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API

function inject<T>(token: ProviderToken<T>): T;
function inject<T>(
  token: ProviderToken<T>,
  options: InjectOptions & { optional?: false | undefined },
): T;
function inject<T>(token: ProviderToken<T>, options: InjectOptions): T | null;
function inject(token: HostAttributeToken): string;
function inject(
  token: HostAttributeToken,
  options: { optional: true },
): string | null;
function inject(
  token: HostAttributeToken,
  options: { optional: false },
): string;

@paramtokenProviderToken<T>

A token that represents a dependency that should be injected.

@returnsT

the injected value if operation is successful, null otherwise.

@paramtokenProviderToken<T>

A token that represents a dependency that should be injected.

@paramoptionsInjectOptions & { optional?: false | undefined; }

Control how injection is executed. Options correspond to injection strategies that can be specified with parameter decorators @Host, @Self, @SkipSelf, and @Optional.

@returnsT

the injected value if operation is successful.

@paramtokenProviderToken<T>

A token that represents a dependency that should be injected.

@paramoptionsInjectOptions

Control how injection is executed. Options correspond to injection strategies that can be specified with parameter decorators @Host, @Self, @SkipSelf, and @Optional.

@returnsT | null

the injected value if operation is successful, null if the token is not found and optional injection has been requested.

@paramtokenHostAttributeToken

A token that represents a static attribute on the host node that should be injected.

@returnsstring

Value of the attribute if it exists.

@paramtokenHostAttributeToken

A token that represents a static attribute on the host node that should be injected.

@paramoptions{ optional: true; }

@returnsstring | null

Value of the attribute if it exists, otherwise null.

@paramtokenHostAttributeToken

A token that represents a static attribute on the host node that should be injected.

@paramoptions{ optional: false; }

@returnsstring

Value of the attribute if it exists.

Usage Notes

In practice the inject() calls are allowed in a constructor, a constructor parameter and a field initializer:

@Injectable({providedIn: 'root'})
export class Car {
  radio: Radio|undefined;
  // OK: field initializer
  spareTyre = inject(Tyre);

  constructor() {
    // OK: constructor body
    this.radio = inject(Radio);
  }
}

It is also legal to call inject from a provider's factory:

providers: [
  {provide: Car, useFactory: () => {
    // OK: a class factory
    const engine = inject(Engine);
    return new Car(engine);
  }}
]

Calls to the inject() function outside of the class creation context will result in error. Most notably, calls to inject() are disallowed after a class instance was created, in methods (including lifecycle hooks):

@Component({ ... })
export class CarComponent {
  ngOnInit() {
    // ERROR: too late, the component instance was already created
    const engine = inject(Engine);
    engine.start();
  }
}

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