How to Get the currently selected cell/range?

Question

Is there a way to get the identification (i.e. A1 notation) of a selected cell or range in Google Sheets?

Something like:

=SELECTION(['Sheet1']) -> "D6"

Solution

This custom function will get the selection at the time you update the cell with the function. But it will not update as the selection changes.

Henrique provides a good explanation of why custom functions like this don't update in Google Apps - script to summarise data not updating.

/**
 * A function that gets the current selection, in A1Notation.
 *
 * @customfunction
 */
function SELECTED_RANGE() {
  return SpreadsheetApp.getActive().getActiveRange().getA1Notation();
}