rxjs - Difference between .unsubscribe to .take(1)

Question

I wonder, if there is any difference in performance between using .take(1) and .unsubscribe when unsubscribe is used right after the subscription:

var observable = Rx.Observable.interval(100);

First:

var subscription = observable.subscribe(function(value) {
   console.log(value);
}).unsubscribe();

Second:

var subscription = observable.take(1).subscribe(function(value) {
    console.log(value);
});

Any ideas of it makes any different regard the performance?

Answer 1

Each serves a different purpose so it's hard to compare them.

In general if you take this source:

const source = range(1,3);

... and consume it with subscribe() followed immediately by unsubscribe():

source.subscribe(
  console.log,
  undefined, 
  () => console.log('complete')
).unsubscribe();

... then all values from source are going to be emitted even though we called unsubscribe() right after subscribing. This is because the code is still strictly sequential (synchronous) and the source is a cold Observable.

1
2
3
complete

Btw, try adding delay(0) operator to make source.pipe(delay(0)).subscribe(...).unsubscribe(). This makes emitting values asynchronous using an actual setTimeout() call and for this reason unsubscribe() is called before any next handlers and is discarded immediately.

In other words unsubscribe() let's you stop receiving values anytime. Even when the source hasn't emitted any value (we never receive any complete notification).

Using take() operator limits the chain to only emit a specific number of values.

source.pipe(
  take(1),
)
.subscribe(
  console.log,
  undefined,
  () => console.log('complete')
);

This just emits a single value and completes:

1
complete

Even if you add .unsubscribe() the result would be the same.

See live demo: https://stackblitz.com/edit/rxjs-tbu5kb

So take() is an operator while unsubscribe() is a method on a Subscription object. These two things are often interchangeable but they never fully substitute each other.

Jan 2019: Updated for RxJS 6