限制文本框输入指定字数内容

<mce:script language="javascript"><!--
function checkLength(which) {
var maxChars = 20;
if (which.value.length > maxChars)
which.value = which.value.substring(0,maxChars);
var curr = maxChars - which.value.length;
document.getElementById("chLeft").innerHTML = curr.toString();
}

// --></mce:script>

<div>
<asp:TextBox ID="txtInput" runat="server" Width="200px" Height="60px" onkeyup="checkLength(this);" TextMode="MultiLine"></asp:TextBox>
<br />
<small>文字最大长度: 20. 还剩: <span id="chLeft">20</span>.</small>
</div>