RxJS: How to not subscribe to initial value and/or undefined?

rxjs replaysubject
behaviorsubject
rxjs subscribe once
behaviorsubject subscribe not working
rxjs-no-subject-value
behaviorsubject default value null
skip rxjs
behaviorsubject initial value angular

Being new to RxJS I often create a subject which holds values in the future, but is initially undefined. It can only be undefined the first time. I currently use a filter to skip undefined values, but this is quite cumbersome as I do it everywhere as I need only once. (Maybe I do something wrong here?) Can I somehow subscribe to mySubject only after it got its first value via onNext?

var mySubject = new Rx.BehaviorSubject(undefined);

mySubject.filter(function(value) {
  return value !== undefined;
}).subscribe(function(value) {
  // do something with the value
});

Use new Rx.ReplaySubject(1) instead of BehaviorSubject.

Get current value from Observable without subscribing (just want , . function stable. Returns an Observable that emits all items emitted by the source Observable that are distinct by comparison from the previous item. In above component code inside ngonInit() service call, this.cashierRiskProfiles inside the subscribe is undefined, but after I put breakpoint before the service callI have that variable available and defined. I have seen lot of people having this issue with component variables with this.variablename getting undefined inside subscribe.


As mentioned by Will you should be able to just skip the first value by using the skip operator:

var mySubject = new Rx.BehaviorSubject(undefined);

mySubject.pipe(skip(1)).subscribe(function(value) {
  // do something with the value
});

Understanding rxjs BehaviorSubject, ReplaySubject and AsyncSubject, Perhaps those first few aren't needed or you are subscribing to a Replay or BehaviorSubject and do not need to act on the initial values. Reach for skip if you are  RxJS: How to not subscribe to initial value and/or undefined? (3) mySubject.pipe (skipWhile (v => !v)); Being new to RxJS I often create a subject which holds values in the future, but is initially undefined.


mySubject.pipe( skipWhile( v => !v ) );

What is the difference between Subject and BehaviorSubject , getValue() always returns initial value if subscribed to an observable Observable from lib: value=undefined getValue()=undefined My subscribe method is the last thing that i will write even i try to not write subscribe in my code. cartant added a commit to cartant/rxjs that referenced this issue on Jan 25. In above component code inside ngonInit() service call, this.cashierRiskProfiles inside the subscribe is undefined, but after I put breakpoint before the service callI have that variable available and defined. I have seen lot of people having this issue with component variables with this.variablename gettting undefined inside subscribe.


For now I am using the filter operator, but I don't know if it's a good solution:

var mySubject = new Rx.BehaviorSubject().filter(x => !!x);

mySubject.subscribe(value => { /* will receive value from below */);

mySubject.next('value');

mySubject.subscribe(value => { /* also receives the value */ });

distinctUntilChanged, What makes RxJS powerful is its ability to produce values using pure With observable.subscribe , the given Observer is not registered as a listener in var subject = new Rx.BehaviorSubject(0); // 0 is the initial value subject.subscribe({ next: (v) small number of messages, RxJS uses no Scheduler, i.e. null or undefined . The value that it emits is not important, it's only important when the value gets emitted because that is what is going to trigger a retry attempt. Immediate Retry Console Output If we now execute this program, we are going to find the following output in the console:


Sometimes there will be a need for behaviourSubject, where the initial value does not matter and the current value is needed asynchronously while working inside a stream, Just in our case multiple chain promises are handled with user cancel while processing or during fetching the data from anywhere inside the stream.

This can be achieved using the following way.

// for user related commands
this.commandSource = new BehaviorSubject(CONTINUE);
// filtering over initial value which is continue to make it as a different pipe
const stopPipe = commandSource.pipe(filter(val => val === STOP));
const fetchStream = Observable.fromPromise(this.fetchDetails);

merge(fetchStream, stopPipe).pipe(
 take(1),
 takeWhile(() => commandSource.value === CONTINUE),
 concatMap((response) => {
  // fetch Another response you can return promise directly in concatMap
  // return array of response [1 ,2 ,3];
  return this.fetchYetAnotherDetails;
 }),
 // we can add this to stop stream in multiple places while processing the response
 takeWhile(() => commandSource.value === CONTINUE),
 // triggers parallelly values from the concatMap that is 1, 2 , 3
 mergeMap(() => // massage the response parallelly using )
 finalize(() => thi
  commandSource.complete())
).subscribe(res => {
 // handle each response 1, 2, 3 mapped
}, () => {
 // handle error
}, () => {
 // handle complete of the stream
});

// when user, clicks cancel, this should stop the stream.
commandSource.next(STOP)

skip, Is this possible in RxJS? How to do this on RxJS? Could use a replay subject with a value of 1 if that is the case (can't It doesn't require an initial value. For the time being, i am not creating, nor planning to use a SSO, my thinking is  Here Observable.empty() completes the Observable stream (and the subscribe is never reached), but you can use any Observable that completes, like Observable.of(null) But, Chris!! That code is so long!


BehaviorSubject.getValue() always returns initial value if subscribed , Empty observables can be dangerous when not understanding how they The subscribe operator in RxJS provides three callbacks: success, error and complete: of(undefined).subscribe( success => console.log('success!'), error combineLatest -> When all input variables have produced one value,  I have a service with a value of BehaviorSubject. I am accessing this value from my service in a TS page. I use this portion of code: Basically if SubjectVariable is not undefined I am adding this value to my local variable of SubjectVariableNew. I want to loop this until the SubjectVariable !== undefined for a timeout of 3 seconds.


RxJS Overview, Optional. Default is undefined . If a seed value is specified, then that value will be used as the initial value for the accumulator. If no seed value is specified, the first item of the source is used as the seed. const seed = 0; const count = ones.​pipe(reduce((acc, one) => acc + one, seed)); count.subscribe(x => console.log(x​));  predicate: Optional. Default is undefined.. An optional function called with each item to test for condition matching. defaultValue: Optional. Default is undefined.. The default value emitted in case no valid value was found on the source.


how to create an BehaviorSubject without an initial value? : Angular2, Default is undefined . The initial If a seed value is specified, then that value will be used as the initial value for the accumulator. If no seed value is specified, the first item of the source is used as the seed. const seed = 0; const count = ones.​pipe(scan((acc, one) => acc + one, seed)); count.subscribe(x => console.log(x));​  My subscribe method is the last thing that i will write even i try to not write subscribe in my code. That's a different scenario and mind set which doesn't belong to me and i will try to get the best from it.