Douglas Crockford classified the "class methods" in JavaScript into three types:

private,

privileged and

public.

Public methods have an obvious meaning: they can be accessed by the public.

Private methods' meaning are also clear: they cannot be accessed by the public.

So what are

privileged methods? To understand that, we should have a short review of how we implement public and private methods in JavaScript first.

Recall that, unlike languages like C++, JavaScript does not have "class". Moreover, it does not have

access specifiers like

public,

protected and

private. Lack of these features make the creation of private and public methods less clear than it should be.

To write a

public method in JavaScript, we make use of the

.prototype property of the constructor function. For example:

function FootballPlayer(){}; // declare a functionFootballPlayer.prototype.kick = function(){ alert("kick!"); }; // create a public method kick in .prototypevar Messi = new FootballPlayer();Messi.kick();

From my , you have learnt that

FootballPlayer.prototype is an object that is built automatically when the function is created. Object constructed with the FootballPlayer, Messi, search through his

__proto__ chain when we tell him to kick. Obviously

FootballPlayer.prototype is on the chain so Messi knows how to kick. All objects created by the

constructor function share the same

FootballPlayer.prototype so they all can invoke the

public method kick!

Private methods are kinda tricky. We declare private variable in a

constructor using the keyword

var:

function man() {	var wealth;	var bath = function(){};	function kiss(){}}

In this case, none of wealth, bath or kiss are accessible outsite the function man. Even man's

public methods

cannot access them.

However, the

privileged methods can access the

private members due to the existence of

closures. They are very useful as they can act as a bridge between the outsite world and the inner status of the object.

Consider the following example:

var func = function(a,b) {    this.a = a;    this.getB = function(){return b}; // a privileged method    this.setB = function(n){b=n;}; // another privileged method    var b = b;};func.prototype.getB2 = function(){return b*2}; // public methodvar obj = new func(1,2);alert(obj.getB()); // privileged method can access private balert(obj.getB2()); // error: public method cannot access private bobj.setB(11);alert(obj.getB());

So actually we can create

privileged methods easily by using the keyword

this!

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by RedcapCoder

 by Douglas Crockford