A pure lens implementation strongly based on ramda implementation, made to teaching my internships at Codeminer42.com

lens.js

o = { user: { name: 'foo' } }
  
const get = key => obj => obj[key]

const set = key => (x, obj) =>
  ({ ...obj, [key]: x })


Identity = v => ({
  value: v,
  map: mapping => Identity(mapping(v))
})

const lens = (getter, setter) => toFunctor => data =>
  toFunctor(getter(data))
    .map(focus => setter(focus, data))

const overLens = setter =>
  lens(x => x, focus => setter(focus))(Identity)

const nameLens = lens(get('name'), set('name'));
const userNameLens = f => lens(get('user'), set('user'))(nameLens(f))

const over = (l, mapping, obj) =>
  l(overLens(mapping))(obj).value


console.log(
  over(userNameLens, x => "fooofoo", o)
);