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What is an Application?

In LoopBack 4, the Application class is the central class for setting up all of your module’s components, controllers, servers and bindings. The Application class extends Context and provides the controls for starting and stopping itself and its associated servers.

When using LoopBack 4, we strongly encourage you to create your own subclass of Application to better organize your configuration and setup.

Making your own application class

By making your own application class, you can perform several additional tasks as a part of your setup:

  • Pass the configuration into the base class constructor
  • Perform asynchronous startup functions before starting the application
  • Perform graceful cleanup functions when the application stops


import {Application} from '@loopback/core';
import {RestComponent} from '@loopback/rest';
import {UserController, ShoppingCartController} from './controllers';

export class WidgetApplication extends Application {
  constructor() {
    // This is where you would pass configuration to the base constructor
    // (as well as handle your own!)
      rest: {
        port: 8080,

    const app = this; // For clarity.
    // You can bind to the Application-level context here.
    // app.bind('foo').to(bar);

  async stop() {
    // This is where you would do whatever is necessary before stopping your
    // app (graceful closing of connections, flushing buffers, etc)
    console.log('Widget application is shutting down...');
    // The superclass stop method will call stop on all servers that are
    // bound to the application.
    await super.stop();

Configuring your application

Your application can be configured with constructor arguments, bindings, or a combination of both.

Binding configuration

Binding is the most commonly-demonstrated form of application configuration throughout our examples. Binding is the recommended method for setting up your application.

In addition to the binding functions provided by Context, the Application class also provides some sugar functions for commonly used bindings, like component, server and controller:

export class MyApplication extends Application {
  constructor() {
    this.server(RestServer, 'public');
    this.server(RestServer, 'private');


You can find a complete list of these functions on the Application API docs page.

Additionally, you can use more advanced forms of binding to fine-tune your application’s configuration:

export class MyApplication extends Application {
  constructor() {

In the above example:

  • injection calls for fooCorp.logger will be handled by the LogProvider class.
  • injection calls for repositories.widget will be handled by a singleton instance of the WidgetRepository class.



The component function allows binding of component constructors within your Application instance’s context.

For more information on how to make use of components, see Using Components.



Much like the component function, the controller function allows binding of Controllers to the Application context.


app.servers([MyServer, GrpcServer]);

The server function is much like the previous functions, but bulk bindings are possible with Servers through the function servers.

const app = new Application();
app.server(RestServer, 'public'); // {'public': RestServer}
app.server(RestServer, 'private'); // {'private': RestServer}

In the above example, the two server instances would be bound to the Application context under the keys servers.public and servers.private, respectively.

Constructor configuration

The Application class constructor also accepts an ApplicationConfig object which contains component-level configurations such as RestServerConfig. It will automatically create bindings for these configurations and later be injected through dependency injections. Visit Dependency Injection for more information.

export class MyApplication extends RestApplication {
  constructor() {
      rest: {
        port: 4000,
        host: 'my-host',

Tips for application setup

Here are some tips for application setup to help avoid common pitfalls and mistakes.

Extend from RestApplication when using RestServer

If you want to use RestServer from the @loopback/rest package, we recommend extending RestApplication in your app instead of manually binding RestServer or RestComponent. RestApplication already uses RestComponent and makes useful functions in RestServer like handler() available at the app level. This means you can call the RestServer functions to perform all of your server-level setups in the app constructor without having to explicitly retrieve an instance of your server.

Serve static files

The RestServer allows static files to be served. It can be set up by calling the static() API.

app.static('/html', rootDirForHtml);


server.static(['/html', '/public'], rootDirForHtml);

Static assets are not allowed to be mounted on / to avoid performance penalty as / matches all paths and incurs file system access for each HTTP request.

The static() API delegates to serve-static to serve static files. Please see and for details.


The static assets are served before LoopBack sequence of actions. If an error is thrown, the reject action will NOT be triggered.

Use unique bindings

Use binding names that are prefixed with a unique string that does not overlap with LoopBack’s bindings. As an example, if your application is built for your employer FooCorp, you can prefix your bindings with fooCorp.

// This is unlikely to conflict with keys used by other component developers
// or within loopback itself!

Avoid use of getSync

We provide the getSync function for scenarios where you cannot asynchronously retrieve your bindings, such as in constructor bodies.

However, the number of scenarios in which you must do this are limited, and you should avoid potential race conditions and retrieve your bindings asynchronously using the get function whenever possible.

Use caution with singleton binding scopes

By default, bindings for controllers will instantiate a new instance whenever they are injected or retrieved from their binding. Your application should only set singleton binding scopes on controllers when it makes sense to do so.