Page Contents

About the project

What’s the vision behind LoopBack 4?

  • Make it even easier to build apps that require complex integrations
  • Enabling an ecosystem of extensions
  • Small, fast, flexible, powerful core
  • Suitable for small and large teams
  • Minimally opinionated, enforce your team’s opinions instead

See Crafting LoopBack 4 for more details.

Is LoopBack 4 a REST framework?

Yes and no. Unlike other REST frameworks, LoopBack 4’s scope spans beyond creating REST APIs, and the individual Node.js packages are designed with this in mind. This leads to increased flexibility for more complex use-cases and enables reuse of concepts across different projects.

LoopBack 4’s focus is still REST APIs and will continue to innovate in improving the API developer experience.

See Can non-REST projects utilize LoopBack 4? for details on how to leverage LoopBack in other projects.

Why introduce so many new concepts?

At first glance, concepts such as Inversion of Control, Dependency Injection, Services and Providers may seem daunting when coming from other typical Node.js projects. However, these have been well-defined and battle-tested in many other programming languages such as C# and Java. Furthermore, they have been rapidly adopted by other Node.js project such as Angular.

These concepts improve code readability and predictability by promoting use of SOLID principles, and by doing so, aid in code maintainability in the long term.

Can non-REST projects utilize LoopBack 4?

Yes they can! LoopBack 4 was designed to be flexible to support non-REST projects. Here’s a few examples:

  • @loopback/context

    Dependency injection framework. Can be used to in any Node.js project to improve code maintainability.

  • @loopback/core

    Builds on top of @loopback/context and introduces higher-level concepts such as Services to further improve code maintainability.

  • @loopback/metadata

    Create and inspect TypeScript decorators to define, merge and inspect metadata.

  • @loopback/repository

    Cross-database ORM and ODM framework. Can be used to create cross-database designs, queries and relations.

  • @loopback/filter

    Contains LoopBack filter typings and builders. Can be used to build queries on LoopBack API consumers.

  • @loopback/graphql

    Brings first-class support for building GraphQL APIs. Can be used alongside or independent of @loopback/rest.

Can I still use require and import without Dependency Injection?

Yes you can! LoopBack 4 projects are still Node.js projects at its core. Hence, developers still can leverage on existing and familiar Node.js concepts to build their applications. We still recommend utilizing the Dependency Injection framework where possible to improve code maintainability.

LoopBack 3 vs LoopBack 4

We recommend that current users migrate to LoopBack 4, which can be done through following the migration guide and new users start with LoopBack 4. See Differences between LoopBack v3 and v4 if you’re interested in the differences between the two versions.

Why TypeScript?

Although developers can still write application logic in either JavaScript or TypeScript, LoopBack 4’s core is written in TypeScript, for the following reasons:

  • Improved developer productivity and scalability. Our customers need a framework that scales to dozens and even hundreds of developers. This scalability is the reason TypeScript exists and is gaining traction.
  • Improved extensibility and flexibility. LoopBack 4’s core is simpler than LoopBack 3.x with well-defined extension points. A lot of responsibility will be shifted to extensions (components), which can be JavaScript or TypeScript.
  • Unified tooling. A good proportion of TypeScript developers use Visual Studio Code. Hence, we provide out-of-the-box Visual Studio Code configurations to make LoopBack application development more convenient.
  • Future-proofing. Ability to leverage the latest and future JavaScript constructs.

TypeScript’s support for static analysis makes more robust tooling possible and is the foundation for its scalability. The ability to easily refactor code without the common human-introduced errors. Dev and Compile time checking. For example, most people don’t have the same expertise and time we do to setup complex linting solutions (for example, a linting config that works across many projects).

For more details, see the lengthy discussion in #6.

About the documentation

Where are the tutorials?

See Examples and Tutorials.

What features are planned?

Our roadmap is captured in here. You can also check out our monthly milestone plans.

For LoopBack 3 users, here is the list of LB3 feature parity we wish to be implemented in the future:

Do you have some other features in mind? Add your feature requests at loopback-next/issues/new.

Technical usage

Does JavaScript still work?

No. While some packages may work with JavaScript, there is no guarantee that all features will be compatible. Please see GitHub issues #560 and #1978 for prior discussion and the issues of supporting a pure-JavaScript experience.

How do I disable the API Explorer?

LoopBack provides a self-hosted and a redirect to an external API Explorer.

Documentation to disable both API Explorers:

How do I send a custom response?

Return a custom response body

The response is usually determined by value returned by the controller function that’s invoked. A good example of this is the default PingController scaffolded with every LoopBack 4 application:

// Note: code shortened for bevity

// Map to `GET /ping`
@get('/ping', {
  responses: {
    '200': PING_RESPONSE,
ping(): object {
  // Reply with a greeting, the current time, the url, and request headers
  return {
    greeting: 'Hello from LoopBack',
    date: new Date(),
    url: this.req.url,
    headers: Object.assign({}, this.req.headers),

In this example, we can see that the ping() function returns a custom object. This would be reflected in the response body when an API consumer makes a request to /ping.

Further customize the response

Sometimes, other parts of the response needs to be modified (such as the HTTP headers). This can be accomplished by injecting the Response object into the controller:

import {inject} from '@loopback/core';
import {get, Response, RestBindings} from '@loopback/rest';

export class PingController {
  constructor(@inject(RestBindings.Http.RESPONSE) private res: Response) {}

  // Map to `GET /ping`
  @get('/ping', {
    responses: {
      '200': {
        description: 'Ping Response',
        content: {
          'application/json': {
            schema: {
              type: 'object',
              title: 'PingResponse',
              properties: {
                greeting: {type: 'string'},
                additionalProperties: false,
  ping(): object {
    this.res.setHeader('x-secret-sauce', 'Sugar, spice and everything nice.');

    // Reply with a greeting
    return {
      greeting: 'Hello from LoopBack',

This will result in a custom response body and a new header, x-secret-sauce.

Where do I find the default binding keys?

Binding keys used by @loopback/* packages are consolidated under Reserved binding keys.

What is the difference between general and configuration bindings?

Configuration bindings leverage an existing binding key to transparently standardize the naming convention of the configuration binding keys without needing to create a completely separate key. See Configuration by convention.

Can I attach an Express router?

Yes. See Mounting an Express Router.

Can I mount an Express middleware?

Yes. See Using Express Middleware.

LoopBack middleware vs Express middleware

Where possible, use LoopBack middleware instead. Express middleware are less flexible and LoopBack integration is only meant to be a compatibility layer for existing middleware. See Middleware for more details.