Using regular expressions to complete the function in one line.

function splitNumber(originalStr) {
   return originalStr.split('').reverse().join('').replace(/(\d{3})(\d{2}|\d|\B)/g, ',,').replace(/,$/,'').split('').reverse().join('');   
}

var str = '123456789';
console.log(splitNumber(str));   // 输出 1,234,56,789

var origStr="123456789";
var result="";
result=splitNumber(origStr);
console.log(result);

function splitNumber(s){
    var sArr=s.split("").reverse();
    var b=[];
    for(var i=0;i<sArr.length;i++){
        if(i!=0 && (i%5==0 || i%5==3)){
            b.push(",");
            b.push(sArr[i]);
        }
        else
            b.push(sArr[i]);
    }
    return b.reverse().join("");
}

function change(num,bool){
    function tho(num){return num>1000?String(num).slice(-5,-3)+','+String(num).slice(-3):num}
  return num<100000?tho(num):change(parseInt(num/100000))+","+tho(num)
}