Dfyjkt

I've been wondering if it's safe to assume that after the using the tap operator, the side effect inside it has completed.

My use case is with ngrx.

 ...
tap(() => 
 this.store.dispatch(new SetValue("Hello World"));
 
}),
switchMap(() => this.store),
select(state => state.value),
tap(state => 
 if (state === undefined) 
 throw new Error("Couldn't find value");
 
)

SetValue is an class that implements ngrx

export class SetValue implements Action 
 readonly type = SET_VALUE;
 constructor(public payload: string) 

What I'm trying to implement is to set a value on the store and then check if it's effectively been set.

Can I assume the dispatch has completed after tap operator?

Answer

I used this on Angular Router guards to set initial state by the parameters on the url, so I ended up filtering to only continue when the store has new value

 ...
tap(() => this.store.dispatch(new SetValue("Hello World"))),
switchMap(() => this.store),
select(state => state.value),
filter(value => value === "Hello World"),
take(1)

Most operations in RxJS are synchronous so if this.store.dispatch(new SetValue("Hello World")) won't do any async task it probably will work as expected (it's still just a Subject under the hood).

However, you shouldn't rely on this behavior. NgRx might change it's internals and in general it's better not to rely on synchronicity/asynchronicity of RxJS operators (this has already happened in the past in eg. from() from RxJS 4 to RxJS 5).

If you want to make sure something has been set then change your new SetValue("Hello World") effect to emit another action after it's done doing what it needs to do.