Javascript object encapsulation.
How do we achieve data encapsulation with Javascript objects?
Private member variables in objects.
-
All properties or let us say member variables defined with
the "this"
keyword inside a function object is of public type, which mean
that they can be accessed from outside of a created object.
function Person(n) { // name and getName ar both public this.name = n; this.getName = function() { return this.name; } }
- All variables, which are defined with the "var" keyword and all the methods that a constructor function contains is private and therefore NOT accessible from outside of a created object. The same applies to all function arguments.
- One of the most important things in object-oriented programming (OOP) is data encapsulation, which means to make private properties and then define public access methods to change these.
- All public methods, which are defined inside a function object, have access to all defined private member variables and private methods in a created object.
-
Public methods defined on the outside of a function object can not
access private member variables as they are not defined in the
same scope as those.
Example:<script type="text/javascript"> function Rectangle(h,w) { var width=w; // Both the 'width' and 'w' is private var heigth=h; // Both the 'height' and 'h' is private this.setWidth= function(w){ width=w;} this.setHeight= function(h){ heigth=h;} this.getWidth= function(){ return width;} this.getHeight= function(){ return heigth;} this.constructor.prototype.getDiagonal=function() { return Math.sqrt(heigth*heigth+width*width); }; } Rectangle.prototype.getArea=function() { // We must use accessors in a prototype kind of method, // then these methods can not access the private members // of a created object. return this.getWidth()*this.getHeight(); }; var rect = new Rectangle(60,70); rect.setHeight(20); document.write("The rectangle area is: "+rect.getArea()+"<br>"); document.write("The rectangle diagonal is: "+rect.getDiagonal()+"<br>"); </script>
-
Private methods have no directly access to properties that are
defined to be public with "this" keyword in a function object.
To achieve this, one can define a variable that has the same reference as "this" refers to.
Example:<script type="text/javascript"> function Person(n, y) { var name=n; var year=y; // Set a variable to the same as this var thisObj=this; this.setName= function(n){ name=n;} this.getName= function(){ return name;} this.setYear= function(y){ year=y;} this.getYear= function(){ return year;} var born = function() { var nYear=new Date().getFullYear(); // Use the thisObj variable which is the same as this refer to. return nYear-thisObj.getYear(); // The "this" keyword inside this function refers to an object // created by the constructor function of this function object. } this.getBornYear= function() { return born(); } } var person = new Person("Nikita",60); document.write(person.getName()+" was born in "+person.getBornYear()+"<br>"); </script>
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