Lens.js

const Set = (lens, value, target) => lens.set(value, target)
const View = (lens, target) => lens.get(target)
const Over = (lens, func, target) =>
  Set(lens, func(View(lens, target), target), target)

const Compose = (...lenses) => {
  lenses = lenses.reverse()
  const itar = (i, target, value) => {
    if (i === lenses.length) return value
    return lenses[i].set(itar(i + 1, lenses[i].get(target), value), target)
  }
  return {
    __isLens: true,
    get: state => lenses.reduce((state, i) => i.get(state), state),
    set: (value, target) => itar(0, target, value)
  }
}

const Identity = () => ({
  __isLens: true,
  get: target => target,
  set: (focus, target) => target
})

const Focus = (...args) => {
  if (args.length === 0) return Identity
  if (args.length === 1) {
    const o = args[0]
    if (o.constructor === Array) return Compose.apply(null, o.map(l => Focus(l)))
    if (o.constructor === Number) return Index(o)
    if (o.constructor === String) return Prop(o)
    if (o.constructor === Function) return Where(o)
    if (o.__isLens) return o
  }
  if (args.length > 1) return Focus(args)
  return Identity
}

const Prop = name => ({
  __isLens: true,
  get (target) {
    return target[name]
  },
  set (value, target) {
    return { ...target, [name]: value }
  }
})

const Index = id => ({
  __isLens: true,
  get (target) {
    return target[id]
  },
  set (value, target) {
    return target.map((k, i) => (i === id ? value : k))
  }
})

const Where = pred => ({
  __isLens: true,
  get (target) {
    const el = target.find(pred)
    if (el === undefined) throw Error('Lens[Where] could not find any element')
    return el
  },
  set (value, target) {
    return target.map((k, i) => (pred(k) ? value : k))
  }
})