This page shows how to create private variables and methods in "classes" in Javascript through the rather simple example of a person. (This is really a very rough approximation of the classical OOP pattern, and not necessarily a "best-practice" for JavaScript programming.) Part 2 covers inheritance.

Summary

private variables are declared with the 'var' keyword inside the object, and can only be accessed by private functions and privileged methods.

are declared with the 'var' keyword inside the object, and can only be accessed by private functions and privileged methods. private functions are declared inline inside the object's constructor (or alternatively may be defined via var functionName=function(){...} ) and may only be called by privileged methods (including the object's constructor).

are declared inline inside the object's constructor (or alternatively may be defined via ) and may only be called by privileged methods (including the object's constructor). privileged methods are declared with this.methodName=function(){...} and may invoked by code external to the object.

are declared with and may invoked by code external to the object. public properties are declared with this.variableName and may be read/written from outside the object.

are declared with and may be read/written from outside the object. public methods are defined by Classname.prototype.methodName = function(){...} and may be called from outside the object.

are defined by and may be called from outside the object. prototype properties are defined by Classname.prototype.propertyName = someValue

are defined by static properties are defined by Classname.propertyName = someValue

Example

In this example, a person's name and race are set at birth and may never be changed. When created, a person starts out at year 1 and a hidden maximum age is determined for that person. The person has a weight which is modified by eating (tripling their weight) or exercising (halfing it). Every time the person eats or exercises, they grow a year older. The person object has a publicly accessible 'clothing' property which anyone can modify, as well as a dirtFactor which can be modified manually (throwing dirt on or scrubbing it off), but which increases every time the person eats or exercises, and is reduced by the use of the shower() method.

Run Gavin's Life <- click to run the example code below (the code block being run appears after the Person constructor).

The Example Code

function Person(n,race){ this.constructor.population++; // ************************************************************************ // PRIVATE VARIABLES AND FUNCTIONS // ONLY PRIVELEGED METHODS MAY VIEW/EDIT/INVOKE // *********************************************************************** var alive=true, age=1; var maxAge=70+Math.round(Math.random()*15)+Math.round(Math.random()*15); function makeOlder(){ return alive = (++age <= maxAge) } var myName=n?n:"John Doe"; var weight=1; // ************************************************************************ // PRIVILEGED METHODS // MAY BE INVOKED PUBLICLY AND MAY ACCESS PRIVATE ITEMS // MAY NOT BE CHANGED; MAY BE REPLACED WITH PUBLIC FLAVORS // ************************************************************************ this.toString=this.getName=function(){ return myName } this.eat=function(){ if (makeOlder()){ this.dirtFactor++; return weight*=3; } else alert(myName+" can't eat, he's dead!"); } this.exercise=function(){ if (makeOlder()){ this.dirtFactor++; return weight/=2; } else alert(myName+" can't exercise, he's dead!"); } this.weigh=function(){ return weight } this.getRace=function(){ return race } this.getAge=function(){ return age } this.muchTimePasses=function(){ age+=50; this.dirtFactor=10; } // ************************************************************************ // PUBLIC PROPERTIES -- ANYONE MAY READ/WRITE // ************************************************************************ this.clothing="nothing/naked"; this.dirtFactor=0; } // ************************************************************************ // PUBLIC METHODS -- ANYONE MAY READ/WRITE // ************************************************************************ Person.prototype.beCool = function(){ this.clothing="khakis and black shirt" } Person.prototype.shower = function(){ this.dirtFactor=2 } Person.prototype.showLegs = function(){ alert(this+" has "+this.legs+" legs") } Person.prototype.amputate = function(){ this.legs-- } // ************************************************************************ // PROTOTYOPE PROERTIES -- ANYONE MAY READ/WRITE (but may be overridden) // ************************************************************************ Person.prototype.legs=2; // ************************************************************************ // STATIC PROPERTIES -- ANYONE MAY READ/WRITE // ************************************************************************ Person.population = 0; // Here is the code that uses the Person class function RunGavinsLife(){ var gk=new Person("Gavin","caucasian"); //New instance of the Person object created. var lk=new Person("Lisa","caucasian"); //New instance of the Person object created. alert("There are now "+Person.population+" people"); gk.showLegs(); lk.showLegs(); //Both share the common 'Person.prototype.legs' variable when looking at 'this.legs' gk.race = "hispanic"; //Sets a public variable, but does not overwrite private 'race' variable. alert(gk+"'s real race is "+gk.getRace()); //Returns 'caucasian' from private 'race' variable set at create time. gk.eat(); gk.eat(); gk.eat(); //weight is 3...then 9...then 27 alert(gk+" weighs "+gk.weigh()+" pounds and has a dirt factor of "+gk.dirtFactor); gk.exercise(); //weight is now 13.5 gk.beCool(); //clothing has been update to current fashionable levels gk.clothing="Pimp Outfit"; //clothing is a public variable that can be updated to any funky value gk.shower(); alert("Existing shower technology has gotten "+gk+" to a dirt factor of "+gk.dirtFactor); gk.muchTimePasses(); //50 Years Pass Person.prototype.shower=function(){ //Shower technology improves for everyone this.dirtFactor=0; } gk.beCool=function(){ //Gavin alone gets new fashion ideas this.clothing="tinfoil"; }; gk.beCool(); gk.shower(); alert("Fashionable "+gk+" at " +gk.getAge()+" years old is now wearing " +gk.clothing+" with dirt factor " +gk.dirtFactor); gk.amputate(); //Uses the prototype property and makes a public property gk.showLegs(); lk.showLegs(); //Lisa still has the prototype property gk.muchTimePasses(); //50 Years Pass...Gavin is now over 100 years old. gk.eat(); //Complains about extreme age, death, and inability to eat. }

Notes