JS如何获取input中被选取的字符?

IE下:

<script>
function setValue(obj){
    var rng = document.selection.createRange();   
    document.getElementById("txt1").value=rng.htmlText; 
}
</script>
<input type="text" value="set" nselect="setValue(this);">
<input type="text" id="txt1">

FireFox下:

function setValue(obj){
    var start = obj.selectionStart;
    var end = obj.selectionEnd;
    document.getElementById("txt1").value=obj.value.substring(start,ed);
}