lens.js

function lens(get, set) {
  var f = function (a) { return get(a); };
  f.set = set;
  f.mod = function (f, a) { return set(a, f(get(a))); };
  return f;
}

var first = lens(
  function (a) { return a[0]; },
  function (a, b) { return [b].concat(a.slice(1)); }
);

console.log(first([1, 2, 3])); // outputs 1
console.log(first.set([1, 2, 3], 5)); // outputs [5, 2, 3]

function tenTimes(x) { return x * 10 }

console.log(first.mod(tenTimes, [1, 2, 3])); // outputs [10, 2, 3]