SOURCE

var a="1234123123123123123123123123";
var b="1";
function getNumber(a,b){
  let result=[];
  let reservea=a.split("").reverse();
  let reserveb=b.split("").reverse();
  let min=reservea.length<=reserveb.length?reservea:reserveb;
  let max=reservea.length>reserveb.length?reservea:reserveb;
  for(let i=0;i<max.length;i++){
    if(i<min.length){
      result.push(parseInt(min[i])+parseInt(max[i]))
    }
    else{
      result.push(parseInt(max[i]))
    }
  }
  result.forEach(function(item,index){
    result[index]=item%10;
    if((index+1)<result.length){
      result[index+1]+=Math.floor(item/10);
    }
  })
return result.reverse().join("");
}
console.log(getNumber(a,b))