Live example here

Are observables synchronous or asynchronous?

It depends on how they are built.

If you look over RxJS dev docs you'll find this:

Observables are lazy Push collections of multiple values.

I've seen that it's a lot of confusion about this on the internet as a lot of people assume that they are asynchronous because observables are presented as a new way of handling asynchronous code. Let me further explain why I say that it depends by using two examples with a custom rxjs operator:

RxJS operator:
In RxJS an operator is just a function that accepts as
input and observable and further returns a new Observable.

1.Synchronous observable:

const simpleBlockingOperator = (noOfLoops: number) => <T>(source: Observable<T>): Observable<T> =>{
  return new Observable(observable => {
    source.subscribe({
      next: no =>{
        let loops = 0;
        while(loops !== noOfLoops) {
          loops++;
        }
        console.log("Done loooping" + loops, " ", noOfLoops)
        return observable.next(no)
      },
      error: error => observable.error(error),
      complete: () => observable.complete()
    })
  })
};

In the above example we've created an rxjs operator which will loop as many times until we pass noOfLoops. Because our
code is synchronous will result in a blocking operation and trying to click in other parts of the page will not be possible as the main thread is blocked with our process of data.

2.Asynchronous observable:

const simpleNonBlockingOperator = (noOfLoops: number) => <T>(source: Observable<T>): Observable<T> =>{
  return new Observable(observable => {
    source.subscribe({
      next: no =>{
        setTimeout(() => {
          let loops = 0;
          while(loops !== noOfLoops) {
            loops++;
          }
          console.log("Done loooping" + loops, " ", noOfLoops)
          return observable.next(no)
        }, 0);
      },
      error: error => observable.error(error),
      complete: () => observable.complete()
    })
  })
};

In our async operator we've used setTimeout, this setTimeout will assure that the main thread will not get blocked because setTimeout
is a browser API and our code will be moved to the event loop.

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Conclusion

As you have already seen the observables are not by default async and it strongly depends on how they are written.

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