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The first step on the component migration journey is to reorganize the project files, change component’s entry point and documentation for adding the component to the target application.

LoopBack 3 component layout & mounting

A LB3 component is implemented as a function accepting the target app object and the configuration options.

A minimal component consists of a single index.js file with the following content:

module.exports = function initializeComponent(loopbackApplication, options) {
  // implementation

A component is typically added to a LB3 application by creating a new entry in server/component-config.json, see LoopBack components.

For example:

  "loopback-explorer": {
    "mountPath": "/explorer"

This allows the component to receive configuration. App developers can provide environment-specific configuration by using component-config.{env}.json files.

LoopBack 4 layout

As explained in Creating components and Using components, a typical LoopBack 4 component is an npm package exporting a Component class.

The component class is usually implemented inside src/{component-name}.component.ts file, for example src/metrics.component.ts:

import {Application, Component, CoreBindings} from '@loopback/core';
import {bind, config, ContextTags, inject} from '@loopback/context';
import {MetricsBindings} from './keys';
import {DEFAULT_METRICS_OPTIONS, MetricsOptions} from './types';

@bind({tags: {[ContextTags.KEY]: MetricsBindings.COMPONENT}})
export class MetricsComponent implements Component {
    private application: Application,
    options: MetricsOptions = DEFAULT_METRICS_OPTIONS,
  ) {
    // ...
  // ...

The code snippet above also shows how a LoopBack 4 component can receive configuration and the target application object.

Usage instructions

LoopBack 4 components are added to applications inside application constructor.

First, the application file needs to import the component class:

import {MetricsComponent} from '@loopback/extension-metrics';

Then in the constructor, add the component to your application:


Finally, the application can configure the component by adding the following code to its constructor:

  // the configuration

Migration steps

It is not feasible to migrate a LoopBack 3 component project to a LoopBack 4 component project in a series of incremental changes done within the same repository. We recommend to create a new project using lb4 extension CLI command and then incrementally migrate artifacts from the original LoopBack 3 component to the new project.