Optional injection

By default, when you try to inject something that isn't provided, Needle DI will throw an error.

Alternatively, you can use optional injection, by passing { optional: true }. Instead of throwing an error, it will now return the requested service, or undefined if not found:

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

class MyService {
  constructor(
    private fooService = inject(FooService),
    //      ^? Type will be inferred as `FooService`
    private barService = inject(BarService, { optional: true }),
    //      ^? Type will be inferred as `BarService | undefined`
  ) {}
}

Outside the injection context

When you construct an instance of MyService manually outside the injection context, and you don't pass any argument for an optional dependency, the inject() function will not throw an error, but gracefully return undefined instead:

class MyService {
    constructor(
        private barService = inject(BarService, { optional: true }),
    ) {}
}
const myService = new MyService(); // will cause no issues