Really simple van Laarhoven lenses in Javascript

Lens.js

const IdModule = {
  fmap: (fn) => (x) => fn(x),
}

const ConstModule = {
  fmap: (fn) => (x) => x,
}

/**
 * Construct a lens from a getter and a setter
 *
 * @param get {S -> A}
 * @param set {A -> (S -> S)}
 */
const lens = (get) => (set) => (mod) => (inj) => (s) =>
  mod.fmap((a) => set(a)(s))(inj(get(s)));

/**
 * View a lens as a getter.
 *
 * @param lens
 * @param s - the "whole" value
 */
const get = (lens) => (s) => // we can eta expand this def if needed
  lens(ConstModule)((a) => a)(s)

/**
 * View a lens as a setter.
 *
 * @param lens
 * @param fn {A -> B}
 * @param s - the whole value
 */
const set = (lens) => (fn) => (s) => // we can eta expand this def if needed
  lens(IdModule)(fn)(s)