Javascript 選択文字の取得

function getText()
{
var txt = '';
if (window.getSelection)
{
txt = window.getSelection();
}
else if (document.getSelection)
{
txt = document.getSelection();
}
else if (document.selection)
{
txt = document.selection.createRange().text;
}
else return;
document.getElementById("msg").innerHTML = txt;
}