A simple Express application with LoopBack 4 REST API
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Express is an unopinionated Node.js framework. LoopBack REST API can be mounted to an Express application and be used as middleware. This way the user can mix and match features from both frameworks to suit their needs.

This tutorial assumes familiarity with scaffolding a LoopBack 4 application, Models, DataSources, Repositories, and Controllers. To see how they’re used in a LoopBack application, please see the Todo tutorial.

Try it out

If you’d like to see the final results of this tutorial as an example application, follow these steps:

  1. Run the lb4 example command to select and clone the express-composition repository:

    lb4 example express-composition
  2. Switch to the directory.

    cd loopback4-example-express-composition
  3. Finally, start the application!

    $ npm start
    Server is running at

Create your LoopBack Application

Scaffold your Application

Run lb4 app note to scaffold your application and fill out the following prompts as follows:

$ lb4 app note
? Project description: An application for recording notes.
? Project root directory: (note)
? Application class name: (NoteApplication)
 ◉ Enable eslint: add a linter with pre-configured lint rules
 ◉ Enable prettier: install prettier to format code conforming to rules
 ◉ Enable mocha: install mocha to run tests
 ◉ Enable loopbackBuild: use @loopback/build helpers (e.g. lb-eslint)
 ◉ Enable vscode: add VSCode config files
❯◯ Enable docker: include Dockerfile and .dockerignore
 ◉ Enable repositories: include repository imports and RepositoryMixin
 ◉ Enable services: include service-proxy imports and ServiceMixin
 # npm will install dependencies now
 Application note was created in note.

Add Note Model

Inside the project folder, run lb4 model to create the Note model with Entity model base class. Add an id property with type number, a required title property with type string, and a content property of type string.

Add a DataSource

Now, let’s create a simple in-memory datasource by running the lb4 datasource ds command and the following full path to file: ./data/ds.json.

Similar to the Todo example, let’s create the ds.json by creating a data folder at the application’s root.

$ mkdir data
$ touch data/ds.json

Then copy and paste the following into the ds.json file:

  "ids": {
    "Note": 3
  "models": {
    "Note": {
      "1": "{\"title\":\"Things I need to buy\",\"content\":\"milk, cereal, and waffles\",\"id\":1}",
      "2": "{\"title\":\"Great frameworks\",\"content\":\"LoopBack is a great framework\",\"id\":2}"

Add Note Repository

To create the repository, run the lb4 repository command and choose the DsDataSource, as the datasource, Note model as the model, and DefaultCrudRepository as the repository base class.

Add Note Controller

To complete the Note application, create a controller using the lb4 controller note command, with the REST Controller with CRUD functions type, Note model, and NoteRepository repository. The id’s type will be number and base HTTP path name is the default /notes.

Create a Facade Express Application

Let’s start by installing dependencies for the express module:

npm install --save express
npm install --save-dev @types/express

Create a new file src/server.ts to create your Express class:


import express from 'express';

export class ExpressServer {
  constructor() {}

Create two properties, the Express application instance and LoopBack application instance:

import {NoteApplication} from './application';
import {ApplicationConfig} from '@loopback/core';
import express from 'express';

export class ExpressServer {
  private app: express.Application;
  private lbApp: NoteApplication;

  constructor(options: ApplicationConfig = {}) {
    this.app = express();
    this.lbApp = new NoteApplication(options);

Now, inside the constructor, we’re going to add the basepath and expose the front-end assets via Express:

this.app.use('/api', this.lbApp.requestHandler);

Let’s also modify public/index.html to update the base path:


<h3>OpenAPI spec: <a href="/api/openapi.json">/openapi.json</a></h3>
<h3>API Explorer: <a href="/api/explorer">/explorer</a></h3>

Then, we can add some custom Express routes, as follows:

import {Request, Response} from 'express';
import path from 'path';

export class ExpressServer {
  private app: express.Application;
  private lbApp: NoteApplication;

  constructor(options: ApplicationConfig = {}) {
    // earlier code

    // Custom Express routes
    this.app.get('/', function (_req: Request, res: Response) {
    this.app.get('/hello', function (_req: Request, res: Response) {
      res.send('Hello world!');

And add the public/express.html file to your project.

Let’s also install p-event to make sure the server is listening:

npm install --save p-event

Finally, we can add functions to boot the Note application and start the Express application:

import pEvent from 'p-event';

export class ExpressServer {
  private app: express.Application;
  private lbApp: NoteApplication;

  constructor(options: ApplicationConfig = {}) {

  async boot() {
    await this.lbApp.boot();

  public async start() {
    await this.lbApp.start();
    const port = this.lbApp.restServer.config.port || 3000;
    const host = this.lbApp.restServer.config.host || '';
    this.server = this.app.listen(port, host);
    await pEvent(this.server, 'listening');

  // For testing purposes
  public async stop() {
    if (!this.server) return;
    await this.lbApp.stop();
    await pEvent(this.server, 'close');
    this.server = undefined;

Now that our src/server.ts file is ready, then we can modify our src/index.ts file to start the application:


import {ExpressServer} from './server';
import {ApplicationConfig} from '@loopback/core';

export {ExpressServer, NoteApplication};

export async function main(options: ApplicationConfig = {}) {
  const server = new ExpressServer(options);
  await server.boot();
  await server.start();
  console.log('Server is running at');


const application = require('./dist');

module.exports = application;

if (require.main === module) {
  // Run the application
  const config = {
    rest: {
      port: +process.env.PORT || 3000,
      host: process.env.HOST || 'localhost',
      openApiSpec: {
        // useful when used with OpenAPI-to-GraphQL to locate your application
        setServersFromRequest: true,
      // Use the LB4 application as a route. It should not be listening.
      listenOnStart: false,
  application.main(config).catch(err => {
    console.error('Cannot start the application.', err);

Please note listenOnStart is set to false to instruct the LB4 application is not listening on HTTP when it’s started as the Express server will be listening.

Now let’s start the application and visit

npm start

Server is running at

If we go to the Explorer, we can make requests for our LoopBack application. Notice how the server is

To view our custom /hello Express route, go to and you should see ‘Hello world!’.

To serve static files in your application, add the following to the end of your constructor:


export class ExpressServer {
  private app: express.Application;
  private lbApp: NoteApplication;

  constructor(options: ApplicationConfig = {}) {
    // earlier code

    // Serve static files in the public folder

  // other functions

Now, you can load any static files in the public/ folder. For example, add the following public/notes.html file to your project, run npm start again, and visit You can now see a static file that will display your Notes in a table format. For more information, please visit Serving static files in Express.

Congratulations, you just mounted LoopBack 4 REST API onto a simple Express application.