"a.b.c.d" -> {a: {b: {c: "d"}}}

gistfile1.js

function p(s) {
  var a = s.split('.').reverse(),
  _p = function(a, d, o) {
    var i=o?0:1; 
    if(d>0) {
      var t={}; t[a[i]]=o||a[0];
      return _p(a.slice(1+i), d-1-i, t);
    } else return o;
  };
  return _p(a, a.length);
}

console.log(p('a.b.c.d.e.f.g'))