SOURCE

function addTwoNumbers (l1, l2) {
    var sum1 = 0;
    var sum2 = 0;
    var sum3 = 0;
    var arr = []
    var l11  = l1.reverse()
    var l22 = l2.reverse()
    for(var i =0 ; i < l11.length ; i++) {
        var num1 = l11[i]*Math.pow(10,i)
        sum1 += num1;
    }
    for(var j = 0; j < l22.length ; j++) {
        var num2 = l22[j]*Math.pow(10,j)
        sum2 += num2;
    }
    sum3 = sum1 + sum2
    return sum3.toString().split("").reverse().map(Number)
}

var l1 = [1,2,3]
var l2 = [1,2,3]
addTwoNumbers(l1,l2)
console.log(addTwoNumbers(l1,l2))