Angular - Struggling to retrieve data from parent component

Question

Angular - Struggling to retrieve data from parent component

Passing a function as parameter from parent to child component has resulted in the trigger of the parent component's function when a click event occurs. However, all properties of the parent component are undefined in this scenario. For instance,

Parent Component

export class AppComponent implements OnInit {
    constructor( private notificationService: NotificationService ) {}

    unreadNotification(): Observable<any> {
        // here this.notificationService is undefined
        console.log( this.notificationService );
    }
}

Parent html

<notification-menu [unread]= "unreadNotification"></notification-menu>

child Component

export class NotificationMenuComponent implements OnInit {
    @Input() updateUnread: Function;
}

child html

<button type="button" class="icon-button" (click)="updateUnread()">
</button>

When clicking on the notification button, unreadNotification is triggered, but the value of this.notificationService in the console.log remains undefined.

How can this issue be resolved?

javascript angular typescript

Answer 1

Answer №1

It is recommended to utilize @Input() in order to transfer data from the parent component to the child component, and use @Output() to send data from the child component back to the parent component.

Child HTML:

<button type="button" class="icon-button" (click)="update()">
</button>

Child Component:

export class NotificationMenuComponent implements OnInit {
    @Output() updateUnread = new EventEmitter<string>();

    update() {
        this.updateUnread.emit("I am working man!");
    }
}

Parent HTML:

<notification-menu (updateUnread)= "unreadNotification($event)"></notification-menu>

Parent Component:

export class AppComponent implements OnInit {
    constructor(private notificationService: NotificationService) {}

    unreadNotification(dataFromChild: string) {
        console.log(dataFromChild);
    }
}

Answer 2

It is recommended to utilize @Input() in order to transfer data from the parent component to the child component, and use @Output() to send data from the child component back to the parent component.

Child HTML:

<button type="button" class="icon-button" (click)="update()">
</button>

Child Component:

export class NotificationMenuComponent implements OnInit {
    @Output() updateUnread = new EventEmitter<string>();

    update() {
        this.updateUnread.emit("I am working man!");
    }
}

Parent HTML:

<notification-menu (updateUnread)= "unreadNotification($event)"></notification-menu>

Parent Component:

export class AppComponent implements OnInit {
    constructor(private notificationService: NotificationService) {}

    unreadNotification(dataFromChild: string) {
        console.log(dataFromChild);
    }
}

Answer 3

Answer №2

Great advice given by @nimeresam on using an @Output for this purpose.

It's important to understand that the reason your initial solution doesn't work is due to how JavaScript handles the this context.

When you write (click)="updateUnread()", it is essentially saying this.updateUnread() with "this" referring to NotificationMenuComponent. Since notificationService isn't part of NotificationMenuComponent, you encounter the undefined error.

To make sure the parent component's context is utilized, you must bind the context to the updateUnread function before passing it into the child component.

This can be accomplished by either converting the function into an arrow function or utilizing Function.bind.

Refer to these resources:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_objects/Function/bind

Enabling the TypeScript option --noImplicitThis is usually recommended to help detect such errors (although its effectiveness in this scenario is uncertain).

Answer 4

Great advice given by @nimeresam on using an @Output for this purpose.

It's important to understand that the reason your initial solution doesn't work is due to how JavaScript handles the this context.

When you write (click)="updateUnread()", it is essentially saying this.updateUnread() with "this" referring to NotificationMenuComponent. Since notificationService isn't part of NotificationMenuComponent, you encounter the undefined error.

To make sure the parent component's context is utilized, you must bind the context to the updateUnread function before passing it into the child component.

This can be accomplished by either converting the function into an arrow function or utilizing Function.bind.

Refer to these resources:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_objects/Function/bind

Enabling the TypeScript option --noImplicitThis is usually recommended to help detect such errors (although its effectiveness in this scenario is uncertain).

Answer 5

Answer №3

To utilize the parent component's information, consider using an arrow function. Here is an example of how you can implement it:

updateUnreadNotification = () => {
   // Using arrow function allows access to notificationService information
   console.log( this.notificationService );
}

Hopefully this solution helps resolve your issue.

Answer 6

To utilize the parent component's information, consider using an arrow function. Here is an example of how you can implement it:

updateUnreadNotification = () => {
   // Using arrow function allows access to notificationService information
   console.log( this.notificationService );
}

Hopefully this solution helps resolve your issue.

Angular - Struggling to retrieve data from parent component

Answer №1

Answer №2

Answer №3

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