Private Members in JavaScript

Douglas Crockford
www.crockford.com

JavaScript is the
world's most misunderstood programming language. Some believe
that it lacks the property of information hiding because
objects cannot have private instance variables and methods. But this
is a misunderstanding. JavaScript objects can have private members.
Here's how.

Objects

JavaScript is fundamentally about objects. Arrays are
objects. Functions are objects. Objects are objects. So what are
objects? Objects are collections of name-value pairs. The names are
strings, and the values are strings, numbers, booleans, and objects
(including arrays and functions). Objects are usually implemented as
hashtables so values can be retrieved quickly.

If a value is a function, we can consider it a method. When
a method of an object is invoked, the this variable is set
to the object. The method can then access the instance variables
through the this variable.

Objects can be produced by constructors, which are
functions which initialize objects. Constructors provide the features
that classes provide in other languages, including static variables
and methods.

Public

The members of an object are all public members. Any
function can access, modify, or delete those members, or add new
members. There are two main ways of putting members in a new
object:

In the constructor

This technique is usually used to initialize public instance
variables. The constructor's this variable is used to add
members to the object.

function Container(param) {
    this.member = param;
}

So, if we construct a new object

var myContainer = new Container('abc');

then myContainer.member contains 'abc'.

In the prototype

This technique is usually used to add public methods. When a member is sought and it isn't found in the object itself, then it is taken from the object's constructor's prototype member. The prototype mechanism is used for inheritance. It also conserves memory. To add a method to all objects made by a constructor, add a function to the constructor's prototype:

Container.prototype.stamp = function (string) {
    return this.member + string;
}

So, we can invoke the method

myContainer.stamp('def')

which produces 'abcdef'.

Private

Private members are made by the constructor. Ordinary vars and parameters of the constructor becomes the private members.

function Container(param) {
    this.member = param;
    var secret = 3;
    var that = this;
}

This constructor makes three private instance variables: param, secret, and that. They are attached to the object, but they are not accessible to the outside, nor are they accessible to the object's own public methods. They are accessible to private methods. Private methods are inner functions of the constructor.

function Container(param) {
 
    function dec() {
        if (secret > 0) {
            secret -= 1;
            return true;
        } else {
            return false;
        }
    }
 
    this.member = param;
    var secret = 3;
    var that = this;
}

The private method dec examines the secret instance variable. If it is greater than zero, it decrements secret and returns true. Otherwise it returns false. It can be used to make this object limited to three uses.

By convention, we make a private that variable. This is used to make the object available to the private methods. This is a workaround for an error in the ECMAScript Language Specification which causes this to be set incorrectly for inner functions.

Private methods cannot be called by public methods. To make private methods useful, we need to introduce a privileged method.

Privileged

A privileged method is able to access the private variables and methods, and is itself accessible to the public methods and the outside. It is possible to delete or replace a privileged method, but it is not possible to alter it, or to force it to give up its secrets.

Privileged methods are assigned with this within the constructor.

function Container(param) {
 
    function dec() {
        if (secret > 0) {
            secret -= 1;
            return true;
        } else {
            return false;
        }
    }
 
    this.member = param;
    var secret = 3;
    var that = this;
 
    this.service = function () {
        return dec() ? that.member : null;
    };
}

service is a privileged method. Calling myContainer.service() will return 'abc' the first three times it is called. After that, it will return null. service calls the private dec method which accesses the private secret variable. service is available to other objects and methods, but it does not allow direct access to the private members.

Closures

This pattern of public, private, and privileged members is possible because JavaScript has closures. What this means is that an inner function always has access to the vars and parameters of its outer function, even after the outer function has returned. This is an extremely powerful property of the language. There is no book currently available on JavaScript programming that shows how to exploit it. Most don't even mention it.

Private and privileged members can only be made when an object is constructed. Public members can be added at any time.

Patterns

Public

function Constructor(...) {

this.membername = value;

}
Constructor.prototype.membername=value;

Private

function Constructor(...) {

var that = this; varmembername = value;

function membername(...) {...}

}

Note: The function statement

functionmembername(...) {...}

is shorthand for

var membername = function membername(...) {...};

Privileged

function Constructor(...) {

this.membername = function (...) {...};

}