如何用JS 限制text框里面最多为 10个字母或5个汉字，即10字节

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<input type="text"   name="explain"  id="explain" onkeyup="limitLength(value,10,'语义说明','explain')" ><script>function limitLength(value, byteLength, title, attribute) {        var newvalue = value.replace(/[^\x00-\xff]/g, "**");        var length = newvalue.length;          //当填写的字节数小于设置的字节数       if (length * 1 <=byteLength * 1){             return;       }       var limitDate = newvalue.substr(0, byteLength);       var count = 0;       var limitvalue = "";      for (var i = 0; i < limitDate.length; i++) {              var flat = limitDate.substr(i, 1);             if (flat == "*") {                   count++;             }      }      var size = 0;      var istar = newvalue.substr(byteLength * 1 - 1, 1);//校验点是否为“×”        //if 基点是×; 判断在基点内有×为偶数还是奇数       if (count % 2 == 0) {               //当为偶数时             size = count / 2 + (byteLength * 1 - count);             limitvalue = value.substr(0, size);     } else {             //当为奇数时             size = (count - 1) / 2 + (byteLength * 1 - count);             limitvalue = value.substr(0, size);     }    alert(title + "最大输入" + byteLength + "个字节（相当于"+byteLength /2+"个汉字）！");    document.getElementById(attribute).value = limitvalue;    return; }</script>