A bunch of examples from the Traversable episode.

traversable.js

const Task = require('data.task')
const Either = require('fantasy-eithers')

// data.task actually uses `ap` with reversed
// arguments, as the spec originally did, so
// this is an "old-fashioned" lift2.
const lift2 = (f, a, b) => a.map(f).ap(b)

const queens = [ 'Alyssa', 'Katya', 'Willam' ]

// Let's pretend this does some AJAX...
const getById = id => new Task((rej, res) =>
  queens[id] ? res(queens[id])
             : rej('Out of range'))

const append = x => xs => [... xs, x]

// insideOut :: Applicative f
//           => [f a] -> f [a]
const insideOut = (T, xs) => xs.reduce(
  (acc, x) => lift2(append, x, acc),
  T.of([]))

// paralleliseTaskArray :: [Int] -> Task e [User]
const paralleliseTaskArray = users =>
  insideOut(Task, users.map(API.getById))

Array.prototype.traverse = function (T, f) {
  return this.reduce(
    //    Here's the map bit! vvvv
    (acc, x) => lift2(append, f(x), acc),
    T.of([]))
}

const sequence = (T, xs) => xs.traverse(T, x => x)

// [ 'Alyssa', 'Katya' ]
;[0, 1].traverse(Task, getById)
       .fork( e => console.log('ERROR')
            , x => console.log(x) )

// toChar :: Int -> Either String Char
const toChar = n => n < 0 || n > 25
  ? Either.Left(n + ' is out of bounds!')
  : Either.Right(String.fromCharCode(n + 65))

// Right(['A', 'B', 'C', 'D'])
console.log(
  [0,  1,  2,  3]
  .traverse(Either, toChar))

// Left('-2 is out of bounds!')
console.log(
  [0, 15, 21, -2]
  .traverse(Either, toChar))