Gtk.TextBuffer.prototype.get_selection_bounds

function get_selection_bounds():Boolean {
    // Gjs wrapper for gtk_text_buffer_get_selection_bounds()
}

Returns true if some text is selected; places the bounds of the selection in start and end (if the selection has length 0, then start and end are filled in with the same value). start and end will be in ascending order. If start and end are NULL, then they are not filled in, but the return value still indicates whether text is selected.