Arithmetic operations on numbers represented as strings on javascript

stringOperations.js

/**
 * Positive addition: both operands are positive
*/
function sum(s0, s1) {
  'use strict';
  let result = '';
  let carryOver = 0;
  for(let i = 0, l = Math.max(s0.length, s1.length); i < l; i++) {
    let sum = carryOver + parseInt(s0.charAt(s0.length - 1 - i ) || 0) + parseInt(s1.charAt(s1.length - 1 - i ) || 0);
    carryOver = sum > 9 ? 1 : 0;
    result = String(sum%10).concat(result);
  }
  if(carryOver) result = String(carryOver).concat(result);
  return result;
}

/**
 * Single digit multiplication
 */
 function mulSingleDigit(num, digit) {
  'use strict';
  let result = '';
  // carryOver is at most 8
  let carryOver = 0;
  for(let i = 0, l = num.length; i < l; i++) {
    let mul = carryOver + parseInt(num.charAt(num.length - 1 - i ) || 0) * digit;
    carryOver = Math.floor(mul/10);
    result = String(mul%10).concat(result);
  }
  if(carryOver) result = String(carryOver).concat(result);
  return result;
}

/**
 * General multiplication of two positive integers
 */
 function mulIntegers(m1, m2) {
  'use strict';
  let m2Digits = m2.split('').map(Number);
  let addends = m2Digits.map((d, i) => mulSingleDigit(m1, d).concat('0'.repeat(m2Digits.length - 1 - i)));
  return addends.reduce((s, e) => sum(s, e), '0');
}