.errorMessage);
+  }
+
+  public async handle(): Promise<never> {
+    return this.canHandle();
+  }
+}

test/unit/util/UnsupportedAsyncHandler.test.ts

@@ -0,0 +1,18 @@
+import { NotImplementedHttpError } from '../../../src/util/errors/NotImplementedHttpError';
+import { UnsupportedAsyncHandler } from '../../../src/util/UnsupportedAsyncHandler';
+
+describe('An UnsupportedAsyncHandler', (): void => {
+  it('throws a default error when no message is set.', async(): Promise<void> => {
+    const handler = new UnsupportedAsyncHandler();
+    await expect(handler.canHandle()).rejects.toThrow(NotImplementedHttpError);
+    await expect(handler.handle()).rejects.toThrow(NotImplementedHttpError);
+    await expect(handler.handleSafe(null)).rejects.toThrow(NotImplementedHttpError);
+  });
+
+  it('throws the specified error when a message is set.', async(): Promise<void> => {
+    const handler = new UnsupportedAsyncHandler('custom error');
+    await expect(handler.canHandle()).rejects.toThrow('custom error');
+    await expect(handler.handle()).rejects.toThrow('custom error');
+    await expect(handler.handleSafe(null)).rejects.toThrow('custom error');
+  });
+});