判断文本框还有多少文字的插件

///判断文本框还有多少文字的插件
/// <author>jimmy,2015-11-12</author>
function leaveWord(option) {
var defaultOption = {
len: 400
};
var param = $.extend({}, defaultOption, option);
if (param.id.val().length > 0) {
var txt = param.id.val(),
len = txt.length;
var s = param.len - len;
option.leaveWord.text(s);
};
param.id.keyup(function (event) {
var txt = $(this).val(), len = txt.length;
if (len > param.len) {
$(this).val(txt.substring(0, param.len));
option.leaveWord.text(0);
} else {
var s = param.len - len;
option.leaveWord.text(s);
}
});
}