lenses.js

const bigBird = {
  name: 'Big Bird',
  address: { name: 'Whatever' },
  age:6,
  comments:[
    {body:'sing, sing a song',title:'Line 1'},
    {body:'make it simple',title:'Line 2'},
    {body:'sing out strong',title:'Line 3'}]
};

const pipe = (fn,...fns) => (...args) => fns.reduce( (acc, f) => f(acc), fn(...args));
const compose = (...fns) => pipe(...fns.reverse());

const curry = (f, ...args) => (f.length <= args.length) ?
  f(...args) :
  (...more) => curry(f, ...args, ...more);

const update = R.update; //for Arrays
const assoc = R.assoc; //for Objects
const fmap = curry((f, mappable) => mappable.map(f));

const arrayLens = curry(
  (index, f, xs) => fmap(replacement => update(index, replacement, xs), f(xs[index]))
);

const objectLens = curry(
  (key, f, xs) => {
    return fmap(replacement => assoc(key, replacement, xs), f(xs[key]))
  }
);

const _K = x => (_) => x;
const Const = x => ({value: x, map(){ return this; }});
const Identity = x => ({value: x, map: mapf => Identity(mapf(x)) });
const Maybe = x => ({ value: x, map: function (mapf) { return x ? Maybe(mapf(x)) : this; } });

const view = curry( (lens, targetData) => lens(Const)(targetData).value );
const over = curry( (lens, apf, targetData) => lens(y => Identity(apf(y)))(targetData).value );
const set = curry( (lens, val, targetData) => over(lens, _K(val), targetData) );

// console.log(over(objectLens('name'), (v) => v.toUpperCase(), bigBird));
// console.log(view(compose(objectLens('comments'), arrayLens(0)), bigBird));