How to use the "window" property in Angular?

Question

I'm trying to implement this "Scroll back to top" button found here:

https://www.w3schools.com/howto/howto_js_scroll_to_top.asp

---

I'm new to Angular and my attempts to implement this keep getting me errors and type errors.

This is my code:

home.component.html

<div class="country-card-container">
   <button (click)="topFunction()" class="scroll-top">TOP</button>
   <app-country-card *ngFor="let country of displayCountries" [country]="country" class="country-cards"></app-country-card>
</div>

home.component.ts

@Component({
  selector: 'app-home',
  templateUrl: './home.component.html',
  styleUrls: ['./home.component.sass']
})
export class HomeComponent {

    mybutton = document.getElementsByClassName("scroll-top");
    
    // When the user scrolls down 20px from the top of the document, show the button
    window.onscroll = this.scrollFunction();
    
    scrollFunction() {
      if (document.body.scrollTop > 20 || document.documentElement.scrollTop > 20) {
        this.mybutton.style.display = "block";
     } else {
        this.mybutton.style.display = "none";
     }
    }
    
    // When the user clicks on the button, scroll to the top of the document
    topFunction() {
      document.body.scrollTop = 0; // For Safari
     document.documentElement.scrollTop = 0; // For Chrome, Firefox, IE and Opera
    }
}

---

I'm getting the errors:

Unexpected keyword or identifier.ts(1434)

Member 'window' implicitly has an 'any' type.ts(7008)

Cannot find name 'scrollFunction'.ts(2304)

Property 'style' does not exist on type 'HTMLCollectionOf<Element>'.ts(2339)

---

I also tried putting

window.onscroll = this.scrollFunction();

in ngOnInit like this:

ngOnInit(){

    window.onscroll = this.scrollFunction();
}

but that doesn't work either.

---

How do I implement this? What did I do wrong and how do you fix it?

Answer 1

Well, it looks like the problem is that you try to declare a window property of your component by doing this:

export class HomeComponent {
  // this code is not valid, because you try to declare a class property here, not to get the window reference
  window.onscroll = this.scrollFunction();
}

Angular has a directive made specially for those purposes, called @HostListener . I would recommend you to consider it.

@Component({...})
export class HomeComponent {
  @HostListener('window:scroll', ['$event'])
  onWindowScroll(event: Event) {
    // do whatever you want here, including manipulations with the window object as it's available here
    console.log(window);
  }
}

Here is a reference to the official docs: https://angular.io/api/core/HostListener

Good luck:)