Check out example codes for "angular best practices unsubscribe". It will help you in understanding the concepts better.

Code Example 1

/* Best practice is to create a base class and move gererik handling to it */

import { Subscription, Observable } from 'rxjs';
import { OnDestroy } from '@angular/core';


export abstract class ComponentBase implements OnDestroy {

  set subs(subscription: Subscription) {
    this.subscriptioions.add(subscription);
  }

  private subscriptioions = new Subscription();

  constructor() {};

  subscribe(service: Observable<any>, successFn: (value: any) => void, errorHandler?: (value: any) => void) {
    this.subs = service.subscribe(successFn, errorHandler || this.errorHandler);
  }

  ngOnDestroy() {
    this.subscriptioions.unsubscribe();
  }

  private errorHandler(error) {
    // TODO: generic error handling
  }
}

/* implementation in a component */
// No need to explicitly subscribe or unsubscribe on destroy

const product$ = this.productService.getProducts(productId);
this.subscribe(product$, (product) => {
	// TODO: use product
});

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