javascript 正确截取单字节和双字节混和字符串的方法

<script type="text/javascript">

    function substr(str, len) {

        if (!str || !len) {

            return '';

        }

        var a = 0;

        var i = 0; //循环计数  

        var temp = ''; //临时字串

        for (i = 0; i < str.length; i++) {

            if (str.charCodeAt(i) > 255) {

                a += 2; //按照预期计数增加2

            } else {

                a++;

            }

            if (a > len) { //如果增加计数后长度大于限定长度，就直接返回临时字符串

                return temp;

            }

            temp += str.charAt(i); //将当前内容加到临时字符串

        }

        return str; //如果全部是单字节字符，就直接返回源字符串

    }

    var str = "呵呵he哈哈嘿嘿and";

    document.write("字符串:<font color='red'>" + str + "</font><br /><br />");

    document.write("javascript 默认截取10个字节的返回值：<br />");

    document.write("<font color='red'>" + str.substr(0, 10) + "</font><br /><br />");

    document.write("用本文介绍的方法截取10个字节的返回值：<br />");

    document.write("<font color='red'>" + substr(str, 10) + "</font><br /><br />");

    document.write("截取9个字节的返回值：<br />");

    document.write("<font color='red'>" + substr(str, 9) + "</font><br /><br />");

</script>