Injection

In most cases you may want to inject your dependencies inside a class. There are several ways to do this.

Constructor injection

Needle DI strongly recommends constructor injection, since it makes the dependencies of your class explicit, more type-safe, and allows for easier unit testing.

Instead of using container.get(token), you can use the inject(token) function here, so no reference to an actual container is needed.

import { inject, injectable } from "@needle-di/core";

@injectable()
class MyService {
  constructor(
    private fooService = inject(FooService),
    private barService = inject(BarService),
    //      ^? Type will be inferred as `BarService`
  ) {}

  // ...
}

TIP

If you don't know what a token is: consider it the unique reference for your binding. In this case, its just the class reference, but there are many more tokens possible.

Learn more about tokens.

NOTE

Needle DI uses default parameter values for constructor injection. This maximizes type-safety and removes the need for parameter decorators, which aren't yet standardized in ECMAScript.

Although experimental parameter decorators allow for static analysis, this design was chosen on purpose to reduce complexity and bundle size.

Initializer injection

Alternatively, you can also initialize your dependencies as (private) fields:

import { inject, injectable } from "@needle-di/core";

@injectable()
class MyService {
  private fooService = inject(FooService);
  private barService = inject(BarService);
  //      ^? Type will be inferred as `BarService`

  // ...
}

Although this is less verbose, this will not allow you to pass in those dependencies when you construct the cass manually, e.g. in unit tests.

About the inject() function

Note that the inject() function is only available in the "injection context":

• During construction of a class being instantiated by the DI container;
• In the initializer for fields of such classes;
• In a factory function specified for useFactory of a provider;
• In the factory function specified for an InjectionToken.

If you try to use this function outside this context, it will throw an error. This is because Needle DI needs a reference to a DI container when constructing services globally.