promisizer.js

function promisize(func) {
  return function () {
    try {
      var promise = func.apply(this, arguments);
      if (! promise.isThenable) {
        var result = new Resolver();
        result.resolve(promise);
        return result.promise;
      } else {
        return promise;
      }
    } catch (e) {
      var result = new Resolver();
      result.reject(e);
      return result.promise;
    }
  };
}

// Usage:
SomeObect.prototype.method = promisizer(function (arg) {
  if (typeof arg != "string") {
    throw "Bad argument";
  }
  if (arg in this._cache) {
    return this._cache[arg];
  }
  var data = loadFromStorage();
  var result = new Resolver();
  result.then(
    function (data) {result.resolve(munge(data));},
    result.reject.bind(result));
  return result.promise;
});