Build Large-Scale Node.js Projects with LoopBack 4 Core

Quick Start: Minimal Extensible Application

import {
  Application,
  BindingKey,
  Component,
  Binding,
  createBindingFromClass,
  extensionPoint,
  extensions,
  BindingTemplate,
  extensionFor,
  injectable,
  Getter,
  config,
} from '@loopback/core';

// 1. Define the extension contract
export interface Greeter {
  language: string;
  greet(name: string): string;
}

export const GREETER_EXTENSION_POINT_NAME = 'greeters';

export const asGreeter: BindingTemplate = binding => {
  extensionFor(GREETER_EXTENSION_POINT_NAME)(binding);
  binding.tag({namespace: 'greeters'});
};

// 2. Define the extension point
@extensionPoint(GREETER_EXTENSION_POINT_NAME)
export class GreetingService {
  constructor(
    @extensions() private getGreeters: Getter<Greeter[]>,
    @config() public readonly options?: {color: string},
  ) {}
  async greet(language: string, name: string): Promise<string> {
    const greeters = await this.getGreeters();
    const greeter = greeters.find(g => g.language === language);
    return greeter ? greeter.greet(name) : `Hello, ${name}!`;
  }
}

// 3. Implement extensions
@injectable(asGreeter)
export class EnglishGreeter implements Greeter {
  language = 'en';
  greet(name: string) {
    return `Hello, ${name}!`;
  }
}

@injectable(asGreeter)
export class ChineseGreeter implements Greeter {
  language = 'zh';
  greet(name: string) {
    return `${name}，你好！`;
  }
}

// 4. Bundle into a component
export const GREETING_SERVICE = BindingKey.create<GreetingService>(
  'services.GreetingService',
);

export class GreetingComponent implements Component {
  bindings: Binding[] = [
    createBindingFromClass(GreetingService, {key: GREETING_SERVICE}),
    createBindingFromClass(EnglishGreeter),
    createBindingFromClass(ChineseGreeter),
  ];
}

// 5. Compose components via nesting
export class CoreComponent implements Component {
  // services: auto-registered service/provider classes
  services = [SomeUtilityService];
}

export class AppComponent implements Component {
  // components: nested components (registered recursively)
  components = [CoreComponent, GreetingComponent];
  // services: additional services for this layer
  services = [AppSpecificService];
}

// 6. Wire up the application
export class MyApp extends Application {
  constructor() {
    super({shutdown: {signals: ['SIGINT']}});
    this.component(AppComponent);
  }
  async main() {
    const svc = await this.get(GREETING_SERVICE);
    console.log(await svc.greet('en', 'World'));
  }
}

Core Patterns Summary

Key Rules

• Always import from @loopback/core, not @loopback/context
• Use BindingKey.create<T>() for strongly-typed keys
• Extension injection: @extensions() returns Getter<T[]> (lazy, picks up dynamic additions); @extensions.list() returns T[] (eager, simpler when extensions are static at startup)
• Use @config() for artifact configuration, app.configure(key).to(value) to set it
• Use @injectable(bindingTemplate) to decorate extension classes — can combine scope and extension: @injectable({scope: BindingScope.SINGLETON}, extensionFor(POINT))
• Use createBindingFromClass() to create bindings that respect @injectable metadata
• Compose components hierarchically: components[] for nesting, services[] for auto-registration, bindings[] for custom bindings
• Use BindingScope.SINGLETON for shared stateful services
• Use CoreBindings.APPLICATION_INSTANCE to inject the Application itself
• Use ContextTags.KEY to tag bindings with a stable key: tags: {[ContextTags.KEY]: MY_KEY}
• Lifecycle observer groups are sorted alphabetically — use numbered prefixes (e.g., '03-setup', '10-app') to control startup order

References

• Context & Bindings: references/context-and-bindings.md — creating contexts, binding types, scopes, finding bindings, context hierarchy, views, components
• Dependency Injection: references/dependency-injection.md — constructor/property/method injection, getters, views, custom decorators, custom injectors
• Extension Points: references/extension-points.md — defining contracts, extension point classes, implementing/registering extensions, configuration
• Interceptors & Observers: references/interceptors-and-observers.md — global interceptors, interceptor proxies, life cycle observers, dynamic config via ContextView
• Configuration: references/configuration.md — @config(), @config.view(), dynamic config, custom resolvers, sync vs async
• Advanced Recipes: references/advanced-recipes.md — custom decorators, custom injectors, parameterized class factories, application scaffolding