An example for https://medium.com/@pacdiv/hi-roman-ostolosh-439c13969b38

filterMapReduce.js

'use strict'

const characters = [
  { name: 'ironman', env: 'marvel' },
  { name: 'black_widow', env: 'marvel' },
  { name: 'wonder_woman', env: 'dc_comics' },
]

characters
  .filter( propEq('env', 'marvel') )
  .map( merge({ alsoSeenIn: ['Avengers'] }) )

characters
  .reduce((acc, character) =>
    propEq('env', 'marvel', character)
      ? acc.concat( merge({ alsoSeenIn: ['Avengers'] }, character) )
      : acc
  , [])