Sum Large Numbers in JavaScript

largeSum.js

function add(num1, num2) {
	num1 = num1.split('');
  num2 = num2.split('');

  num1 = num1.map(function (num) {
  	return parseInt(num, 10);
  });

  num2 = num2.map(function (num) {
  	return parseInt(num, 10);
  });

	if (num2.length > num1.length) {
  	return _add(num2, num1);
  } else {
  	return _add(num1, num2)
  }
}

function _add(num1, num2) {
	var num1_idx = num1.length-1;
  var num2_idx = num2.length-1;
  var remainder = 0;
  
  for (; num1_idx > -1; num1_idx--, num2_idx--) {
  	var sum = num1[num1_idx] + remainder;

    if (num2_idx > -1) {
    	sum += num2[num2_idx];
    }

		if (sum <= 9 || num1_idx === 0) {
    	remainder = 0;
    	num1[num1_idx] = sum;
    } else if (sum >= 10) {
    	remainder = 1;
      num1[num1_idx] = sum - 10;
    }
    
    console.log(remainder);
  }
  
  return num1.join('');
}

document.querySelector('#sum').innerHTML = add('1923129312616892123789632134615237899999999999999999999999999999995', '19273612943202027304826153899999999999999999999999999999999999999999999999999999999');

Thanks for your time and effort!