RxJS, understanding defer

Question

I searched for the usage of defer in RxJS but still I don't understand why and when to use it.

As I understand neither Observable methods is fired before someone subscribes to it.

If that's the case then why do we need to wrap an Observable method with defer?

An example

I'm still wondering why it wrapped Observable with defer? Does it make any difference?

var source = Rx.Observable.defer(function () {   return Rx.Observable.return(42); });  var subscription = source.subscribe(   function (x) { console.log('Next: ' + x); },   function (err) { console.log('Error: ' + err); },   function () { console.log('Completed'); } ); 

Answer 1

Quite simply, because Observables can encapsulate many different types of sources and those sources don't necessarily have to obey that interface. Some like Promises always attempt to eagerly compete.

Consider:

var promise = $.get('https://www.google.com'); 

The promise in this case is already executing before any handlers have been connected. If we want this to act more like an Observable then we need some way of deferring the creation of the promise until there is a subscription.

Hence we use defer to create a block that only gets executed when the resulting Observable is subscribed to.

Observable.defer(() => $.get('https://www.google.com')); 

The above will not create the Promise until the Observable gets subscribed to and will thus behaves much more in line with the standard Observable interface.