Sum of big integer strings

sum-arr.js

/*!
 *  Big Integer Sum Algorithm
 *
 *
 *  Result sum( '5' * 10^7, '8' * 10^7)
 *  real    0m2.995s
 *  user    0m2.928s
 *  sys     0m0.060s
 */

function sum(n1, n2) {
    // Create new result array
    var re = [];
    // Point to the last index
    var ix = n1.length - 1;
    // Carry number along calculation
    var ca = 0;
    // Start adding
    for (var su; ~ix; --ix) {
        // Calculate sum between the two right most numbers and carry
        sum = n1[ix] || 0 + n2[ix] || 0 + ca;
        // Shit right most number
        re[ix] = sum % 10;
        // Add carry if needed
        ca = +(sum > 9);
    }
    // Check if there is any carry remaining
    //  if (ca) return [ca].concat(re);
    return re;
}

// Create array numbers
var n1 = "5".repeat(10000000).split('').map(Number, Number.call);
var n2 = "8".repeat(10000000).split('').map(Number, Number.call);
// Start the operation
var n = Date.now();
var re = sum(n1, n2);
console.log(Date.now() - n);
// Print result
console.log("Done!");

sum-basic.js

/*! 
 *  Big Integer Sum Algorithm
 *  
 *  Kind of slow algorithm, sums each digit of a string number using
 *  carry operations
 *
 *  Result sum( '5' * 10^7, '8' * 10^7)
 *  real    0m1.274s
 *  user    0m1.208s
 *  sys     0m0.092s
 */

function sum(left, right) {
    // Add leading zeros to make them the same length
    if (left.length > right.length) {
        right = '0'.repeat(left.length - right.length) + right;
    } else {
        left  = '0'.repeat(right.length - left.length) + left;
    }
    // var result
    var result = "";
    // Point to the last index
    var ptr = right.length - 1;
    // Carry number along calculation
    var carry = 0;
    // Start calculation
    for (var sum ; ptr >= 0 ; ptr--) {
        // Calculate the sum between the two right most numbers
        sum = +left[ptr] + +right[ptr];
        // Add the carry
        sum = sum + carry;
        // Check if number is result needs more carry
        if (sum > 9) {
            // Shift the right most digit
            result = sum % 10 + result;
            // Add other digits to carry
            carry = 1;
        } else {
            // If it doesn't shift the result
            result = sum + result;
            // Set carry to 0
            carry = 0;
        }
    }
    // Check if there is any carry remaining
    if (carry) {
        result = carry + result;
    }
    // Return resulting number as string
    return result;
}

// Start the operation
var s1 = "5".repeat(10000000);
var s2 = "8".repeat(10000000);
var n = Date.now();
var re = sum(s1,s2);
console.log(Date.now() - n);
// Print result
console.log("Starts in " + re.slice(0, 4) + " and ends in " + re.slice(-4));

sum-buffer.js

/*! 
 *  Big Integer Sum Algorithm
 *
 *  Result sum( '5' * 10^7, '8' * 10^7)
 *  real    0m0.756s
 *  user    0m0.724s
 *  sys 0m0.028s
 */

function sum(left, right) {
    // Creates the resulting buffer
    var bres = Buffer(left.length);
    // Point to the last element
    var ptr = bres.length - 1;
    // Carry number along calculation
    var car = 0;
    // Start calculation
    for (var sum; ptr >= 0; --ptr) {
        // Sum of two right most digits
        sum = left[ptr] + right[ptr] + car;
        // If result needs more carry
        if (sum > 9) {
            // Shift the right most digit
            bres[ptr] = sum % 10;
            // Add other digits to carry
            car = 1;
        } else {
            // Shift all
            bres[ptr] = sum;
            // Has nothign else to carry
            car = 0;
        }
    }
    return car ? Buffer.concat([Buffer([car]), bres]) : bres;
}

var bleft  = Buffer("5".repeat(10000000).split('')),
    bright = Buffer("8".repeat(10000000).split(''));
var n = Date.now();
var re = sum(bleft, bright);
console.log(Date.now() - n);