Both Promises and Observables provide us with abstractions that help us deal with the asynchronous nature of our applications. The difference between them was pointed out clearly by Günter and @Relu.

Since a code snippet is worth a thousand words, let’s go through the below example to understand them easier.

Thanks @Christoph Burgdorf for the awesome article

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

Suppose that you are building a search function that should instantly show you results as you type. It sounds familiar, but 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 parameters 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.

The demo will simply consist of two files: app.ts and wikipedia-service.ts. In a real world scenario, we would most likely split things further up, though.

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

wikipedia-service.ts

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

@Injectable()
export class WikipediaService {
  constructor(private jsonp: Jsonp) {}

  search (term: string) {
    var 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<Response> to a Promise<Response>. Eventually end up with a Promise<Array<string>> as the return type of our search method.

app.ts

// check the plnkr for the full list of imports
import {...} from '...';

@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<string>;

  constructor(private wikipediaService: WikipediaService) {}

  search(term) {
    this.wikipediaService.search(term)
                         .then(items => this.items = items);
  }
}

There is not much of a surprise here either. We inject our WikipediaService and expose its 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 WikipediaService 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.

See the example of Promise-based implementation on Plunker

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<string> 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.ts

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”.

<input type="text" [formControl]="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<string> as property valueChanges that we can subscribe to. Now that we have an Observable<string>, overcoming the user input is as easy as calling debounceTime(400) on our Observable. This will return a new Observable<string> that will only emit a new value when there haven’t been coming new values for 400 ms.

export class App {
  items: Array<string>;
  term = new FormControl();
  constructor(private wikipediaService: WikipediaService) {
    this.term.valueChanges
              .debounceTime(400)        // wait for 400 ms pause in events
              .distinctUntilChanged()   // ignore if next search term is same as previous
              .subscribe(term => this.wikipediaService.search(term).then(items => this.items = items));
  }
}

It would be a waste of resources to send out another request for a search term that our application 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)