let solve = () => {
  let sum = arr => arr.reduce((a, b) => a + b)
  let min = arr => Math.min(...arr)
  let max = arr => Math.max(...arr)
  let product = arr => arr.reduce((a, b) => a * b)
  let join = arr => arr.join('')
  return { sum, min, max, product, join }
}

let aggregator = solve()
console.log(aggregator.sum([1, 2, 3, 4, 5]))
console.log(aggregator.min([1, 2, 3, 4, 5]))
console.log(aggregator.max([1, 2, 3, 4, 5]))
console.log(aggregator.product([1, 2, 3, 4, 5]))
console.log(aggregator.join([1, 2, 3, 4, 5]))