如何获得鼠标选中的值

<!DOCTYPE html>
<html>
    <head></head>
    <body>
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    <script>
        var funcGetSelectText = function(){
            var txt = '';
            if(document.selection){
                txt = document.selection.createRange().text;//ie
            }else{
                txt = document.getSelection();
            }
            return txt.toString();
        }
        var container = container || document;
        container.onmouseup = function(){
            var txt = funcGetSelectText();
            if(txt)
            {
                alert(txt);
            }
        }
    </script>
    </body>
</html>