中文字符截取

Code
//计算字符串长度
String.prototype.strLen = function() {
    var len = 0;
    for (var i = 0; i < this.length; i++) {
        if (this.charCodeAt(i) > 255 || this.charCodeAt(i) < 0) len += 2; else len ++;
    }
    return len;
}
//将字符串拆成字符，并存到数组中
String.prototype.strToChars = function(){
    var chars = new Array();
    for (var i = 0; i < this.length; i++){
        chars[i] = [this.substr(i, 1), this.isCHS(i)];
    }
    String.prototype.charsArray = chars;
    return chars;
}
//判断某个字符是否是汉字
String.prototype.isCHS = function(i){
    if (this.charCodeAt(i) > 255 || this.charCodeAt(i) < 0)
        return true;
    else
        return false;
}
//截取字符串（从start字节到end字节）
String.prototype.subCHString = function(start, end){
    var len = 0;
    var str = "";
    this.strToChars();
    for (var i = 0; i < this.length; i++) {
        if(this.charsArray[i][1])
            len += 2;
        else
            len++;
        if (end < len)
            return str;
        else if (start < len)
            str += this.charsArray[i][0];
    }
    return str;
}
//截取字符串（从start字节截取length个字节）
String.prototype.subCHStr = function(start, length){
    return this.subCHString(start, start + length);
}// JavaScript Document