<!DOCTYPE html>
<html>
<head>
<title>137</title>
<meta charset="utf-8">
<script type="text/javascript">
window.onload=function()
{
var a,b,w;
a=document.getElementById("a");
b=document.getElementById("b");
a.onkeyup=function()
{
w=a.value;
var s=w.length;
b.innerHTML=s;
}
}
</script>
</head>
<body>
<input type="text" id="a" size="20">
字符的个数是 <span id="b">0</span>
</body>
</html>