typewriter.js

import {
  compose,
  concat,
  curry,
  filter,
  isEmpty,
  join,
  length,
  map,
  not,
  nth,
  reduce,
  repeat,
  replace,
  split,
  take,
  takeWhile,
  transpose,
  trim
} from 'rambda'

const clear = compose(
  reduce(
    compose(
      replace(/.?[\b]/, ''),
      concat
    ),
    ''
  ),
  split('')
)

const backspaces = compose(join(''), repeat('\b'))

const collisions = compose(
  length,
  takeWhile(v => nth(0, v) === nth(1, v)),
  transpose,
  map(split(''))
)

const diff = curry((replace, origin) => {
  const c = collisions([replace, origin])
  return backspaces(origin.length - c) + replace.slice(c)
})

const diffAll = compose(
  join(''),
  reduce((a, v) => [
    ...a,
    compose(
      diff(v),
      clear,
      join('')
    )(a)
  ], [])
)

export default (node, { speed = 50, separator }) => {
  const children = compose(
    filter(compose(not, isEmpty)),
    map(trim),
    split(separator),
    join(''),
    map(({ textContent }) => textContent),
    filter(({ nodeType }) => nodeType === Node.TEXT_NODE),
    Array.from
  )(node.childNodes)
  
  const text = diffAll(children)
  const duration = text.length * speed
  
  return {
    duration,
    tick: t => {
      const i = ~~(text.length * t)
      node.textContent = clear(take(i, text))
    }
  }
}