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Overview

Express is the most popular web framework for Node.js developers. As quoted below from the Express web site, middleware are the basic building blocks for Express applications.

Express is a routing and middleware web framework that has minimal functionality of its own: An Express application is essentially a series of middleware function calls.

LookBack 4 leverages Express behind the scenes for its REST server implementation. We decided to not expose middleware capabilities to users while we pursue an elegant and non-invasive way to fit Express middleware into the LoopBack 4 programming model nicely. Meanwhile, we have received various requests and questions from our users on how to use Express middleware with LoopBack 4 or migrate their usage of Express middleware from LoopBack 3 to LoopBack 4.

Use cases

The following use cases are identified to allow Express middleware to work with LoopBack 4:

For application developers

  1. Invoke one or more Express middleware handler functions in the REST sequence before, after, or between existing actions.
  2. Apply Express middleware globally to all controllers, or locally to certain controller classes and methods.
  3. Migrate LoopBack 3 applications that use Express middleware
  4. Be able to separate Express middleware configuration from its registration and allow configuration changes at runtime to be effective without restarting the application.

For extension developers

  1. Allow new actions to be added to the REST sequence without requiring code changes to the application as long as the extension component is mounted to the application. For example, @loopback/authentication contributes an Authenticate action. It should be possible to build an extension module for Helmet to provide better protection for LoopBack.
  2. Allow actions to be invoked in a configurable order.
  3. Allow extensions to leverage LoopBack’s dependency injection and extension point/extension capabilities.

Let’s start with a few examples to illustrate how we can bring Express middleware to LoopBack applications with minimal effort.

Use Express middleware within the sequence of actions

Express middleware can now be plugged into the REST sequence with an InvokeMiddleware action being injected to the default sequence.

Invoke Express middleware explicitly in the sequence

The custom sequence class below invokes two Express middleware (helmet and morgan) handler functions as the first step.

src/sequence.ts

import helmet from 'helmet'; // For security
import morgan from 'morgan'; // For http access logging

const middlewareList: ExpressRequestHandler[] = [
  helmet({}), // options for helmet is fixed and cannot be changed at runtime
  morgan('combined', {}), // options for morgan is fixed and cannot be changed at runtime
];

export class MySequence extends DefaultSequence {
  async handle(context: RequestContext): Promise<void> {
    try {
      const {request, response} = context;
      // `this.invokeMiddleware` is an injected function to invoke a list of
      // Express middleware handler functions
      const finished = await this.invokeMiddleware(context, middlewareList);
      if (finished) {
        // The http response has already been produced by one of the Express
        // middleware. We should not call further actions.
        return;
      }
      const route = this.findRoute(request);
      const args = await this.parseParams(request, route);
      const result = await this.invoke(route, args);

      this.send(response, result);
    } catch (error) {
      this.reject(context, error);
    }
  }
}

Register Express middleware to be executed by InvokeMiddleware actions

While the explicit Express middleware invocation is easy and simple, there are some limitations.

  1. The list of Express middleware has to be hard-coded in src/sequence.ts. It’s not easy to plug in a new middleware.
  2. The configuration of Express middleware is fixed unless we use dependency injections for such values in src/sequence.ts.

We provide another option to make invocation of Express middleware more flexible and extensible. The InvokeMiddleware actions within the sequence can discover registered middleware and invoke them in a chain.

We first register middleware against the default or a named chain using APIs from RestApplication. It can happen in the constructor of an application.

src/application.ts

import morgan from 'morgan';
import {ApplicationConfig} from '@loopback/core';
import {RestApplication} from '@loopback/rest';

export class MyApplication extends RestApplication {
  constructor(config: ApplicationConfig) {
    this.expressMiddleware(
      morgan,
      {}, // default config
      {
        // Allow configuration to be injected to allow dynamic changes to
        // morgan logging by configuring `middleware.morgan` to a new value
        injectConfiguration: 'watch',
        key: 'middleware.morgan',
      },
    );
  }
}

Use Express middleware as interceptors for controllers

The LoopBack global and local interceptors now also serve as an avenue to attach middleware logic to specific points of controller invocations, such as global, class, or method levels.

There are a few options to wrap an Express middleware module into an LoopBack 4 interceptor.

  • toInterceptor: wraps an Express handler function to a LoopBack interceptor function
  • createInterceptor: creates a LoopBack interceptor function from an Express factory function with configuration
  • defineInterceptorProvider: creates a LoopBack provider class for interceptors from an Express factory function with configuration. This is only necessary that injection and/or change of configuration is needed. The provider class then needs to be bound to the application context hierarchy as a global or local interceptor.

