function cutstr(str, len) {

    var str_length = 0;

    var str_len = 0;

    str_cut = new String();

    str_len = str.length;

    for (var i = 0; i < str_len; i++) {

        a = str.charAt(i);

        str_length++;

        if (escape(a).length > 4) {

            //中文字符的长度经编码之后大于4

            str_length++;

        }

        str_cut = str_cut.concat(a);

        if (str_length >= len) {

            str_cut = str_cut.concat("...");

            return str_cut;

        }

    }

    //如果给定字符串小于指定长度，则返回源字符串；

    if (str_length < len) {

        return str;

    }

}

var str= "656345555464563";

var s = cutstr(str,5)

console.log(s)