.filter().map() pedantry!

test.js

let values = []
let scores = []

function prepare (length = 10_000_000) {
  values = (new Array(length)).fill(1).map((e, i) => i)
}

function forIn () {
  const output = []
  for (let i = 0; i < values.length; i++) {
    if (values[i] % 2) {
      output.push({ value: values[i] })
    }
  }
  return output
}

function forOf () {
  const output = []
  for (const value of values) {
    if (value % 2) {
      output.push({ value })
    }
  }
  return output
}

function filterMap () {
  return values
    .filter(value => value % 2)
    .map(value => ({ value }))
}

function mapFilter () {
  return values
    .map(value => ({ value }))
    .filter(el => el.value % 2)
}

function reducePush () {
  return values
    .reduce((output, value) => {
      if (value % 2) {
        output.push({ value })
      }
      return output
    }, [])
}

function reduceConcat () {
  return values
    .reduce((output, value) => {
      return value % 2
        ? output.concat({ value })
        : output
    }, [])
}

function reduceSpread () {
  return values
    .reduce((output, value) => {
      return value % 2
        ? [...output, { value }]
        : output
    }, [])
}

function test (fn) {
  const d = Date.now()
  fn()
  const name = fn.name
  const time = Date.now() - d
  scores.push({ name, time })
  console.log(name, time)
}

function results () {
  const results = scores
    .filter(result => result.name !== 'prepare')
    .sort((a, b) => {
      a = a.time
      b = b.time
      return a < b
        ? -1
        : a > b
          ? 1
          : 0
    })
  console.table(results)
}

// build data
test(prepare)

// tests
test(forIn)
test(forOf)
test(filterMap)
test(mapFilter)
test(reducePush)
// test(reduceConcat) // 1500 x slower than reduce push
// test(reduceSpread) // 10500 x slower than reduce push

// results
results()