<script>
var OldValue = "";
function txt_Change(NewValue)
{
if (OldValue != NewValue)
{
alert(NewValue);
OldValue = NewValue;
}
}
</script>
<body">
<input type="text" onKeyUp="txt_Change(this.value)">
</body>
原文：http://www.cnblogs.com/gaocong/p/5066491.html