String Calculator Kata in JS

string-calculator.js

const calculator = (function(){
  
  function add(numberString){
    
    if (numberString === ''){
      return 0
    }
    const delimiter = getDelimiter(numberString)
    const formattedInput = formatInput(numberString)
    return calculateSum(getNumbers(formattedInput, delimiter))    
   
  }
  
  function formatInput(input){
    const delimiterRegExp = /^(\/\/.*\n)/
    const matches = delimiterRegExp.exec(input)
    if(matches && matches.length > 0){
      return input.replace(delimiterRegExp,'')
    }
    return input
  }
  
  function getDelimiter(input) {
    const delimiters = []
    const multipleDelimiterRegexp = /(?:^\/\/)?\[([^\[\]]+)\]\n?/g
    let matches = multipleDelimiterRegexp.exec(input)
    while(matches !== null){
      delimiters.push(matches[1])
      matches = multipleDelimiterRegexp.exec(input)
    }
    if(delimiters.length > 0){
      return new RegExp('['+delimiters.join('')+']')
    }
    matches = /^\/\/(.*)\n/.exec(input)
    if(matches && matches[1]){
      return matches[1]
    }
    return /[\n,]/
    
  }
  
  function getNumbers(string, delimiter){
    return string.split(delimiter)
      .filter(n => n !== '')
      .map(n => parseInt(n))
  }
  
  function calculateSum(numbers){
    const negatives = []
    const finalSum = numbers.reduce((sum, n) =>{
      if(n > 1000){
        return 0
      }
      if(n < 0){
        negatives.push(n)
        return 0
      }
      return sum + n
    },0)
    if(negatives.length > 0){
      throw new Error('Negatives not allowed: '+negatives.join(','))
    }
    return finalSum
  }
  
  
  return {add}
  
}())

console.info(calculator.add('')) // 0
console.info(calculator.add('1,2,3')) // 6
console.info(calculator.add('1\n2,3')) // 6
console.info(calculator.add('//;\n1;2;3')) // 6
console.info(calculator.add('1001,2')) // 6
console.info(calculator.add('//[**]\n1**2**3')) // 6
console.info(calculator.add('//[*][%]\n1*2%3')) // 6
console.info(calculator.add('//[..][%%]\n1..2%%3')) // 6
console.info(calculator.add('-1,-3,2')) // 6