OOP JavaScript: Accessing Public Methods in Private Methods
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Join For FreeAs you know in JavaScript when you define a variable with the special word “var” the scope of this variable is within the function. So when you simply wite “var a = 5″ the variable named “a” has a global scope and can be accessed in any function in the global scope.
var a = 5; function f() { return a; } // returns 5
Thus, f will return the value of “a” which equals to 5. You can also change the value of the global variable in the function body.
var a = 5; function f() { a = 10; return a; } console.log(a); // equals to 10
Now after we call the function f the value of “a” will equal to 10.
This is because we reference the global variable “a” into the function
body without using the keyword “var”. This means that if you put the
“var” keyword the variable “a” inside the function body is no longer the
same variable as the variable defined outside the body. It becames
“local” and it’s visible only inside the function.
Objects & JavaScript
Actually in JavaScript functions can be also objects. Any variable defined with the “var” keyword becomes private, because it’s only visible inside the function body, while by using the “this” keyword we can define global variables visible outside the function body. Let’s see na example.
var f = function() { var a = 10; this.b = 5; } var myfunc = new f(); myfunc.a; // is undefined because a is private myfunc.b; // equals to 5 because b is public
Public and Private Methods in JavaScript
As you may know in JS you can define functions inside other functions. Actually this is how you can define classes in JavaScript.
var myClass = function() { var f = function() { return 10; } }
But this in fact defines a local (private) function into myClass and we cannot access it from the outside world. Here’s a fully functional class with one public and one private method.
var myClass = function() { // public method this.a = function() { return 10; } // private method var b = function() { return 5; } } var c = new myClass(); c.a(); // will return 10 c.b(); // is undefined, because is private
Accessing Private Methods from Public Methods
Easily we can access private methods from public ones.
var myClass = function() { // public method this.a = function() { return b(); } // private method var b = function() { return 5; } }
However accessing public mtehods in private ones is more difficult. That’s because we cannot use “this” into the private methods, just because “this” refers to the private method itself and not to the global method, which is “myClass” in our case.
var myClass = function() { // public method this.a = function() { return 10; } // private method var b = function() { return this.a(); // this will result in an error } }
Accessing Public Methods from Private Methods
To access public methods in private methods you need to define a variable that points to the global “this” object.
var myClass = function() { var self = this; // public method this.a = function() { return 10; } // private method var b = function() { return self.a(); // this will return 10 } }
This variable gives you the bridge between private methods and global “this” pointer.
Published at DZone with permission of Stoimen Popov, DZone MVB. See the original article here.
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