Parameter decorator to be used on constructor parameters, which marks the parameter as being an optional dependency. The DI framework provides null if the dependency is not found.

Can be used together with other parameter decorators that modify how dependency injection operates.

The following code allows the possibility of a null result:

class Engine {}

        @Injectable()
        class Car {
          constructor(@Optional() public engine: Engine) {}
        }

        const injector = Injector.create({
          providers: [{provide: Car, deps: [[new Optional(), Engine]]}],
        });
        expect(injector.get(Car).engine).toBeNull();