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I do not understand from the docs. Could anyone explain it to me?

Indraraj26
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    Please be more specific about what you do and to not understand from that document. – jonrsharpe Jan 21 '19 at 12:02
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    `tap` handles `(next), (error), (complete)` events, so you can perform actions. https://www.learnrxjs.io/operators/utility/do.html If that doesn't help you, as **johnrsharpe** said, please, be more specific. – Florian Jan 21 '19 at 12:14
  • so basically we use it for console.log, can you give me more example other than console.log – Indraraj26 Jan 21 '19 at 12:38
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    if you need to maintain a side effect that is tied to the *emission* of the observable but not directly tied to its result. For example, you could start an animation. Or you could have a separate variable that tracks the number of times an observable emitted a value. There's no way to give an "example", it's an utility function that act as a middle-callback inside an Observable's pipeline. – Badashi Jan 22 '19 at 00:41

3 Answers3

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Most of the operators are working in streamed sequence, for example: 

source$.pipe(
  map((a: string) => changeAndReturnArray(a)),
  filter((b: string[]) => giveMeOnlySymbolsThatAreAfterNInAlphabet(b)),
  switchMap((c: string[]) => putToSomeObservable(c))
  ....
);

In that example you are not 'breaking' the stream, or jumping outside of it to do some external action. Jumping outside of stream is possible with 'tap' operator, where you can:

  • call functions that will cause some side effect, that might be visible to end user (for example - display dialog, show snackbar, redirect to different route (but in my opinion it's not recommended to use tap in that way))
  • dispatch actions for store (if you are using any - for example ngrx store)
  • debug you're code -> console.log()
  • anything what can be considered as 'side effect' for your stream.

My personal opinion - use 'tap' only if you can't find any better solution. Jumping outside of stream and calling some side effect can be double edged sword, especially when your dealing with some bigger application. Side effect are always harder to maintain, and you can finish with application that is doing magic stuff without any reason.

Pawel Kiszka
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You can use it to perform a side effect for example. Or you can use it to see what's the current value that is being passed around without affecting/modifying the Observable. So something like a console.log() but inside the stream.

Dzhavat Ushev
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    so basically we use it for console.log, can you give me more example other than console.log – Indraraj26 Jan 21 '19 at 12:36
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    If you use state management like NGXS then you'll most likely use `tap` to set/update the store. Here's an example (https://ngxs.gitbook.io/ngxs/concepts/state#async-actions) (take a look at the code and read the text afterwards). – Dzhavat Ushev Jan 21 '19 at 12:42
  • Glad I could help. Can you mark one of the answers as accepted by clicking on the checkmark on the left? That way people will know you've figured out your problem. – Dzhavat Ushev Jan 22 '19 at 07:59
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Decalration

public tap(nextOrObserver: Observer | function, error: function, complete: function): Observable

tap is replacement of do operator which returns observable identical to your source observable. for each value emitted, perform a side-effect. it has 3 optional parameters.

  • nextOrObserver: A normal Observable object to perform side effect.
  • error: Callback for errors in source Observable.
  • complete: Callback for completion of the source.

Recommended for debugging purpose.

Vinay
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