Private Members

Links

Private Members in JavaScript copy of article by Douglas Crockford.
Constructors (Class)

Objects

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

Public Members

There are two main ways of putting members in a new object:

In the constructor:

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:

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 var s 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.

By convention, we make a private that parameter. 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 () {
        if (dec()) {
            return that.member;
        } else {
            return 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.