如何用javascript计算文本框还能输入多少个字符

//输入计数

//count：能?入的????量
  function Calculation(v, count) {
    var span = $(v).next();
    var valLength = $(v).val().length;
    if (valLength > count) {
      $(v).val($(v).val().substr(0, count));
      valLength = count;
    }
    span.text("您已?入" + valLength + "字元,?剩下" + (count - valLength) + "字元");
  }