# 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


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