A CodePen by nuweb. Increment large numbers in JavaScript

increment-large-nos.js

// increment/decrement large integers in JavaScript
var num = "900719925474599945678999";
function incdecLargeInt(str, operation){
  var arr = str.split("");
  var lastInt = Number(arr.pop());
  if(lastInt===9){
    // implement carry over
    fixCarryOver(num.split(''));
    var zx = res.reverse().join('');
    // edge case when all are '9's
      if(Number(zx)===0) {
          zx = "1" + zx;
      }
      console.log(zx);
  } else {
    lastInt++;
    console.log(arr.join('') + lastInt.toString());
  }
}

var res = [], CARRY = true;
function fixCarryOver(arr){
  if(arr.length){
      var tstr = arr.pop();
      if(tstr==="9" && CARRY) {
          res.push("0");
          CARRY = true;
      } else {
          if(CARRY) {
              tstr = Number(tstr);
              tstr++;
          }
          res.push(tstr.toString());
          CARRY = false;
      }
  } else {
    return res;
  }
  fixCarryOver(arr);
}

incdecLargeInt(num, "inc");