js 限制input输入字节长度

<script>
function WidthCheck(str, maxLen){ 
    var w = 0; 
    var tempCount = 0; 
    //length 获取字数数，不区分汉子和英文 
    for (var i=0; i<str.value.length; i++) { 
        //charCodeAt()获取字符串中某一个字符的编码 
        var c = str.value.charCodeAt(i); 
        //单字节加1 
        if ((c >= 0x0001 && c <= 0x007e) || (0xff60<=c && c<=0xff9f)) { 
            w++; 
        } else { 
            w+=2; 
        } 
        if (w > maxLen) { 
            str.value = str.value.substr(0,i); 
            break; 
         } 
    } 
} 
</script> 
<input type="text" id="nickname" class="nickname" value="" onkeyup="WidthCheck(this,8);"/>