Both  Promises  and  Observables  provide us with abstractions that help us deal with the  asynchronous  nature of our applications.Often Observable is preferred over Promise because it provides the features of Promise and more. With Observable it doesn't matter if you want to handle 0, 1, or multiple events. You can utilize the same API in each case.

Promise

A  Promise handles a  single event  when an async operation completes or fails.Note: There are Promise libraries out there that support cancellation, but ES6 Promise doesn't so far.

Observable

An Observable is like a Stream  (in many languages) and allows to pass zero or more events where the callback is called for each event. Observable also has the advantage over Promise to be cancelable. If the result of an HTTP request to a server or some other expensive async operation isn't needed anymore, the Subscription of an Observable allows to cancel the subscription, while a Promise will eventually call the success or failed callback even when you don't need the notification or the result it provides anymore.

Observable provides operators like map, forEach, reduce, ... similar to an array

There are also powerful operators like retry(), or replay(), ... that are often quite handy.

Lets take a look at a use case scenario. We'll be building a search functionality. We are going to use just two files; search.service.ts and search.component.ts

Angular uses Rx.js Observables instead of promises for dealing with HTTP.

Suppose that you are building a search function that should instantly show results as you type, there are a lot of challenges that come with that task.

• We don't want to hit the server endpoint every time user presses a key, it should flood them with a storm of HTTP requests. Basically, we only want to hit it once the user has stopped typing instead of with every keystroke.
• Don’t hit the search endpoint with the same query params for subsequent requests.
• Deal with out-of-order responses. When we have multiple requests in-flight at the same time we must account for cases where they come back in unexpected order. Imagine we first type computer, stop, a request goes out, we type car, stop, a request goes out. Now we have two requests in-flight. Unfortunately, the request that carries the results for computer comes back after the request that carries the results for car.

Below is Promise-based implementation that doesn’t handle any of the described edge cases.

search-service.ts

//javascript
import { Injectable } from '@angular/core';
import { URLSearchParams, Jsonp } from '@angular/http';

@Injectable()
export class SearchService {
  constructor(private jsonp: Jsonp) {}
search (term: string) {
    const search = new URLSearchParams()
    search.set('action', 'opensearch');
    search.set('search', term);
    search.set('format', 'json');
    return this.jsonp
                .get('http://en.wikipedia.org/w/api.php?callback=JSONP_CALLBACK', { search })
                .toPromise()
                .then((response) => response.json()[1]);
  }
}

We are injecting the Jsonp service to make a GET request against the Wikipedia API with a given search term. Notice that we call toPromise in order to get from an Observable to a Promise. Eventually end up with a Promise> as the return type of our search method.

search.component.ts

//javascript
import { Component } from '@angular/core';
import { SearchService } from './search.service';
@Component({
  selector: 'my-app',
  template: <div>
      <h2>Wikipedia Search</h2>
      <input #term type="text" (keyup)="search(term.value)">
      <ul>
        <li *ngFor="let item of items">{{item}}</li>
      </ul>
    </div>
})
export class AppComponent {
  items: Array;

constructor(private searchService: SearchService) {}
search(term) {
    this.searchService.search(term)
                         .then(items => this.items = items);
  }
}

We inject our SearchService and expose it’s functionality via a search method to the template. The template simply binds to keyup and calls search(term.value).

We unwrap the result of the Promise that the search method of the SearchService returns and expose it as a simple Array of strings to the template so that we can have *ngFor loop through it and build up a list for us.

Where Observables really shine

Let’s change our code to not hammer the endpoint with every keystroke but instead only send a request when the user stopped typing for 400 ms

To unveil such super powers we first need to get an Observable that carries the search term that the user types in. Instead of manually binding to the keyup event, we can take advantage of Angular’s formControl directive. To use this directive, we first need to import the ReactiveFormsModule into our application module.

app.module.ts

//javascript
import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { JsonpModule } from '@angular/http';
import { ReactiveFormsModule } from '@angular/forms';

@NgModule({
  imports: [BrowserModule, JsonpModule, ReactiveFormsModule]
  declarations: [AppComponent],
  bootstrap: [AppComponent]
})
export class AppModule {}

Once imported, we can use formControl from within our template and set it to the name "term".

In our component, we create an instance of FormControl from @angular/form and expose it as a field under the name term on our component.

Behind the scenes, term automatically exposes an Observable as property valueChangesthat we can subscribe to. Now that we have an Observable, overcoming the user input is as easy as calling debounceTime(400) on our Observable. This will return a new Observablethat will only emit a new value when there haven’t been coming new values for 400ms.

//javascript
import { Component } from '@angular/core';
import { SearchService } from './search.service';
import { FormControl } from '@angular/forms';
@Component({
  selector: 'app-search',
  template: <div>
      <h2>Wikipedia Search</h2>
      <input type="text" [formControl]="term">
      <ul>
        <li *ngFor="let item of items | async">{{item}}</li>
      </ul>
    </div>
})
export class AppSearchComponent {

items: Observable>;
  term = new FormControl();
constructor(private searchService: SearchService) {}
ngOnInit() {
    this.items = this.term.valueChanges
                 .debounceTime(400)
                 .distinctUntilChanged()
                 .switchMap(term => this.searchService.search(term));
  }
}

It would've been a waste of resources to send out another request for a search term that our app already shows the results for. All we have to do to achieve the desired behavior is to call the distinctUntilChanged operator right after we called debounceTime(400).