> 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/groupby.md).

# groupBy

Groups the elements of an observable sequence according to a specified key selector function and comparer and selects the resulting elements by using a specified function.

## Arguments

1. `keySelector` *(`Function`)*: A function to extract the key for each element.
2. `[elementSelector]` *(`Function`)*: A function to map each source element to an element in an observable group.
3. `[comparer]` *(`Any`)*: Used to compare objects. If not specified, the default comparer is used.

## Returns

*(`Observable`)*: A sequence of observable groups, each of which corresponds to a unique key value, containing all elements that share that same key value.

## Example