sum of two big numbers (still in testing)

sum.js

function doSum(a, b){
    var aLen = ("" + a).length, bLen = ("" + b).length;
    var large = Math.max(aLen, bLen);
    var dummyZeroes = "";
    for(var j = 0; j < large; j++) {
        dummyZeroes += "0";
    }
    a = (dummyZeroes + a).slice(-large);
    b = (dummyZeroes + b).slice(-large);
    var result = "";
    var extraNum = 0;
    for(var i = large-1; i >= 0; i--){
        var aChar = a[i] || 0, bChar = b[i] || 0;
        var sResult = ("0" + (+extraNum - - aChar - - bChar)).slice(-2);
        result = sResult[1] + result;
        extraNum = sResult[0];
    }
    return (result[0] - - extraNum) + "" + result.slice(1);
}