Using Router Events To Detect Back And Forward Browser Navigation In Angular 7.0.4

// Import the core angular services.
import { Component } from "@angular/core";
import { Event as NavigationEvent } from "@angular/router";
import { filter } from "rxjs/operators";
import { NavigationStart } from "@angular/router";
import { Router } from "@angular/router";

// ----------------------------------------------------------------------------------- //
// ----------------------------------------------------------------------------------- //

@Component({
	selector: "my-app",
	styleUrls: [ "./app.component.less" ],
	template: `
		<nav class="nav">
			<a routerLink="./section-a" class="nav__item">Section A</a>
			<a routerLink="./section-b" class="nav__item">Section B</a>
			<a routerLink="./section-c" class="nav__item">Section C</a>
		</nav>

		<router-outlet></router-outlet>
	`
})
export class AppComponent {

	// I initialize the app component.
	constructor( router: Router ) {

		router.events
			.pipe(
				// The "events" stream contains all the navigation events. For this demo,
				// though, we only care about the NavigationStart event as it contains
				// information about what initiated the navigation sequence.
				filter(
					( event: NavigationEvent ) => {

						return( event instanceof NavigationStart );

					}
				)
			)
			.subscribe(
				( event: NavigationStart ) => {

					console.group( "NavigationStart Event" );
					// Every navigation sequence is given a unique ID. Even "popstate"
					// navigations are really just "roll forward" navigations that get
					// a new, unique ID.
					console.log( "navigation id:", event.id );
					console.log( "route:", event.url );
					// The "navigationTrigger" will be one of:
					// --
					// - imperative (ie, user clicked a link).
					// - popstate (ie, browser controlled change such as Back button).
					// - hashchange
					// --
					// NOTE: I am not sure what triggers the "hashchange" type.
					console.log( "trigger:", event.navigationTrigger );

					// This "restoredState" property is defined when the navigation
					// event is triggered by a "popstate" event (ex, back / forward
					// buttons). It will contain the ID of the earlier navigation event
					// to which the browser is returning.
					// --
					// CAUTION: This ID may not be part of the current page rendering.
					// This value is pulled out of the browser; and, may exist across
					// page refreshes.
					if ( event.restoredState ) {

						console.warn(
							"restoring navigation id:",
							event.restoredState.navigationId
						);

					}

					console.groupEnd();

				}
			)
		;

	}

}

// Import the core angular services.
import { BrowserModule } from "@angular/platform-browser";
import { NgModule } from "@angular/core";
import { RouterModule } from "@angular/router";

// Import the application components and services.
import { AppComponent } from "./app.component";
import { SectionAComponent } from "./section-a.component";
import { SectionBComponent } from "./section-b.component";
import { SectionCComponent } from "./section-c.component";

// ----------------------------------------------------------------------------------- //
// ----------------------------------------------------------------------------------- //

@NgModule({
	imports: [
		BrowserModule,
		RouterModule.forRoot(
			[
				{
					path: "section-a",
					component: SectionAComponent
				},
				{
					path: "section-b",
					component: SectionBComponent
				},
				{
					path: "section-c",
					component: SectionCComponent
				}
			],
			{
				// Tell the router to use the hash instead of HTML5 pushstate.
				useHash: true,

				// These aren't necessary for this demo - they are just here to provide
				// a more natural experience and test harness.
				scrollPositionRestoration: "enabled",
				anchorScrolling: "enabled",
				enableTracing: false
			}
		)
	],
	declarations: [
		AppComponent,
		SectionAComponent,
		SectionBComponent,
		SectionCComponent
	],
	providers: [
		// CAUTION: We don't need to specify the LocationStrategy because we are setting
		// the "useHash" property in the Router module above (which will be setting the
		// strategy for us).
		// --
		// {
		// provide: LocationStrategy,
		// useClass: HashLocationStrategy
		// }
	],
	bootstrap: [
		AppComponent
	]
})
export class AppModule {
	// ...
}