Let’s walk through a few examples:

Adapt an Express middleware handler function to an interceptor

If the Express middleware only exposes the handler function without a factory or a single instance is desired, use toInterceptor.

import {toInterceptor} from '@loopback/express';
import morgan from 'morgan';

const morganInterceptor = toInterceptor(morgan('combined'));

Create an interceptor from Express middleware factory function and configuration

When the Express middleware module exports a factory function that takes an optional argument for configuration, use createInterceptor.

import {createInterceptor} from '@loopback/express';
import helmet, {IHelmetConfiguration} from 'helmet';
const helmetConfig: IHelmetConfiguration = {};
const helmetInterceptor = createInterceptor(helmet, helmetConfig);

If the Express middleware module does not expose a factory function conforming to the ExpressMiddlewareFactory signature, a wrapper can be created. For example:

import morgan from 'morgan';

// Register `morgan` express middleware
// Create a middleware factory wrapper for `morgan(format, options)`
const morganFactory = (config?: morgan.Options) => morgan('combined', config);

Define a provider class for middleware-based interceptor

It’s often desirable to allow dependency injection of middleware configuration for the middleware. We can use defineInterceptorProvider to simplify definition of such provider classes.

import {defineInterceptorProvider} from '@loopback/express';
import helmet, {IHelmetConfiguration} from 'helmet';

const helmetProviderClass = defineInterceptorProvider<IHelmetConfiguration>(
  helmet,
  {}, // default config
);

Alternatively, we can create a subclass of ExpressMiddlewareInterceptorProvider.

import {config} from '@loopback/context';
import {
  ExpressMiddlewareInterceptorProvider,
  createMiddlewareInterceptorBinding,
} from '@loopback/express';
import helmet, {IHelmetConfiguration} from 'helmet';

class HelmetInterceptorProvider extends ExpressMiddlewareInterceptorProvider<
  IHelmetConfiguration
> {
  constructor(@config() helmetConfig?: IHelmetConfiguration) {
    super(helmet, helmetConfig);
  }
}

The provider class can then be registered to the application. For example, the code below can be used in the constructor of your Application subclass.

const binding = createMiddlewareInterceptorBinding(HelmetInterceptorProvider);
this.add(binding);

Apply Express middleware as invocation interceptors

With the ability to wrap Express middleware as LoopBack 4 interceptors, we can use the same programming model to register middleware as global interceptors or local interceptors denoted by @intercept decorators at class and method levels.

The middleware interceptor function can be directly referenced by @intercept.

import morgan from 'morgan';
const morganInterceptor = toInterceptor(morgan('combined'));
class MyController {
  @intercept(morganInterceptor)
  hello(msg: string) {
    return `Hello, ${msg}`;
  }
}

It’s also possible to bind the middleware to a context as a local or global interceptor.

import helmet, {IHelmetConfiguration} from 'helmet';
const binding = registerExpressMiddlewareInterceptor(
  app,
  helmet,
  {},
  {
    // As a global interceptor
    global: true,
    key: 'interceptors.helmet',
  },
);

For a bound local interceptor, the binding key can now be used with @intercept.

@intercept('interceptors.helmet')
class MyController {
  hello(msg: string) {
    return `Hello, ${msg}`;
  }
}

Use lb4 interceptor command to create interceptors for Express middleware

The lb4 interceptor can be used to generate a skeleton implementation of global or local interceptors. We can update the generated code to plug in Express middleware. For example, to add helmet as the security middleware:

lb4 interceptor
? Interceptor name: Helmet
? Is it a global interceptor? Yes
? Group name for the global interceptor: ('') middleware
   create src/interceptors/helmet.interceptor.ts
   update src/interceptors/index.ts

Interceptor Helmet was created in src/interceptors/

Let’s update `src/interceptors/helmet.interceptor.ts:

import {config, globalInterceptor} from '@loopback/core';
import helmet, {IHelmetConfiguration} from 'helmet';
import {ExpressMiddlewareInterceptorProvider} from '@loopback/express';

@globalInterceptor('middleware', {tags: {name: 'Helmet'}})
export class MorganInterceptor extends ExpressMiddlewareInterceptorProvider<
  IHelmetConfiguration
> {
  constructor(
    @config()
    options: IHelmetConfiguration = {
      hidePoweredBy: true,
    },
  ) {
    super(helmet, options);
  }
}

Use Express Router to enable

Express allows HTTP verbs to be used to set up routes, such as app.post('/hello', ...). See http://expressjs.com/en/4x/api.html#app.METHOD.

To allow a similar usage in LoopBack, we can create an Express router and register it to LoopBack as follows:

import {ExpressRequestHandler, Router} from '@loopback/express';

const handler: ExpressRequestHandler = async (req, res, next) => {
  res.send(req.path);
};

const router = Router();
router.post('/greet', handler);
router.get('/hello', handler);
const binding = server.expressMiddleware('middleware.express.greeting', router);

What’s behind the scenes

Middleware and Interceptor are key concepts that allow Express middleware into LoopBack seamlessly. Please read the following pages to better understand the architecture.