Angular 6: The importance of access modifiers when injecting services

Question

Angular 6: The importance of access modifiers when injecting services

As I make my way through the tutorial, an interesting scenario has arisen.

I've noticed that when injecting a service into my component without specifying an access modifier, it results in an error as shown below. However, adding either "private" or "public" fixes the issue.

Is there no default scope in Angular if we forget to specify the access modifier?

export class UserDetailsComponent implements OnInit {

  name="";
  lastName="";
  constructor(userService : UserServiceService) { }

  ngOnInit() {
  }

  save(){
    this.userService.saveUser();

  }

}

Property 'userService' does not exist on type 'UserDetailsComponent'.

angular typescript angular6

Answer 1

Answer №1

In TypeScript, if you add an access modifier (private, protected, or public) or readonly before a constructor parameter, it automatically becomes a class property. This feature is known as constructor parameter properties.

On the other hand, if you don't use the prefix, the constructor parameter remains just that - a method parameter. You will then need to manually assign it to a declared class property within the constructor.

According to the information provided in the handbook:

TypeScript has a specific syntax for transforming a constructor parameter into a class property with the same name and value. These are referred to as parameter properties and are created by adding one of the visibility modifiers public, private, protected, or readonly before a constructor argument. The resulting field inherits those modifier(s):

Answer 2

In TypeScript, if you add an access modifier (private, protected, or public) or readonly before a constructor parameter, it automatically becomes a class property. This feature is known as constructor parameter properties.

On the other hand, if you don't use the prefix, the constructor parameter remains just that - a method parameter. You will then need to manually assign it to a declared class property within the constructor.

According to the information provided in the handbook:

TypeScript has a specific syntax for transforming a constructor parameter into a class property with the same name and value. These are referred to as parameter properties and are created by adding one of the visibility modifiers public, private, protected, or readonly before a constructor argument. The resulting field inherits those modifier(s):

Answer 3

Answer №2

It's clear that the answers given so far are informative and easy to understand. In simple terms, here's what's going on:

A constructor in any programming language is a function where parameters only exist within that function's scope. This means that a parameter passed into a constructor is not accessible throughout the entire class unless it's explicitly assigned to a class member (as shown in Samuel J Matthew's code snippet).

The access modifiers on the parameter tell Angular to create a class member with the same name as the parameter, set with the specified access modifier, and initialize it with an instance of the parameter type. Essentially, the parameter becomes a class property. As mentioned before, this concept is known as a parameter property, which is a fitting name for it.

Ken's provided documentation is incredibly useful and definitely worth checking out.

Answer 4

It's clear that the answers given so far are informative and easy to understand. In simple terms, here's what's going on:

A constructor in any programming language is a function where parameters only exist within that function's scope. This means that a parameter passed into a constructor is not accessible throughout the entire class unless it's explicitly assigned to a class member (as shown in Samuel J Matthew's code snippet).

The access modifiers on the parameter tell Angular to create a class member with the same name as the parameter, set with the specified access modifier, and initialize it with an instance of the parameter type. Essentially, the parameter becomes a class property. As mentioned before, this concept is known as a parameter property, which is a fitting name for it.

Ken's provided documentation is incredibly useful and definitely worth checking out.

Answer 5

Answer №3

Typescript offers a convenient shortcut for managing dependency injection patterns.

You have the option to either include this in your constructor:

this.userService = userService

And also declare a property:

userService: UserService;

Alternatively, you can simply add 'public' or 'private' before the constructor parameter and Typescript will handle the rest. However, be cautious when using a private property in your template as AOT may encounter issues.

Learn more

Answer 6

Typescript offers a convenient shortcut for managing dependency injection patterns.

You have the option to either include this in your constructor:

this.userService = userService

And also declare a property:

userService: UserService;

Alternatively, you can simply add 'public' or 'private' before the constructor parameter and Typescript will handle the rest. However, be cautious when using a private property in your template as AOT may encounter issues.

Learn more

Answer 7

Answer №4

Issues arise when you neglect to specify access modifiers for constructor parameters, causing them to only be accessible within the constructor function itself. To make these parameters available throughout the entire class, access modifiers need to be provided.

For instance:

export class UserDetailsComponent implements OnInit {
  name="";
  lastName="";
  constructor(userService : UserServiceService) {
    console.log(userService)
  }
  ngOnInit() {
  }
  save(){
    this.userService.saveUser();
  }
}

In this example, the userService is restricted to the constructor scope. Running a console.log(userService) within the constructor will result in output to the console.

If you prefer not to use access modifiers or wish to assign the parameter to another class variable, you can do so as shown below:

export class UserDetailsComponent implements OnInit {
  name="";
  lastName="";
  private _service
  constructor(userService : UserServiceService) { 
    this._service = userService;
  }
  ngOnInit() {
  }
  save(){
    this._service.saveUser();
  }
}

In the above code, I have reassigned the injected userService to a different class member named _service.

Answer 8

Issues arise when you neglect to specify access modifiers for constructor parameters, causing them to only be accessible within the constructor function itself. To make these parameters available throughout the entire class, access modifiers need to be provided.

For instance:

export class UserDetailsComponent implements OnInit {
  name="";
  lastName="";
  constructor(userService : UserServiceService) {
    console.log(userService)
  }
  ngOnInit() {
  }
  save(){
    this.userService.saveUser();
  }
}

In this example, the userService is restricted to the constructor scope. Running a console.log(userService) within the constructor will result in output to the console.

If you prefer not to use access modifiers or wish to assign the parameter to another class variable, you can do so as shown below:

export class UserDetailsComponent implements OnInit {
  name="";
  lastName="";
  private _service
  constructor(userService : UserServiceService) { 
    this._service = userService;
  }
  ngOnInit() {
  }
  save(){
    this._service.saveUser();
  }
}

In the above code, I have reassigned the injected userService to a different class member named _service.

Angular 6: The importance of access modifiers when injecting services

Answer №1

Answer №2

Answer №3

Answer №4

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