index.js

var resolve = function (key, object) {
  var split = key.split('.')

  for (var i = 0; i < split.length; i += 1) {
    var key = split[i];
    if ('undefined' === object[key]) break;
    object = object[key];
  }
  return object;
};

var set = function (key, object, value) {
  var split = key.split('.')
    , penultimate = split.slice(0, -1).join('.')
    , ultimate = split.slice(-1).join('.');

  var obj = resolve(penultimate, object);

  if (obj) {
    // If we can resolve it, set the value
    obj[ultimate] = value;
  } else {
    // If not, we have to mkdirp
    mkdirp(penultimate, object);
    resolve(penultimate, object)[ultimate] = value;
  }
};

var mkdirp = function (key, object) {
  var split = key.split('.');
  if (!object) return;

  for (var i = 0; i < split.length; i += 1) {
    var key = split[i];
    // For each layer, create a new object if none exists
    if (!object[key]) object[key] = {};
    // And drill down into it
    object = object[key];
  }
};

var a = {
  a: {
    b: {
      c: 'd'
    }
  }
};

console.log(resolve('a.b.c', a));
console.log(a);

set('a.c.d', a, {herp: 'derp'});

console.log(a.a.c.d);