Calculate extra large factorial

factorial.js

function factorial(userInt)
{
  if(userInt===0)
    return '1'

  if(!userInt)
    return ''

  var i, nextNumber, carret,

  result = userInt.toString().split('').reverse().map(Number)

  while(--userInt){
    i = carret = 0

    while((nextNumber = result[i++]) !== undefined || carret) {
      carret = (nextNumber || 0) * userInt + carret
      result[i-1] = carret % 10
      carret = parseInt(carret/10)
    }
  }

  return result.reverse().join('')
}

how did u do that ?care to explain?

Best and concise decision that I've seen so far, thank you.