@Inject decorator

TypeDI
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Usage
@Inject decorator
The @Inject() decorator is a property and parameter decorator used to resolve dependencies on a property of a class or a constructor parameter. By default it infers the type of the property or argument and initializes an instance of the detected type, however, this behavior can be overwritten via specifying a custom constructable type, Token, or named service as the first parameter.
Property injection
This decorator is mandatory on properties where a class instance is desired (aka: without the decorator, the property will stay undefined). The type of the property is automatically inferred so there is no need to define the desired value as the decorator parameter.
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import 'reflect-metadata';
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import { Container, Inject, Service } from 'typedi';
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​
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@Service()
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class InjectedExampleClass {
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  print() {
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    console.log('I am alive!');
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  }
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}
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​
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@Service()
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class ExampleClass {
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  @Inject()
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  withDecorator: InjectedExampleClass;
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​
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  withoutDecorator: InjectedExampleClass;
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}
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​
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const instance = Container.get(ExampleClass);
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​
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/**
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 * The `instance` variable is an ExampleClass instance with the `withDecorator`
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 * property containing an InjectedExampleClass instance and `withoutDecorator`
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 * property being undefined.
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 */
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console.log(instance);
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​
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instance.withDecorator.print();
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// prints "I am alive!" (InjectedExampleClass.print function)
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console.log(instance.withoutDecorator);
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// logs undefined, as this property was not marked with an @Inject decorator
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Constructor Injection
The @Inject decorator is not required in constructor injection when a class is marked with the @Service decorator. TypeDI will automatically infer and inject the correct class instances for every constructor argument. However, it can be used to overwrite the injected type.
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import 'reflect-metadata';
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import { Container, Inject, Service } from 'typedi';
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​
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@Service()
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class InjectedExampleClass {
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  print() {
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    console.log('I am alive!');
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  }
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}
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​
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@Service()
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class ExampleClass {
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  constructor(
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    @Inject()
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    public withDecorator: InjectedExampleClass,
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    public withoutDecorator: InjectedExampleClass
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  ) {}
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}
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​
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const instance = Container.get(ExampleClass);
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​
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/**
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 * The `instance` variable is an ExampleClass instance with both the
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 * `withDecorator` and `withoutDecorator` property containing an
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 * InjectedExampleClass instance.
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 */
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console.log(instance);
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​
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instance.withDecorator.print();
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// prints "I am alive!" (InjectedExampleClass.print function)
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instance.withoutDecorator.print();
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// prints "I am alive!" (InjectedExampleClass.print function)
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Explicitly requesting target type
By default, TypeDI will try to infer the type of property and arguments and inject the proper class instance. When this is possible (eg: the property type is an interface) there is three-way to overwrite type of the injected value:
via @Inject(() => type) where type is a constructable value (eg: a class definition)
via @Inject(myToken) where myToken is an instance of Token class
via @Inject(serviceName) where serviceName is a string which is already registered via Container.set(serviceName, value)
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import 'reflect-metadata';
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import { Container, Inject, Service } from 'typedi';
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​
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@Service()
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class InjectedExampleClass {
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  print() {
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    console.log('I am alive!');
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  }
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}
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​
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@Service()
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class BetterInjectedClass {
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  print() {
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    console.log('I am a different class!');
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  }
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}
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​
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@Service()
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class ExampleClass {
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  @Inject()
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  inferredPropertyInjection: InjectedExampleClass;
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​
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  /**
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   * We tell TypeDI that initialize the `BetterInjectedClass` class
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   * regardless of what is the inferred type.
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   */
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  @Inject(() => BetterInjectedClass)
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  explicitPropertyInjection: InjectedExampleClass;
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​
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  constructor(
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    public inferredArgumentInjection: InjectedExampleClass,
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    /**
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     * We tell TypeDI that initialize the `BetterInjectedClass` class
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     * regardless of what is the inferred type.
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     */
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    @Inject(() => BetterInjectedClass)
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    public explicitArgumentInjection: InjectedExampleClass
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  ) {}
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}
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​
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/**
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 * The `instance` variable is an ExampleClass instance with both the
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 *  - `inferredPropertyInjection` and `inferredArgumentInjection` property
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 *    containing an `InjectedExampleClass` instance
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 *  - `explicitPropertyInjection` and `explicitArgumentInjection` property
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 *    containing a `BetterInjectedClass` instance.
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 */
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const instance = Container.get(ExampleClass);
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​
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instance.inferredPropertyInjection.print();
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// prints "I am alive!" (InjectedExampleClass.print function)
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instance.explicitPropertyInjection.print();
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// prints "I am a different class!" (BetterInjectedClass.print function)
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instance.inferredArgumentInjection.print();
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// prints "I am alive!" (InjectedExampleClass.print function)
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instance.explicitArgumentInjection.print();
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// prints "I am a different class!" (BetterInjectedClass.print function)
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@Service decorator
Service Tokens
Last modified 3mo ago
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Contents
Property injection
Constructor Injection
Explicitly requesting target type