Overview

Components play an important role in the extensibility of LoopBack 4. AComponent makes it easy for independent developers to contribute additionalfeatures to LoopBack. Components serve as a vehicle to group extensioncontributions such as Context Bindings and various artifacts toallow easier extensibility of your Application.

A typical LoopBack component is an npm packageexporting a Component class which can be added to your application.

Apart from its own properties, Component class can have the followingproperties:

• controllers - An array of controller classes.
• providers - A map of providers to be bound to the applicationcontext.
• classes - A map of TypeScipt classes to be bound to the application context.
• servers - A map of name/class pairs for servers.
• lifeCycleObservers - An array of life cycle observers.
• bindings - An array of bindings to be aded to the applicationcontext. A good example of using bindings to extend the functionality of aLoopBack 4 app iscontributing an additional body parser.These properties contribute to the application to add additional features andcapabilities.

LoopBack 4 was built with extensibility in mind and this includes Components,which can be allowed to contribute additional artifacts by adding a Mixin toyour Application class. This doesn’t change how a Component is registered(app.component()) but it enables the Component to contribute additionalartifacts. For example:

• Repositories can be contributed by a Component by addingRepositoryMixin from @loopback/repository to your Application
• Booters can be contributed by a Componentby adding BootMixin from @loopback/boot to your Application

Note:

Always check a component’s instructions to see if it requires the useof a Mixin. A Mixin may automatically register a Component, saving you thetrouble of having to do so manually. Again it’s best to check the documentationfor the given Component/Mixin.

See Using components andCreating components for more information.