Functional Lenses (Van Laarhoven)

lens.js

// Combinators
const K         = a => b => a
const I         = a => a

// Operators
const eq        = a => b => a === b

// Types
const typeOf    = a => typeof a
const isArray   = a => Array.isArray(a)

// Lists
const reverse   = xs => xs.slice().reverse()
const reduce    = f => a => xs => xs.reduce(f, a)

// Functions
const compose   = (...fns) => x => reduce((acc, fn) => fn(acc))(x)(reverse(fns))

// Objects
const assign    = Object.assign
const prop      = key => x => x[key]
const assoc     = key => value => x => assign(isArray(x) ? [] : {}, x, {[key]: value})

// Functors
const Identity  = value => ({ join: K(value), map: fn => Identity(fn(value))})
const Constant  = value => ({ join: K(value), map: () => Constant(value)})

// Lenses
const Lens      = (get, set) => fn => x => fn(get(x || {})).map((value) => set(value)(x))
const Prop      = name => Lens(prop(name), assoc(name))
const Get       = lens => x => lens(Constant)(x).join()
const Over      = lens => f => x => lens(compose(Identity, f))(x).join()
const Set       = lens => v => x => Over(lens)(K(v))(x)

// Example
const o = {a: {b: {c: 0}}}

const a = Prop('a')
const b = Prop('b')
const c = Prop('c')
const abc = compose(a, b, c)

console.log(Get(abc)(o))
console.log(Get(abc)({}))
console.log(Set(abc)(1)(o))