feat: create propagate error to client error handle middleware

Author: gregorygaines

src/middleware/propagate_error_to_client_error_handle.ts

@@ -0,0 +1,101 @@
+// Copyright 2019 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+import {Request, Response, NextFunction, Express} from 'express';
+import {HandlerFunction} from '../functions';
+import {ILayer} from 'express-serve-static-core';
+
+/**
+ * Common properties that exists on Express object instances. Extracted by calling
+ * `Object.getOwnPropertyNames` on an express instance.
+ */
+const COMMON_EXPRESS_OBJECT_PROPERTIES = [
+  '_router',
+  'use',
+  'get',
+  'post',
+  'put',
+  'delete',
+];
+
+/** The number of parameters on an express error handler. */
+const EXPRESS_ERROR_HANDLE_PARAM_LENGTH = 4;
+
+/** A express app error handle. */
+interface ErrorHandle {
+  (err: Error, req: Request, res: Response, next: NextFunction): any;
+}
+
+/**
+ * Express middleware to propagate framework errors to the user function error handle.
+ * This enables users to handle framework errors that would otherwise be handled by the
+ * default Express error handle. If the user function doesn't have an error handle,
+ * it falls back to the default Express error handle.
+ * @param userFunction - User handler function
+ */
+export const createPropagateErrorToClientErrorHandleMiddleware = (
+  userFunction: HandlerFunction
+): ErrorHandle => {
+  const userFunctionErrorHandle =
+    getFirstUserFunctionErrorHandleMiddleware(userFunction);
+
+  return function (
+    err: Error,
+    req: Request,
+    res: Response,
+    next: NextFunction
+  ) {
+    // Propagate error to user function error handle.
+    if (userFunctionErrorHandle) {
+      return userFunctionErrorHandle(err, req, res, next);
+    }
+
+    // Propagate error to default Express error handle.
+    return next();
+  };
+};
+
+/**
+ * Returns the first user handler function defined error handle, if available.
+ * @param userFunction - User handler function
+ */
+const getFirstUserFunctionErrorHandleMiddleware = (
+  userFunction: HandlerFunction
+): ErrorHandle | null => {
+  if (!isExpressApp(userFunction)) {
+    return null;
+  }
+
+  const middlewares: ILayer[] = (userFunction as Express)._router.stack;
+  for (const middleware of middlewares) {
+    if (
+      middleware.handle &&
+      middleware.handle.length === EXPRESS_ERROR_HANDLE_PARAM_LENGTH
+    ) {
+      return middleware.handle as unknown as ErrorHandle;
+    }
+  }
+
+  return null;
+};
+
+/**
+ * Returns if the user function contains common properties of an Express app.
+ * @param userFunction
+ */
+const isExpressApp = (userFunction: HandlerFunction): boolean => {
+  const userFunctionProperties = Object.getOwnPropertyNames(userFunction);
+  return COMMON_EXPRESS_OBJECT_PROPERTIES.every(prop =>
+    userFunctionProperties.includes(prop)
+  );
+};

test/middleware/propagate_error_to_client_error_handle.ts

@@ -0,0 +1,68 @@
+import * as assert from 'assert';
+import * as sinon from 'sinon';
+import {NextFunction} from 'express';
+import {Request, Response} from '../../src';
+import {createPropagateErrorToClientErrorHandleMiddleware} from '../../src/middleware/propagate_error_to_client_error_handle';
+import * as express from 'express';
+
+describe('propagateErrorToClientErrorHandleMiddleware', () => {
+  let request: Request;
+  let response: Response;
+  let next: NextFunction;
+  let errListener = () => {};
+  let error: Error;
+  beforeEach(() => {
+    error = Error('Something went wrong!');
+    request = {
+      setTimeout: sinon.spy(),
+      on: sinon.spy(),
+    } as unknown as Request;
+    response = {
+      on: sinon.spy(),
+    } as unknown as Response;
+    next = sinon.spy();
+    errListener = sinon.spy();
+  });
+
+  it('user express app with error handle calls user function app error handle', () => {
+    const app = express();
+    app.use(
+      (
+        _err: Error,
+        _req: Request,
+        _res: Response,
+        _next: NextFunction
+      ): any => {
+        errListener();
+      }
+    );
+
+    const middleware = createPropagateErrorToClientErrorHandleMiddleware(app);
+    middleware(error, request, response, next);
+
+    assert.strictEqual((errListener as sinon.SinonSpy).called, true);
+    assert.strictEqual((next as sinon.SinonSpy).called, false);
+  });
+
+  it('user express app without error handle calls default express error handle', () => {
+    const app = express();
+
+    const middleware = createPropagateErrorToClientErrorHandleMiddleware(app);
+    middleware(error, request, response, next);
+
+    assert.strictEqual((errListener as sinon.SinonSpy).called, false);
+    assert.strictEqual((next as sinon.SinonSpy).called, true);
+  });
+
+  it('non-express user app calls default express error handle', () => {
+    const app = (_req: Request, res: Response) => {
+      res.send('Hello, World!');
+    };
+
+    const middleware = createPropagateErrorToClientErrorHandleMiddleware(app);
+    middleware(error, request, response, next);
+
+    assert.strictEqual((errListener as sinon.SinonSpy).called, false);
+    assert.strictEqual((next as sinon.SinonSpy).called, true);
+  });
+});