A JavaScript utility for managing (really) true per object private state.

JavaScript, private, safe factory, pattern
bower install private-state


Private State

A JavaScript utility for managing (really) true per object private state.

private-state.js is an implementation of the Safe Factory Pattern, for node.js and the browser environment. You can read more about the pattern here.

Unlike with other "private" state utilities, in which privacy is only a word expressing an actually unachieved intent (or in which it is achieved by leaking memory, the objects and/or the stored secrets), with the safe factory pattern you actually get to store, with each object, in some predetermined property, an unbreakable safe that stores the object's private state. This technique does not prevent an object from being garbage collected; when it is so, the private state goes with it.

Knowledge of the name of the property where a safe is stored is of no use. The only way to open a safe stored in an object is with its associated key function. This function must be kept in a "safe" place — usually a closure, the scope of the JavaScript module that creates the objects holding private state.


It's best to just show an example. The following is the content of a file named "robot.js":

define(['private-state'], function(privState) {
  // Create a key and, from it,
  // a derived one that reads/stores safes
  // from a randomly named property.
  var keyProp = privState.key().property();

  function Robot() {
    // This instance's private state.
    var priv = {
        state:  'stopped',
        target: null

    // Store it publicly, in the instance itself,
    // in the randomly named property.
    keyProp.init(this, priv);

  Robot.prototype.move = function(to) {
    var priv = keyProp(this);
    switch(priv.state) {
      case 'stopped':
        priv.state  = 'moving'; = to;
        this._startMoving(); // not shown...
      case 'moving':
        var from =;
        if(from !== to) {
 = to;
          this._changeTarget(to, from); // not shown...

  // Export the class, but **keep `keyProp` for yourself**.
  return SomeClass;


For node.js, just execute the following on the command line:

$ npm install private-state

For the browser environment, private-state is available in two flavors:

Source and Tests

Check the source code and tests at github. To build or run the tests you need node.js.

Clone the repository and install all dev-time dependencies by executing:

$ git clone git://
$ cd private-state
$ npm install

Build it:

$ grunt

Test it:

$ karma start