A Simple Example About Privileged Methods in JavaScript

Posted 2020-09-28 blfbuaa

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

From my previous article, 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 method

var obj = new func(1,2);
alert(obj.getB()); // privileged method can access private b
alert(obj.getB2()); // error: public method cannot access private b
obj.setB(11);
alert(obj.getB());

So actually we can create privileged methods easily by using the keyword this!

Read More:

Prototype and Constructor in JavaScript by RedcapCoder

Private Members in JavaScript by Douglas Crockford