How to add two big numbers in javascript?

I have two text boxes. Each will take input up to thousand digits.

Now i want to add these two numbers. My question is what data type should i use to store the result?

I have tried this:

<script>
   var x = 'Thousand digit of number'
    var y = 'Thousand digit of number'
    var z = x + y
</script>

but i am getting result in exponential form. How to store the result and display it?

Answer

Input the numbers as string and add each characters each other as array something like this:

 function add() {
        document.getElementById("demo").innerHTML = "";
        var x = document.getElementById("txt1").value;
        var y = document.getElementById("txt2").value;
        var len;
        var lenx = x.length;
        var leny = y.length;
        var x1,y1,rem,div=0;
        if(lenx>leny) len = lenx; else len = leny;

        for(var i=0;i<len;i++){
            if(i>=lenx) x1  = 0;
            else x1 = parseInt(x[lenx-i-1]);
            if(i>=leny) y1 = 0;
            else y1 = parseInt(y[leny-i-1]);
            rem = (x1+y1+div)%10;
            div = Math.floor((x1 + y1+div)/10);
            document.getElementById("demo").innerHTML = rem + document.getElementById("demo").innerHTML;
        }
       if(div>0){
            document.getElementById("demo").innerHTML = div + document.getElementById("demo").innerHTML;
       }
    }

Here the code: https://jsfiddle.net/mtsL1k2x/5/

Note: this is only for natural numbers. You can modify depending on your inputs