JavaScript function to sum two strings as numbers and return the full string (no scientific notation).

sumStringsAsNumbers.js

function sumStrings(a,b) {

  var arrayA = a.split('').reverse();
  var arrayB = b.split('').reverse();
  var arrayAB = [];
  var remnants = 0;
  
  for (var i = 0 ; i < Math.max(arrayA.length,arrayB.length); i++){
    
    // if string is longer a or b, will clean up at the end
    if (!arrayA[i]) arrayA[i] = '0';
    if (!arrayB[i]) arrayB[i] = '0';
    
    arrayAB[i] = (parseInt(arrayA[i]) + parseInt(arrayB[i]) + remnants) % 10;
    if (((parseInt(arrayA[i]) + parseInt(arrayB[i])) + remnants) > 9) remnants = 1;
    else remnants = 0;    
  }
  
  var stringNumber =  arrayAB.reverse().join('');
  if (remnants === 1) stringNumber = '1' + stringNumber;
  
  while (stringNumber[0] === '0'){
      stringNumber = stringNumber.slice(1);
    }
  
  return stringNumber;
}