feat(rest): add mountExpressRouter

Co-authored-by: Miroslav Bajtoš <mbajtoss@gmail.com>

docs/site/Routes.md

@@ -226,3 +226,62 @@ export class MyApplication extends RestApplication {
   }
 }
 ```
+
+## Mounting an Express Router
+
+If you have an existing [Express](https://expressjs.com/) application that you
+want to use with LoopBack 4, you can mount the Express application on top of a
+LoopBack 4 application. This way you can mix and match both frameworks, while
+using LoopBack as the host. You can also do the opposite and use Express as the
+host by mounting LoopBack 4 REST API on an Express application. See
+[Creating an Express Application with LoopBack REST API](express-with-lb4-rest-tutorial.md)
+for the tutorial.
+
+Mounting an Express router on a LoopBack 4 application can be done using the
+`mountExpressRouter` function provided by both `RestApplication` and
+`RestServer`.
+
+Example use:
+
+{% include note.html content="
+Make sure [express](https://www.npmjs.com/package/express) is installed.
+" %}
+
+{% include code-caption.html content="src/express-app.ts" %}
+
+```ts
+import {Request, Response} from 'express';
+import * as express from 'express';
+
+const legacyApp = express();
+
+// your existing Express routes
+legacyApp.get('/pug', function(_req: Request, res: Response) {
+  res.send('Pug!');
+});
+
+export {legacyApp};
+```
+
+{% include code-caption.html content="src/application.ts" %}
+
+```ts
+import {RestApplication} from '@loopback/rest';
+
+const legacyApp = require('./express-app').legacyApp;
+
+const legacySpec: RouterSpec = {
+  // insert your spec here, your 'paths', 'components', and 'tags' will be used
+};
+
+class MyApplication extends RestApplication {
+  constructor(/* ... */) {
+    // ...
+
+    this.mountExpressRouter('/dogs', legacyApp, legacySpec);
+  }
+}
+```
+
+Any routes you define in your `legacyApp` will be mounted on top of the `/dogs`
+base path, e.g. if you visit the `/dogs/pug` endpoint, you'll see `Pug!`.

packages/rest/src/__tests__/integration/rest.application.integration.ts

@@ -5,9 +5,17 @@
 
 import {anOperationSpec} from '@loopback/openapi-spec-builder';
 import {Client, createRestAppClient, expect} from '@loopback/testlab';
+import * as express from 'express';
+import {Request, Response} from 'express';
 import * as fs from 'fs';
 import * as path from 'path';
-import {RestApplication, RestServer, RestServerConfig, get} from '../..';
+import {
+  get,
+  RestApplication,
+  RestServer,
+  RestServerConfig,
+  RouterSpec,
+} from '../..';
 
 const ASSETS = path.resolve(__dirname, '../../../fixtures/assets');
 
@@ -163,6 +171,103 @@ describe('RestApplication (integration)', () => {
     await client.get(response.header.location).expect(200, 'Hi');
   });
 
+  context('mounting an Express router on a LoopBack application', async () => {
+    beforeEach('set up RestApplication', async () => {
+      givenApplication();
+      await restApp.start();
+      client = createRestAppClient(restApp);
+    });
+
+    it('gives precedence to an external route over a static route', async () => {
+      const router = express.Router();
+      router.get('/', function(_req: Request, res: Response) {
+        res.send('External dog');
+      });
+
+      restApp.static('/dogs', ASSETS);
+      restApp.mountExpressRouter('/dogs', router);
+
+      await client.get('/dogs/').expect(200, 'External dog');
+    });
+
+    it('mounts an express Router without spec', async () => {
+      const router = express.Router();
+      router.get('/poodle/', function(_req: Request, res: Response) {
+        res.send('Poodle!');
+      });
+      router.get('/pug', function(_req: Request, res: Response) {
+        res.send('Pug!');
+      });
+      restApp.mountExpressRouter('/dogs', router);
+
+      await client.get('/dogs/poodle/').expect(200, 'Poodle!');
+      await client.get('/dogs/pug').expect(200, 'Pug!');
+    });
+
+    it('mounts an express Router with spec', async () => {
+      const router = express.Router();
+      function greetDogs(_req: Request, res: Response) {
+        res.send('Hello dogs!');
+      }
+
+      const spec: RouterSpec = {
+        paths: {
+          '/hello': {
+            get: {
+              responses: {
+                '200': {
+                  description: 'greet the dogs',
+                  content: {
+                    'text/plain': {
+                      schema: {type: 'string'},
+                    },
+                  },
+                },
+              },
+            },
+          },
+        },
+      };
+      router.get('/hello', greetDogs);
+      restApp.mountExpressRouter('/dogs', router, spec);
+      await client.get('/dogs/hello').expect(200, 'Hello dogs!');
+
+      const openApiSpec = restApp.restServer.getApiSpec();
+      expect(openApiSpec.paths).to.deepEqual({
+        '/dogs/hello': {
+          get: {
+            responses: {
+              '200': {
+                description: 'greet the dogs',
