addLongs.js

/**
 * Program to add VERY large numbers in javascript
 * Note - numbers should be passed as strings.
 * example -
 * add("15", "15");  // returns "30"
 *
 */
function add(str1, str2) {
  let sum = ""; // our result will be stored in a string.

  // we'll need these in the program many times.
  let str1Length = str1.length;
  let str2Length = str2.length;

  // if s2 is longer than s1, swap them.
  if (str2Length > str1Length) {
    let temp = str2;
    str2 = str1;
    str1 = temp;
  }

  let carry = 0; // number that is carried to next decimal place, initially zero.
  let a;
  let b;
  let temp;
  let digitSum;
  for (let i = 0; i < str1.length; i++) {
    a = parseInt(str1.charAt(str1.length - 1 - i)); // get ith digit of str1 from right, we store it in a
    b = parseInt(str2.charAt(str2.length - 1 - i)); // get ith digit of str2 from right, we store it in b
    b = b ? b : 0; // make sure b is a number, (this is useful in case, str2 is shorter than str1
    temp = (carry + a + b).toString(); // add a and b along with carry, store it in a temp string.
    digitSum = temp.charAt(temp.length - 1); //
    carry = parseInt(temp.substr(0, temp.length - 1)); // split the string into carry and digitSum ( least significant digit of abSum.
    carry = carry ? carry : 0; // if carry is not number, make it zero.

    sum = i === str1.length - 1 ? temp + sum : digitSum + sum; // append digitSum to 'sum'. If we reach leftmost digit, append abSum which includes carry too.
  }

  return sum; // return sum
}

// console.log(add("2750", "3750"));  // will return 6500
//
// // demo of how the function works
// // console.log(add("5987342879234789234897", "23489072349807239487"));  // will return 6010831951584596474384
// //
// //
// // console.log(add("893427328497983427893248932498034289324", "234859234879342897893427893274"));  // will return 893427328732842662772591830391462182598
//