获取被选中的内容

function getSelText() {
       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;
                return txt;
}