how to use the publishlast function from the rxjs library in javascript

The publishLast operator from the rxjs library is used to multicast the last emitted value of an observable to multiple subscribers. It returns a new observable that shares a single subscription to the source observable, and when the source completes, it emits only the last value to all subscribers.

Here's an example of how to use publishLast:

index.tsx
const { from } = rxjs;
const { publishLast, tap } = rxjs.operators;

// create an observable
const source$ = from([1, 2, 3]);

// apply the publishLast operator
const shared$ = source$.pipe(
  tap(val => console.log(`source emitted: ${val}`)),
  publishLast()
);

// subscribe to the shared observable
shared$.subscribe(val => console.log(`subscriber 1 received: ${val}`));
shared$.subscribe(val => console.log(`subscriber 2 received: ${val}`));

// connect to the source observable
shared$.connect();
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In the example above, we create an observable source$ that emits three values. We then use the tap operator to log each value emitted by the source observable. Next, we apply the publishLast operator to source$ to create a new observable shared$.

We then subscribe twice to shared$. When connect is called on the shared$ observable, it subscribes to source$ and begins emitting values. Since publishLast buffers the last value emitted from source$, both subscribers receive the last emitted value, which is 3.

Note that connect is used to initiate the subscription to source$ and activate the shared$ observable. Without it, nothing would happen because shared$ would not be connected to the source.