Reusing observables

In the previous section, we saw how we can use operators to create new observables, but until this moment, we have never tried reusing an observable. To do this, I propose a change in the program we created in the previous section. We still want to print the string for every date where the seconds part is an even number. But now, let's also print the string a second has passed every second. To do this, we can use another Bacon.interval call to generate a new observable that emits an empty object every second. Once this is done, we can map each object in the observable to the string we want and finally subscribe to print, as you can see in the following code:

Bacon 
    .interval(1000) 
    .map( 
    ()=> 'a second has passed' 
) 
.onValue((str)=> console.log(str)); 

If you add the preceding code to the last program, you will see the following output:

    a second has passed
    The number in the second part of the date 2016-11-23T22:42:18.683Z
         is 18 which is as even number
    a second has passed
    a second has passed
    The number in the second part of the date 2016-11-23T22:42:20.696Z 
         is 20 which is as even number
    a second has passed
    a second has passed
    The number in the second part of the date 2016-11-23T22:42:22.702Z
         is 22 which is as even number  

This solution is okay, but we can reuse the first Bacon.interval(), storing it in a variable. This way, we don't have to keep repeating this code every time we want a source of events emitting every second, using a variable as follows:

var emitsEverySecondStream = Bacon.interval(1000); 

We can create better code by reusing this variable as follows:

var emitsEverySecondStream = Bacon.interval(1000); 
emitsEverySecondStream.map( 
    (i)=> new Date() 
) 
.filter( 
    (date)=>date.getSeconds() % 2 == 0 
) 
.map( 
    (date)=> 'The number in the second part of the date ' +  
    date.toISOString() + ' is ' + date.getSeconds() +  
' which is as even number' 
) 
.onValue((date)=>console.log(date)); 
emitsEverySecondStream.map( 
    ()=> 'a second has passed' 
) 
.onValue((str)=> console.log(str)); 

This code gives us the same kind of output as from the previous example.

Another important feature when reusing observables is the ability to add multiple subscribers to the same observable. You can see this happening in the following code:

var eventStream = Bacon 
        .interval(1000) 
        .map( 
        ()=> 'a second has passed' 
   ); 
eventStream 
    .onValue((str)=>console.log('Subscriber 1 prints => ' + str)); 
eventStream 
    .onValue((str)=>console.log('Subscriber 2 prints => ' + str)); 
eventStream 
    .onValue((str)=>console.log('Subscriber 3 prints => ' + str)); 

In this code, we added three subscriptions to the same EventStream. This will give us the following output:

    Subscriber 1 prints => a second has passed
    Subscriber 2 prints => a second has passed
    Subscriber 3 prints => a second has passed
    Subscriber 1 prints => a second has passed
    Subscriber 2 prints => a second has passed
    Subscriber 3 prints => a second has passed