main.js

function User() {
  // constructor.
}

User.prototype.publicFn = function() {
  // call private function.
  this._privateFn();
};

User.prototype._privateFn = function() {
  // this is private function.
};

var user = new User();
user.publicFn();
user._privateFn(); // damn, It's callable...

main2.js

function User() {
  // constructor.
}

!function(proto) {

  function _privateFn() {
    // this is private function.
  }

  proto.publicFn = function() {
    // call private function.
    _privateFn();
  };

}(User.prototype);

var user = new User();
user.publicFn();
user._privateFn(); // yeah, throws exception!

main3.js

function User() {
  // constructor.
}

!function(proto) {

  function _privateFn() {
    // what is `this`?
    this.is = 'it';
    return Object.prototype.toString.call(this);
  }

  proto.publicFn1 = function() {
    // call private function.
    console.log(_privateFn()); // [object global]
  };

  proto.publicFn2 = function() {
    // call private function with `call()`
    console.log(_privateFn.call(this)); // [object Object]
  };

}(User.prototype);

var user = new User();
user.publicFn1();
console.log(user.is); // `undefined` ... wtf?
console.log(is); // `it` ... aarrrghh!! global leak!

user.publicFn2();
console.log(user.is); // `it` ... yeah!

http://null.ly/post/17762856703/javascript-private-method