The Selection.isCollapsed read-only property returns a boolean value which indicates whether or not there is currently any text selected. No text is selected when the selection's start and end points are at the same position in the content.
Keep in mind that a collapsed selection may still have one (or more, in Gecko) Ranges, so Selection.rangeCount may not be zero. In that scenario, calling a Selection object's getRangeAt() method may return a Range object which is collapsed.