withEval.js

function exporter(publics, privates) {
  privates = privates || {};
  var all = Object.assign({}, publics, privates);
  publics.__test = function (t) {
    var str = t.toString().match(/\{((.|\s)*)\}/)[1];
    with (all) {
      eval(str);
    }
  };
  return publics;
}

function closureFactory(a, b) {
  function privateFunc(foo) {return foo + 1;}

  return exporter({
    func: function () {return a + b}
  }, {privateFunc, a, b})
}

var ins = closureFactory(1, 2);

ins.__test(function () {
  console.log(a, b);
  console.log(privateFunc(5));
});