将一段数字从右到左每隔三位插入一个逗号

群里某个出的题目，听说来自某大公司的面试题，个人感觉是应该来自像土豆网显示用户数，播放数的需求。

window.onload = function(){
    //整个测试由小刀提供
    var testFun = function( callback, str ){
        var tipElem = document.createElement( 'div' ),
        startTime,
        duration = 0;
        for( var j = 5; j > 0; j-- ){
            startTime = +new Date();
            for( var i = 10000; i > 0; i-- ){
                callback();
            }
            duration = ((+new Date()) - startTime) + duration;
        }
        duration = (duration / 5).toFixed(0);
        tipElem.innerHTML = str + '总耗时：' + duration + ' ms';
        document.body.appendChild( tipElem );
    };

    var str = '3345687687876789123';

    var cuter1 = function( str ){//带刀
        var len = str.length,
        lastIndex,
        arr = [];
        while( len > 0 ){
            lastIndex = len;
            len -= 3;
            arr.unshift( str.substring(len, lastIndex) );
        }
        return arr.join(',');
    };


    var cuter2 = function( str ){//abcd
        return str.replace( /\B(?=(?:\d{3})+$)/g, ',' );
    };

    var cuter3 = function( str ){//前叔
        return str.replace(/(.*)(\d{3})$/,function(){
            if(arguments[1]&& arguments[2]){
                return arguments[1].replace(/(.*)(\d{3})$/,arguments.callee)+","+arguments[2];
            } else {
                return arguments[0];
            }
        });
    };

    var cuter4 = function( str ){//Alucelx
        return str.split('').reverse().join('').replace(/(\d{3})/g, '$1,').split('').reverse().join('');
    };

    var cuter5 = function( str ){//司徒正美
        var ret = [];
        while(str){
            str = str.replace(/\d{1,3}$/g,function(a){
                ret.unshift(a)
                return ""
            });
        }
        return ret.join(",");
    };
    var cuter6 = function( str ){//司徒正美
        var n = str.length % 3;
        if(n){
            return str.slice(0,n) +  str.slice(n).replace(/(\d{3})/g,',$1')
        }else{
            return str.replace(/(\d{3})/g,',$1').slice(1)
        }
    };
    var cuter7 = function(str){//司徒正美
                var ret = ""
                for(var i = 0, n = str.length, m = n %3 - 1; i 0;i-=3){
            s2.splice(i, 0 ,',' );
        }
        return s2.join("")
    }
    var cuter9 = function(str){//听说
        var newStr= new Array(str.length+ parseInt(str.length/3));
        newStr[newStr.length-1]=str[str.length-1];
        var currentIndex=str.length-1;
        for(var i = newStr.length-1;i >= 0;i--) {
            if((newStr.length-i)%4==0)
            {
                newStr[i]=",";
            }else{
                newStr[i]=str[currentIndex--];
            }
        }
        return newStr.join("")
    }
    var cuter10 = function(str){//Rekey
        var  len = str.length, str2 = '', max = Math.floor(len / 3);
        for(var i = 0 ; i