take 1 vs first

Question

I found a few implementation of AuthGuards that use take 1   In my project  I used first     Do both work the same way   import  rxjs add operator map   import  rxjs add operator first   import   Observable   from  rxjs Observable    import   Injectable   from   angular core   import   CanActivate  Router  ActivatedRouteSnapshot  RouterStateSnapshot   from   angular router   import   AngularFire   from  angularfire2     Injectable   export class AuthGuard implements CanActivate        constructor private angularFire  AngularFire  private router  Router           canActivate route  ActivatedRouteSnapshot  state  RouterStateSnapshot   Observable lt boolean gt    boolean           return this angularFire auth map               auth    gt                     if  auth                        this router navigate    dashboard                         return false                    else                       return true                                            first       Just change this to  take 1

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Here are three Observables A  B  and C with marble diagrams to explore the difference between  first  take  and single operators       Legend  --o--  value ----   error ----   completion  Play with it at https   thinkrx io rxjs first-vs-take-vs-single     Already having all the answers  I wanted to add a more visual explanation  Hope it helps someone

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Tip  Only use first   if   You consider zero items emitted to be an error condition  eg  completing before emitting   AND if there   s a greater than 0  chance of error you handling it gracefully OR You know 100  that the source observable will emit 1  items  so can never throw    If there are zero emissions and you are not explicitly handling it  with catchError  then that error will get propagated up  possibly cause an unexpected problem somewhere else and can be quite tricky to track down - especially if it s coming from an end user  You re safer off using take 1  for the most part provided that   You re OK with take 1  not emitting anything if the source completes without an emission  You don t need to use an inline predicate  eg  first x   gt  x  gt  10     Note  You can use a predicate with take 1  like this   pipe  filter x   gt  x  gt  10   take 1     There is no error with this if nothing is ever greater than 10  What about single   If you want to be even stricter  and disallow two emissions you can use single   which errors if there are zero or 2  emissions  Again you d need to handle errors in that case  Tip  Single can occasionally be useful if you want to ensure your observable chain isn t doing extra work like calling an http service twice and emitting two observables  Adding single to the end of the pipe will let you know if you made such a mistake  I m using it in a  task runner  where you pass in a task observable that should only emit one value  so I pass the response through single    catchError   to guarantee good behavior   Why not always use first   instead of take 1    aka  How can first potentially cause more errors  If you have an observable that takes something from a service and then pipes it through first   you should be fine most of the time  But if someone comes along to disable the service for whatever reason - and changes it to emit of null  or NEVER then any downstream first   operators would start throwing errors  Now I realize that might be exactly what you want - hence why this is just a tip  The operator first appealed to me because it sounded slightly less  clumsy  than take 1  but you need to be careful about handling errors if there s ever a chance of the source not emitting  Will entirely depend on what you re doing though   If you have a default value  constant   Consider also  pipe defaultIfEmpty 42   first    if you have a default value that should be used if nothing is emitted  This would of course not raise an error because first would always receive a value  Note that defaultIfEmpty is only triggered if the stream is empty  not if the value of what is emitted is null

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Operators first   and take 1  aren t the same   The first   operator takes an optional predicate function and emits an error notification when no value matched when the source completed   For example this will emit an error   import   EMPTY  range   from  rxjs   import   first  take   from  rxjs operators    EMPTY pipe    first      subscribe console log  err   gt  console log  Error   err          as well as this   range 1  5  pipe    first val   gt  val  gt  6     subscribe console log  err   gt  console log  Error   err      While this will match the first value emitted   range 1  5  pipe    first      subscribe console log  err   gt  console log  Error   err      On the other hand take 1  just takes the first value and completes  No further logic is involved   range 1  5  pipe    take 1     subscribe console log  err   gt  console log  Error   err      Then with empty source Observable it won t emit any error   EMPTY pipe    take 1     subscribe console log  err   gt  console log  Error   err      Jan 2019  Updated for RxJS 6

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It turns out there s a very important distinction between the two methods  first   will emit an error if the stream completes before a value is emitted  Or  if you ve provided a predicate  i e  first value   gt  value      foo     it will emit an error if the stream completes before a value that passes the predicate is emitted  take 1   on the other hand  will happily carry on if a value is never emitted from the stream  Here s a simple example  const subject    new Subject        logs  quot no elements in sequence quot  when the subject completes subject  first   subscribe null   err    gt  console log err message        never does anything subject  take 1  subscribe console log    subject  complete     Another example  using a predicate  const observable    of 1  2  3       logs  quot no elements in sequence quot  when the observable completes observable    first  value    gt  value  gt  5    subscribe null   err    gt  console log err message        the above can also be written like this  and will never do    anything because the filter predicate will never return true observable    filter  value    gt  value  gt  5     take 1    subscribe console log    As a newcomer to RxJS  this behavior was very confusing to me  although it was my own fault because I made some incorrect assumptions  If I had bothered to check the docs  I would have seen that the behavior is clearly documented   Throws an error if defaultValue was not provided and a matching element is not found   The reason I ve run into this so frequently is a fairly common Angular 2 pattern where observables are cleaned up manually during the OnDestroy lifecycle hook  class MyComponent implements OnInit  OnDestroy     private stream   Subject   someDelayedStream      private destroy    new Subject       ngOnInit         this stream         takeUntil this destroy          first          subscribe doSomething          ngOnDestroy         this destroy  next true          The code looks harmless at first  but problems arise when the component in destroyed before stream  can emit a value  Because I m using first    an error is thrown when the component is destroyed  I m usually only subscribing to a stream to get a value that is to be used within the component  so I don t care if the component gets destroyed before the stream emits  Because of this  I ve started using take 1  in almost all places where I would have previously used first    filter fn  take 1  is a bit more verbose than first fn   but in most cases I prefer a little more verbosity over handling errors that ultimately have no impact on the application  Also important to note  The same applies for last   and takeLast 1   reference

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It seems that in RxJS 5 2 0 the  first   operator has a bug    Because of that bug  take 1  and  first   can behave quite different if you are using them with switchMap   With take 1  you will get behavior as expected    var x   Rx Observable interval 1000      do  x  gt  console log  One        take 1      switchMap x   gt  Rx Observable interval 1000       do  x  gt  console log  Two        subscribe  x    gt          In the console you will see     One    Two    Two    Two    Two    etc      But with  first   you will get wrong behavior   var x   Rx Observable interval 1000     do  x  gt  console log  One       first      switchMap x   gt  Rx Observable interval 1000      do  x  gt  console log  Two       subscribe  x    gt          In console you will see     One    One    Two    One    Two    One    etc       Here s a link to codepen

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There s one really important difference which is not mentioned anywhere    take 1  emits 1  completes  unsubscribes  first   emits 1  completes  but doesn t unsubscribe   It means that your upstream observable will still be hot after first   which is probably not expected behavior   UPD  This referes to RxJS 5 2 0  This issue might be already fixed

[angular] take(1) vs first()

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