error handling - catch-and-replace Rx Observable stream/sequence -

say have observable compromised of 2 remote ajax calls exposed observable's. these streams flatmaps of sponse sequence of values.

var remotedata = rx.observable.merge(   data1,   data2); 

for sake of argument imagine following definitions:

var data1 = rx.observable.from([1,2,3]);     // multiple items! var data2 = rx.observable.throw("whoops!"); 

i either get data1 , data2, fetched or if there any error use cached data.

however if write var dataorcache = remotedata.catch(cacheddata) append output stream adding cached data after error. because catch merely continues sequence different observable:

continues observable sequence terminated exception next observable sequence.

this problematic because error might result data2 (or later in data1 steam); yet cachedata should superseded , all results err'ing source.

for example, if cachedata = rx.observable.from(["cached"]) final sequence [1,2,3,"cached"] when desired goal ["cached"].

i've looked @ getting started: errors, unable how handle situation not 1 of listed scenarios. 'caching' situation shown assumes fail-fast observables error before returning any results.

i looking way handle via different stream composition or 1 of built-in operator methods , not want unwrap explicit observable source.

not sure understood :

• if want values cache when ajax call fails, can this:

var remotedata = rx.observable.zip( data1.catch(getfromcache('data1')), data2.catch(getfromcache('data2')) )

this uses instance version of catch operator, , signature rx.observable.prototype.catch(handler): cf. https://github.com/reactive-extensions/rxjs/blob/master/doc/api/core/operators/catchproto.md

your getfromcache function must return promise or observable. remotedata emit value when both data1 , data2 fetched, whether cache or ajax call.

• if want take values cache 1 of ajax call fails:

var remotedata = rx.observable.zip(data1, data2).catch(getdatafromcache('data1','data2'))

i haven't tested let me know if worked you.