js如何实现点击标签文字，文字在文本框出现

<input id=txt type=text value=""/>

<span>点我1</span><span>点我2</span><span>点我3</span><span>点我4</span>

<script>

window.onload=function(){

    var ts=document.getElementsByTagName("span");

    for(var i=0;i<ts.length;i++){

        ts[i].onclick=function(){

            document.getElementById("txt").value+=this.innerHTML;

        }

    }

}

</script>