emit the emissions from two or more Observables without interleaving them
Concat waits to subscribe to each additional Observable that you pass to it until the previous Observable completes. Note that because of this, if you try to concatenate a “hot” Observable, that is, one that begins emitting items immediately and before it is subscribed to, Concat will not see, and therefore will not emit, any items that Observable emits before all previous Observables complete and Concat subscribes to the “hot” Observable.
In some ReactiveX implementations there is also a ConcatMap operator (a.k.a. concat_all, concat_map, concatMapObserver, for, forIn/for_in, mapcat, selectConcat, or selectConcatObserver) that transforms the items emitted by a source Observable into corresponding Observables and then concatenates the items emitted by each of these Observables in the order in which they are observed and transformed.
The StartWith operator is similar to Concat, but prepends, rather than appends, items or emissions of items to those emitted by a source Observable.
The Merge operator is also similar. It combines the emissions of two or more Observables, but may interleave them, whereas Concat never interleaves the emissions from multiple Observables.
RxJS implements this operator as concat and concatAll.
concat and concatAll are found in each of the following distributions:
RxKotlin concat concatWith
There is also an instance method, concatWith, such that Observable.concat(a,b) is equivalent to a.concatWith(b).
RxPHP concat concatAll
RxPY concat concatAll
Rxrb concat merge_all
RxScala concat ++
RxScala implements this operator in two ways. There is a concat operator that accepts an Observable of Observables as its parameter, and then concatenates each of these Observables in the order they are emitted. There is also a ++ operator that concatenates one Observable to another.