Solution for Reaktor's programming task at http://reaktor.fi/ura/fast_track/ This solution relies on the fact that JavaScript can solve multiplication for strings (e.g. "2" * "3" -> 6) and thus parseInt is not needed.

reaktor_fast_track.js

function (input) {
  function product(numbers) {
    if (numbers.indexOf("0") !== -1) {
      return 0
    } else {
      return _(numbers).reduce(function(product, number){
        return product * number
      }, 1)
    }
  }
  function getNumberGroup(firstNumber, index, allNumbers) {
    return allNumbers.substr(index, 5)
  }
  function hasFiveNumbers(numbers) {
    return numbers.length === 5
  }

  var biggestProduct =_(input).chain()
    .map(getNumberGroup)
    .filter(hasFiveNumbers)
    .map(product)
    .max()
    .value()
  return biggestProduct
}