runStatsProgram.js

const runStatsProgram = ({kms, writer}) => { 
 const Id = x => ({
   map: f => Id(f(x)),
   fold: f => (f(x)),
 })
 
 const merge = a => b => Object.assign({},a,b)
 const round = x => Math.round(x * 100) / 100
 
 Id(kms)
  .map(x => 
          x.reduce((accumulator, item) => ({
            time: accumulator.time + item,
            ks: accumulator.ks + 1,
          }), { ks:0, time: 0 }))
  .map(stats => 
    Id(stats)
    .map(merge)
    .fold(m =>
        Id(stats.time / stats.ks)
          .map(round)
          .map(average => ({ average }))
          .fold(m)))
  .map(stats => [ 
      `Best time for ${stats.ks}k = ${stats.time}.`,
      `Time per minute ${stats.ks}k = ${stats.average}`
    ])
  .map(messages => 
    messages.map(writer))
}

runStatsProgram({ 
  kms: [4.20,4.09,4.16,4.30,4.20], 
  writer: x => console.log(x)
})