wtf_filterMap.js

const numbers = [ 1, 2, 3 ]

const isOdd = num => {
    console.log(['isOdd', num])
    return num % 2 === 0
}
const double = num => {
    console.log(['double', num])
    return num * 2
}

console.log('.filter.map')

numbers
  .filter(isOdd)
  .map(double) //=> [ 2, 6 ]

console.log('filterMap')

const filterMap = (filter, map) => (acc, x) => {
  if (filter(x)) {
    const value = map(x)
    acc.push(value)
  }
  return acc
}

numbers.reduce(filterMap(isOdd, double), []) //=> [ 2, 6 ]

// --> output
// .filter.map
// ["isOdd", 1]
// ["isOdd", 2]
// ["isOdd", 3]
// ["double", 2]
// filterMap
// ["isOdd", 1]
// ["isOdd", 2]
// ["double", 2]
// ["isOdd", 3]