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Assuming there will always be only one key/value pair, the following piece of code should work.

It may not work this way because you're using concatMap. As you know, it waits for the inner observable to complete before starting to process(to subscribe) the pending ones. It internally uses a buffer, such that if an inner observable is still active(did not complete), the emitted value will be added to that buffer. When the inner observable becomes inactive, the oldest value from the buffer is selected and a new inner observable will be created, based on the provided callback function.

There is also delayWhen, which emits a complete notification after all of its pending observables complete:

It is a convention for an observable. It's not needed, but people have got into the habit of using them, like underscores for private variables.

See: https://stevenschwenke.de/DollarSignsInVariableNamesForObservablesInTypeScript

The original Angular style guide considered taking a stand on this issue, and end up not doing so. See Ward Bell's comment in the archival documentation: https://github.com/angular/angular.io/issues/2039

We do not recommend the $ suffix. It smacks of Hungarian notation. But neither do we wish to prohibit it. A deliberate silence is our course ... for now.