SEO-SPA: Pagination service

pagination-service.ts

import { Injectable, Renderer2, Inject, ElementRef } from "@angular/core";
import { ActivatedRoute } from "@angular/router";
import { DOCUMENT } from "@angular/platform-browser";

//This should be injected in the app component, and any component or service
//that is responsible for pagination
@Injectable()
export class PaginationService {
    private totalPages: number;
    private currentPageNumber: number;

    private baseUrl: string;

    private prevLinkRef: ElementRef;
    private nextLinkRef: ElementRef;

    constructor(private renderer: Renderer2, private activatedRoute: ActivatedRoute,
        @Inject(DOCUMENT) private document
    ) {
        this.activatedRoute.url.subscribe((url) => {
            this.baseUrl = '/' + url.join('/');
        });
    }

    public updatePagination(currentPageNumber: number, totalPages: number) {
        this.currentPageNumber = currentPageNumber;
        this.totalPages = totalPages;
        this.updateLinkTags();
    }

    private updateLinkTags() {
        if (this.totalPages > 1) {
            // We only add the next page if there is more than 1 page, and not on the last page
            if (this.currentPageNumber < this.totalPages - 1) {
                const linkTagNext = this.nextLinkRef ? this.nextLinkRef : this.renderer.createElement('link');
                this.renderer.setAttribute(linkTagNext, 'rel', 'next');
                this.renderer.setAttribute(linkTagNext, 'href', this.baseUrl + '?page=' + (this.currentPageNumber + 1));
                if (!this.nextLinkRef) {
                    this.nextLinkRef = linkTagNext;
                    this.renderer.appendChild(this.document.head, linkTagNext);
                }
            }
            else if (this.nextLinkRef) this.removeNextTag();
            // We only add the prev page if there is more than 1 page, and not on the first page
            if (this.currentPageNumber > 0) {
                const linkTagPrev = this.prevLinkRef ? this.prevLinkRef : this.renderer.createElement('link');
                this.renderer.setAttribute(linkTagPrev, 'rel', 'prev');
                this.renderer.setAttribute(linkTagPrev, 'href', this.baseUrl + '?page=' + (this.currentPageNumber - 1));
                if (!this.prevLinkRef) {
                    this.prevLinkRef = linkTagPrev;
                    this.renderer.appendChild(this.document.head, linkTagPrev);
                }
            }
            else if (this.prevLinkRef) {
                this.removePrevTag();
            }
        }
        else {
            this.removeLinkTags();
        }
    }

    private removeLinkTags() {
        if (this.prevLinkRef) this.removePrevTag();
        if (this.nextLinkRef) this.removeNextTag();
    }

    private removeNextTag() {
        this.renderer.removeChild(this.document.head, this.nextLinkRef);
        this.nextLinkRef = null;
    }

    private removePrevTag() {
        this.renderer.removeChild(this.document.head, this.prevLinkRef);
        this.prevLinkRef = null;
    }
}