inject
Injects a token from the currently active injector.
inject
is only supported in an injection context. It can
be used during:
- Construction (via the
constructor
) of a class being instantiated by the DI system, such as an@Injectable
or@Component
. - In the initializer for fields of such classes.
- In the factory function specified for
useFactory
of aProvider
or an@Injectable
. - In the
factory
function specified for anInjectionToken
. - In a stackframe of a function call in a DI context
inject<T>(token: HostAttributeToken | ProviderToken<T>, flags: InjectOptions | InjectFlags = InjectFlags.Default)
Parameters
token
|
HostAttributeToken | ProviderToken<T> |
A token that represents a dependency that should be injected. |
flags
|
InjectOptions | InjectFlags |
Optional flags that control how injection is executed.
The flags correspond to injection strategies that can be specified with
parameter decorators Optional. Default is |
Throws
Error
if called outside of a supported context.
Overloads
Parameters
Returns
Throws
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Deprecated prefer an options object instead of Parameters
Returns
Throws
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Parameters
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Parameters
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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();
}
}