问请教 getSelection()选中的问题？

//html

<textarea id="aa">1,2</textarea>

<textarea id="bb">3,4</textarea>

<p id="showConut"></p>

//script

var showConut = document.getElementById("showConut");

function aa(){

var selection = window.getSelection().toString();

var n=(selection.split(',')).length-1;

showConut.innerText = n;

};

document.addEventListener("selectionchange", aa);

我想要达到的结果:只在id=aa下选取统计结果