Q: Angular RxJS - Subscribe to Observable only IF subscribing to another Observable gave a certain result I need some kind of special RxJS syntax if it exists for this scenario:
    this.electronicInvoiceService.getElectronicInvoice(this.invoiceId)
            .subscribe(and here I can get .isApproved, for example: 
        (data) => data.isApproved 
        and what I want is IF data.isApproved is false only then 
        I want to subscribe to this 
        next observable I wrote below this);

do this next subscribe only if isApprove is false
  this.electronicInvoiceStagingService.getElectronicInvoice(this.invoiceId).subscribe();

What I tried already: I tried to have some variable in in my ts file and inside subscribe I did this.myVariableName == data.isApproved but this did not work I think because subscribing takes some time and it was bad logic.
[SOLUTION]: The comment below solved it, its silly of me I didnt use it right away (: solution> Why is a simple if statement not sufficient? (data) => {if (!data.isApproved) this.electronicInvoiceService.getElectronicInvoice(this.invoiceId).subscribe();}

A: Subscriptions inside other subscriptions are not considered best practices in RxJs.
I think that a more idiomatic solution could be the following
this.electronicInvoiceService.getElectronicInvoice(this.invoiceId)
// here you start a pipe, i.e. a series of operations that transform the source stream
.pipe(
   // concatMap is the first operator which basically says: wait for upstream
   // to notify and then take the data notified and return a new Observable
   concatMap( data => {
      // here you put your if logic
      if (!data.isApproved) {
        // return the other Observable
        return this.electronicInvoiceStagingService.getElectronicInvoice(this.invoiceId)
      }
      // otherwise return EMPY, which is an Observable that does nothing and just
      // completes immediately
      return EMPTY
   })
)
// eventually you just subscribe to the one and only one stream you have created
// transforming, with the pipe, the original source stream
.subscribe(dataFromStagingService => {
   // do anything you need to do with the data returned by the staging service call
})

