Await on Observable - Javascript

Asynchronous programming is an important aspect of modern web development. In Javascript, we often use Promises to handle asynchronous code. However, Observables provide us with a richer API to deal with asynchronous data.

In this article, we will explore how to use the 'await' keyword on an Observable in Javascript.

Background

Observables are a core part of the Reactive Extensions (Rx) library. They enable us to handle asynchronous events as a stream of values over time. An Observable can emit multiple values over time and subscribers can consume these values as they arrive.

Promises, on the other hand, represent a single value that may become available at some point in the future. They are used to handle asynchronous code by providing a way to chain multiple asynchronous operations together.

Basic Usage

In Javascript, Observables can be created using the RxJS library. To use Observables with the 'await' keyword, we need to convert the Observable to a Promise. Thankfully, the RxJS library provides a 'toPromise' method that does this for us.

For example, let's create an Observable that emits a single value after a delay of 1 second:

import { Observable } from 'rxjs';

const source$ = new Observable(observer => {
  setTimeout(() => {
    observer.next('Hello World!');
    observer.complete();
  }, 1000);
});

To convert this Observable to a Promise, we can call the 'toPromise' method:

const result = await source$.toPromise();
console.log(result); // prints 'Hello World!'

The 'toPromise' method returns a Promise that resolves with the last value emitted by the Observable. In this case, the Observable emits a single value 'Hello World!', which is then returned as the result of the Promise.

Error Handling

When using Promises, we can use the 'catch' method to handle any errors that occur during asynchronous operations. With Observables, we can use the 'catchError' method to achieve the same result.

For example, let's create an Observable that throws an error after a delay of 1 second:

const source$ = new Observable(observer => {
  setTimeout(() => {
    observer.error(new Error('Something went wrong!'));
  }, 1000);
});

To handle errors with the 'await' keyword, we need to wrap the 'toPromise' method call within a 'try...catch' block:

try {
  const result = await source$.toPromise();
  console.log(result); // this line will not be executed
} catch (err) {
  console.log(err.message); // prints 'Something went wrong!'
}

The 'catch' block will handle any errors that occur during the asynchronous operation. In this example, the error message 'Something went wrong!' is printed to the console.

Combining Observables

One of the strengths of Observables is their ability to combine streams of data using operators such as 'merge', 'zip', and 'concat'.

In order to use the 'await' keyword with Observables that have been combined with an operator, we need to convert the Observable to a Promise after the operation has completed.

For example, let's combine two Observables using the 'concat' operator:

const first$ = of('Hello');
const second$ = of('World!');

const result$ = concat(first$, second$);

To convert the combined Observable to a Promise, we can call the 'toPromise' method after the Observable has completed:

const result = await result$.toPromise();
console.log(result); // prints 'Hello World!'

The 'toPromise' method will wait for the Observable to complete before returning the final result.

Conclusion

In this article, we explored how to use the 'await' keyword on an Observable in Javascript. We learned how to convert an Observable to a Promise using the 'toPromise' method, how to handle errors with the 'try...catch' block, and how to combine Observables using operators. Observables provide a powerful API for dealing with asynchronous events, and the 'await' keyword allows us to consume this data in a way that is both concise and readable.