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This article will help existing users understand LoopBack 4:

  • How to connect LoopBack 3.x concepts to LoopBack 4 terms
  • What’s new and exciting in LoopBack 4


At high-level, LoopBack 3.x applications consist of three big “parts”

  • Persistence layer (this includes talking to backend services like SOAP/REST)
  • Outwards facing REST API
  • App-wide setup - Express middleware, boot scripts, etc.

In the persistence layer, users can contribute the following artifacts:

  1. Definitions of Model data types (properties, validations)
  2. Definition of data sources
  3. Configuration of models (which datasource are they attached to)
  4. Operation hooks

At the public API side, users can define:

  1. Which built-in methods should be exposed (think of disableRemoteMethodByName)
  2. Custom remote methods
  3. before/after/afterError hooks at application-level
  4. before/after/afterError hooks at model-level
  5. before/after/afterError hooks at model method level

LoopBack 4 was intentionally designed to allow users to choose their own ORM/persistence solution. The juggler from LoopBack 3 has been packaged into @loopback/repository so that it’s possible for users to reuse their existing model definitions, migrating their application incrementally.

Concept/feature mapping

In Loopback 3.x (and earlier), models were responsible for both accessing data in other systems (databases, SOAP services, etc.) and providing the application’s external REST API. This made it easy to quickly build a REST interface for an existing database, but difficult to customize the REST API and fine-tune it to the needs of application clients.

LoopBack 4 is moving to the well-known Model-(View-)Controller pattern, where the code responsible for data access and manipulation is separated from the code responsible for implementing the REST API.

loopback4-example-microservices demonstrates this loose coupling. Facade is the top-level service that serves the account summary API, and is dependent on the three services Account, Customer, and Transaction. But the facade only aggregates the calls to the three services, and is not tightly coupled with the service implementation; that’s why it is independent of the three services. We can define the APIs in facade the way we want. Thus, code responsible for data access and manipulation is separated from the code responsible for implementing client side APIs.

Concept/Feature LoopBack 3.x LoopBack 4
Programming Language Built with JavaScript ES5
Node.js callback
TypeScript 2.6.x & JavaScript ES2016/2017
Promise & Async/Await
Core foundation Express with LoopBack extensions Home-grown IoC container
Model Definition Models can be defined with JavaScript or JSON Models can be defined with TypeScript/JavaScript/JSON(TBA)
Model Persistence A model can be attached to a datasource backed by a connector that implements CRUD operations Repositories are introduced to represent persistence related operations; a repository binds a model metadata to a datasource
Model Relation Relations can be defined between models (TBA) Relations can be defined between models but they will be realized between repositories
Model Remoting JavaScript/JSON remoting metadata is used to describe method signatures and their mapping to REST/HTTP
Swagger specs are generated after the fact
Remoting metadata can be supplied by OpenAPI JSON/YAML documents or generated automatically through TypeScript decorators
API Spec Swagger 2.0 OpenAPI Spec 3.0 and potentially other API specs such as GraphQL, gRPC, etc.
API Explorer Built-in UI based on swagger-ui (/explorer) (Beta) Expose OpenAPI specs and a browser redirect to Swagger UI hosted by
DataSource JSON and JS JSON/JS/TypeScript
Connector Plain JS JS and TypeScript (TBA)
Mixin Use a utility to add methods from the mixin to the target model class Use ES2015 mixin classes pattern supported by TypeScript 2.2 and above
Middleware Express middleware with phase-based registration and ordering Sequence consisting of actions
Boot script Scripts to be invoked during bootstrapping (TBD)
Remote hooks Before/After hooks for remote methods Sequence/actions
CRUD operation hooks Hooks for CRUD operations Sequence/actions
Built-in models Built-in User/AccessToken/Application/ACL/Role/RoleMapping for AAA (TBD)
Authentication User model as the login provider
(TBA) Authentication component (@loopback/authentication) with extensibility to strategy providers
Authorization Use built-in User/Application/AccessToken model for identity and ACL/Role/RoleMapping for authorization (TBD) Authorization component
Component A very simple implementation to configure and invoke other modules A fully-fledged packaging model that allows contribution of extensions from other modules
Tooling loopback-cli and API Connect UI @loopback/cli

What’s new and exciting in LoopBack 4

Some of the highlights of LoopBack 4 include:

  • Leverage TypeScript for better code quality and productivity
  • Unify and simplify the asynchronous programming model/style around Promise and Async/Await
  • Implement an IoC Container with Dependency Injection for better visibility, extensibility and composability
  • Introduce Component as packaging model for extensions that can be plugged into LoopBack 4 applications
  • Make everything else as components (REST, Authentication, and Authorization)
  • Divide the responsibilities of LoopBack models into
    • Controllers: handle incoming API requests
    • Repositories: provide access to datasources
    • Models: define schemas for business objects
    • Services: interact with existing REST APIs, SOAP Web Services, and other forms of services/microservices
  • Refactor the ORM into separate modules for different concerns

As LoopBack 4 continues to grow, more features are continuously added. You can check out our blog to keep up with these new features.