trying to add two integers in js but somehow if the Integer is big , withoutithout using any built-in library for handling large integers

Question

Given two non-negative integers, num1 and num2 represented as string, return the sum of num1 and num2 as a string.
You must solve the problem without using any built-in library for handling large integers (such as BigInteger). You must also not convert the inputs to integers directly.

Here is My code:

/**
 * @param {string} num1
 * @param {string} num2
 * @return {string}
 */
var addStrings = function(num1, num2) {
  var num1Int = parseInt(num1);
  var num2Int = parseInt(num2);
  var sum = num1Int + num2Int;
  var sumString = sum.toString();
  return sumString;

};
console.log(addStrings("11", "123"));