@woubuc/inject

Dependency Injection Container

A simple global dependency injection container

Basic usage

Simply annotate a service with @injectable() to register it.

Use the inject() function to get the singleton instance of the service. The DI container will automatically construct an instance of the injected class the first time it is injected anywhere.

import { injectable, inject } from '@woubuc/inject';

@injectable()
class MyService {
  doStuff() {}
}

class OtherService {
  private readonly service = inject(MyService);

  public doStuff() {
    this.service.doStuff();
  }
}

Custom tokens

You can use strings or symbols as injection tokens, instead of classes.

import { inject, injectable } from '@woubuc/inject';

const token = 'foo';

@injectable({ token })
class MyService {
  doStuff() {}
}

class OtherService {
  private readonly service = inject<MyService>(token);

  public doStuff() {
    this.service.doStuff();
  }
}

Note: always prefer class injection tokens as they are automatically strongly typed.

The DI Container

You can get a reference to the current container with Container.current(). This allows you to do more dynamic things with the container, besides simply injecting.

Get (inject)

The simple inject() function is actually shorthand for Container.current().get():

import { Container, injectable } from '@woubuc/inject';

@injectable()
class MyService {
  doStuff() {}
}

class OtherService {
  private readonly service = Container.current().get(MyService); // The long version

  public doStuff() {
    this.service.doStuff();
  }
}

Provide

Use provide() to provide custom data for a given injection token. Combined with string tokens, this can be used to provide e.g. configuration values.

import { Container, inject } from '@woubuc/inject';

const urlToken = Symbol('urlToken');

Container.current().provide(urlToken, 'http://example.com/');

class MyApiClientService {
  private readonly url = inject<string>(urlToken);

  public async load() {
    await fetch(this.url);
  }
}

Example: mocking injectables

You can use provide() to provide mock classes for tests.

// Main app
import { inject, injectable } from '@woubuc/inject';

@injectable()
class ApiService {
  public get() {
    return fetch('http://example.com');
  }
}

@injectable()
class TestableService {
  private api = inject(ApiService);

  public async load() {
    this.api.get(); // Does a fetch request
  }
}

//
// In your tests:
//
import { Container } from './lib.js';

test('my test', (t) => {
  class MockApiService {
    public get() {
      return {}; // Doesn't do a fetch request
    }
  }

  // Provide the mock implementation
  Container.current().provide(ApiService, new MockApiService());

  inject(TestableService).load(); // Doesn't do fetch request when calling api.get()
});

Scoped containers

Use scoped() to create a scoped child container and run (async) logic in it.

import { Container, inject } from '@woubuc/inject';

// Inject (and instantiate) an injectable in the global container
inject(MyService);

// Run some code in a local scoped container
await Container.current().scoped('temp-0', () => {
  // When we ask for the same class, the container will see that an instance
  // of it exists in the parent container (global) and return that instance.
  inject(MyService);

  // When we ask for a class that isn't instantiated yet, a new instance will
  // be created in the current container scope.
  inject(OtherService);
});

// Trying to inject the other service in the global container, even after
// injecting it successfully in the scoped container, will create a new instance
// because the global container has no idea what happened in the scoped container.
inject(OtherService);

Optional injectables

When using scoped() and/or provide(), sometimes you may have services that might or might not exist (yet). In that case you can use injectOptional() (or Container.current().tryGet()) to return undefined if no instance of the injectable exists yet - instead of constructing a new instance.

import { Container, injectOptional } from '@woubuc/inject';

const token = 'foo';

injectOptional(token); // undefined

Container.current().provide(token, 'hello world');

injectOptional(token); // 'hello world'

Destructors

If you need to run some cleanup logic when an instance is no longer needed (particularly useful when working with scoped containers), you can implement the OnDestroy interface.

import { Container, inject, injectable, type OnDestroy } from '@woubuc/inject';

@injectable()
class MyService implements OnDestroy {
  public constructor() {
    console.log('hello');
  }

  public onDestroy() {
    console.log('bye');
  }
}

await Container.current().scoped('test', () => {
  let service = inject(MyService); // "hello"

  // We're at the end of the scope so the container is deleted
  // and all local injectable instances are cleared.
  // "bye"
});