+                content: {'text/plain': {schema: {type: 'string'}}},
+              },
+            },
+          },
+        },
+      });
+    });
+
+    it('mounts more than one express Router', async () => {
+      const router = express.Router();
+      router.get('/poodle', function(_req: Request, res: Response) {
+        res.send('Poodle!');
+      });
+
+      restApp.mountExpressRouter('/dogs', router);
+
+      const secondRouter = express.Router();
+
+      secondRouter.get('/persian', function(_req: Request, res: Response) {
+        res.send('Persian cat.');
+      });
+
+      restApp.mountExpressRouter('/cats', secondRouter);
+
+      await client.get('/dogs/poodle').expect(200, 'Poodle!');
+      await client.get('/cats/persian').expect(200, 'Persian cat.');
+    });
+  });
+
   function givenApplication(options?: {rest: RestServerConfig}) {
     options = options || {rest: {port: 0, host: '127.0.0.1'}};
     restApp = new RestApplication(options);

packages/rest/src/__tests__/unit/router/assign-router-spec.unit.ts

@@ -0,0 +1,151 @@
+// Copyright IBM Corp. 2019. All Rights Reserved.
+// Node module: @loopback/rest
+// This file is licensed under the MIT License.
+// License text available at https://opensource.org/licenses/MIT
+
+import {expect} from '@loopback/testlab';
+import {assignRouterSpec, RouterSpec} from '../../../router';
+
+describe('assignRouterSpec', () => {
+  it('duplicates the additions spec if the target spec is empty', async () => {
+    const target: RouterSpec = {paths: {}};
+    const additions: RouterSpec = {
+      paths: {
+        '/': {
+          get: {
+            responses: {
+              '200': {
+                description: 'greeting',
+                content: {
+                  'application/json': {
+                    schema: {type: 'string'},
+                  },
+                },
+              },
+            },
+          },
+        },
+      },
+      components: {
+        schemas: {
+          Greeting: {
+            type: 'object',
+            properties: {
+              message: {
+                type: 'string',
+              },
+            },
+          },
+        },
+      },
+      tags: [{name: 'greeting', description: 'greetings'}],
+    };
+
+    assignRouterSpec(target, additions);
+    expect(target).to.eql(additions);
+  });
+
+  it('does not assign components without schema', async () => {
+    const target: RouterSpec = {
+      paths: {},
+      components: {},
+    };
+
+    const additions: RouterSpec = {
+      paths: {},
+      components: {
+        parameters: {
+          addParam: {
+            name: 'add',
+            in: 'query',
+            description: 'number of items to add',
+            required: true,
+            schema: {
+              type: 'integer',
+              format: 'int32',
+            },
+          },
+        },
+        responses: {
+          Hello: {
+            description: 'Hello.',
+          },
+        },
+      },
+    };
+
+    assignRouterSpec(target, additions);
+    expect(target.components).to.be.empty();
+  });
+
+  it('uses the route registered first', async () => {
+    const target: RouterSpec = {
+      paths: {
+        '/': {
+          get: {
+            responses: {
+              '200': {
+                description: 'greeting',
+                content: {
+                  'application/json': {
+                    schema: {type: 'string'},
+                  },
+                },
+              },
+            },
+          },
+        },
+      },
+    };
+
+    const additions: RouterSpec = {
+      paths: {
+        '/': {
+          get: {
+            responses: {
+              '200': {
+                description: 'additional greeting',
+                content: {
+                  'application/json': {
+                    schema: {type: 'string'},
+                  },
+                },
+              },
+              '404': {
+                description: 'Error: no greeting',
+                content: {
+                  'application/json': {
+                    schema: {type: 'string'},
+                  },
+                },
+              },
+            },
+          },
+        },
+      },
+    };
+
+    assignRouterSpec(target, additions);
+    expect(target.paths).to.equal(target.paths);
+  });
+
+  it('does not duplicate tags from the additional spec', async () => {
+    const target: RouterSpec = {
+      paths: {},
+      tags: [{name: 'greeting', description: 'greetings'}],
+    };
+    const additions: RouterSpec = {
+      paths: {},
+      tags: [
+        {name: 'greeting', description: 'additional greetings'},
+        {name: 'salutation', description: 'salutations!'},
+      ],
+    };
+
+    assignRouterSpec(target, additions);
+    expect(target.tags).to.containDeep([
+      {name: 'greeting', description: 'greetings'},
+      {name: 'salutation', description: 'salutations!'},
+    ]);
+  });
+});