Long String Number Add return as decimal not as scientific notation

longNumberAdd.js

function sumStrings(a,b) { 
    /* 
    What this does is a brute force addition of numbers as strings 
    Purpose: to avoid the conversion to scientific notation and display 
    in Decimal notation as a string.
    Input: a, b  numbers in string format
    Return: total of addition of a and b in string format. 
    */
    
    let x = [];
    let y = [];
    let total = []; //initialise the string. 
    let subtotal = 0;
    
    
    // lots of duplicate code from here. Could probably be added to a helper function.

    for (let adigit of a){
        x.push(adigit);
    }
    for (let bdigit of b){
        y.push(bdigit);
    }
    let tempArr = x.slice();

    for(let checkLeading of x){
        if (checkLeading == 0){
            tempArr.shift();
        } else{
            // break on first occurance of 
            break;
        }
    }
    x = tempArr.slice();
    tempArr = y.slice();
    for(let checkLeading of y){
        if (checkLeading == 0){
            tempArr.shift();
        } else{
            // break on first occurance of 
            break;
        }
    }
    y = tempArr.slice();
    
    x = x.reverse();
    y = y.reverse();
    // all this code up to here is duplicated. This should probably be added to a helper function
 
    let carry = 0;
    //decide which is the longest number for the maximum loop length
    let max_len = x.length > y.length ? x.length : y.length;
    for (let position =0; position < max_len; position++){
        //might get an element from the array that does not exist
        // and you don't know if a or b is the largest number
        //test for this condition
        if (x[position] == undefined){
            subtotal =  parseInt(y[position],10) + 0 + parseInt(carry,10);
            carry = 0; // reset the carry
        }else if (y[position] == undefined){
            subtotal =  parseInt(x[position],10)  + 0 + parseInt(carry,10);
            carry = 0; // reset the carry 
        }else{
            //if neither is undefined, add them with any carry.
            subtotal = parseInt(x[position],10) + parseInt(y[position],10)+ parseInt(carry,10) ;
            carry = 0; // reset the carry
        }
        
        if (subtotal > 9){
            carry = 1; //don't forget to include the carry. 
            //remove the carry from the subtotal.
            subtotal -=10;
            
        }
        //add it to the front of the array so the number is in the correct order
        total.unshift(subtotal);
    }
    //there may be a case where a carry occurs with the last number and the loop is complete
    //add the carry here
    if (carry == 1){
       total.unshift(carry);
    }
    let decimalSum = "";
    for (let strNumber of total){
        decimalSum = decimalSum + String(strNumber);
    }
    return(decimalSum);
}
console.log(sumStrings('00103', '08567'));
console.log(sumStrings('523','556'))
console.log(sumStrings('123','456'))
console.log(sumStrings('712569312664357328695151392', '8100824045303269669937') )
console.log(sumStrings('712569312664357328695151392', '810082404530326966993712121212121') )