Generate power set from a string

powerset.js

function powerset (input) {
  return input
  .split(input ? '' : false)
  .filter(duplicates({}))
  .map(concatEmptyStr)
  .reduce(multiplyMatrix)
}

function concatEmptyStr (str) {
  return str ? ['', str] : ['']
}

function multiplyMatrix (results, array) {
  return results.reduce(function (aggr, item) {
    return aggr.concat(array[0]+item, array[1]+item) // .push .push would be O(1)
  }, [])
}

function duplicates (dupTracker) {
  return function (item) {
    return dupTracker[item] ? false : dupTracker[item] = true
  }
}

console.log(powerset('').length === 1, powerset(''))
console.log(powerset('a').length === 2, powerset('a'))
console.log(powerset('ab').length === 4, powerset('ab'))
console.log(powerset('abc').length === 8, powerset('abc'))
console.log(powerset('abca').length === 8, powerset('abca'))