bigNumbers.js

/*
  Please write a function that adds two large numbers represented by a string , such that the output is not expressed in scientific notation.
*/

function add(a, b) {
  let aNums = breakIntoManageableParts(a);
  let bNums = breakIntoManageableParts(b);
  let remainder = 0, bigSum = "";
  
  while (aNums.length > 0 || bNums.length > 0) {
    let lastA = getNumToAdd(aNums);
    let lastB = getNumToAdd(bNums);
    
    let sum = String(lastA + lastB + remainder);
    bigSum = sum + bigSum; // Adds higher numbers on the left
    remainder = sum.length > 15 ? parseInt(sum.charAt(0)) : 0;
    
    // Remove numbers already added
    aNums.pop();
    bNums.pop();
  }
  
  console.log("Big sum:", bigSum);
  return bigSum;
}

/** Breaks the number up from right to left in segments of 15 */
function breakIntoManageableParts(bigassNum) {
  let smallerParts = [];
  while (bigassNum.length > 15) {
    let endIndex = bigassNum.length - 16;
    smallerParts.unshift(bigassNum.substr(endIndex));
    bigassNum = bigassNum.substr(0, endIndex);
  }

  if (bigassNum.length > 0) smallerParts.unshift(bigassNum);
  return smallerParts;
}

/** Replace non-existant values with 0 for addition purposes */
function getNumToAdd(numArray) {
   return numArray.length > 0 ? parseInt(numArray[numArray.length - 1]) : 0;
}

console.log(add('3156551651651','6513515126516516') === '6516671678168167');
console.log(add('2334234232342342346675467678567676764456345134123523', '23034298372387187498752364987564957845') === '2334234232342365380973840065755175516821332699081368');
console.log(add('1616516516516516516516151551', '466161651651651651616516511') === '2082678168168168168132668062');