> For the complete documentation index, see [llms.txt](https://soufatn.gitbook.io/rxjs-book/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://soufatn.gitbook.io/rxjs-book/summary/observable/observable_instance_methods/concatmapobserver.md).

# concatMapObserver | selectConcatObserver

Projects each notification of an observable sequence to an observable sequence and concats the resulting observable sequences into one observable sequence.

## Arguments

1. `onNext` *(`Function`)*:  A transform function to apply to each element. The selector is called with the following information:
   1. the value of the element
   2. the index of the element
2. `onError` *(`Function`)*: A transform function to apply when an error occurs in the source sequence.
3. `onCompleted` *(`Function`)*: A transform function to apply when the end of the source sequence is reached.
4. `[thisArg]` *(`Any`)*: Object to use as `this` when executing the transform functions.

## Returns

*(`Observable`)*: An observable sequence whose elements are the result of invoking the one-to-many transform function corresponding to each notification in the input sequence.

## Example