20150619仿微博提示剩余可输入字符

html=====

<textarea name="" id="txt" cols="30" rows="10"></textarea>
<div id=‘show‘>剩余字符150字</div>

js=======

var txt=document.getElementById(‘txt‘);
var show=document.getElementById(‘show‘);
txt.onkeyup=function () {
    show.innerHTML=‘剩余字符‘+(150-(this.value.length))+‘字‘
}