Monthly Archives: March 2012

Methods & Properties with Javascript OOP

Note: This isn’t quite OOP. It’s actually called the “module pattern”.

If you come from a language such as Python, PHP, or Java, you’re used to having public and private methods and variables. In Javascript this is possible too, but it’s not nearly as straight-forward. To make this happen, you usually need to wade through Javascript’s prototype inheritance. However, there is another way.

Step 1: Create your class.

function MyClass(somevar) {
}

Now that your class is created, you can instantiate an instance of it by doing the following:

var x = new MyClass(123);

Step 2: Public Methods and Properties

To expose properties and methods to the public, you need to attach them to an object and return it. Some people will attach to this, but I prefer to create a new object called self and return that. It keeps me from getting confused with what this scope I’m in. One thing to remember, is that anything inside of your class that isn’t in a function gets executed as soon as you create a new instance of it.

function MyClass(somevar) {
	var self = {};
 
	// Check to make sure somevar is defined and then
	// assign it to the my_property public property.
	if(somevar === undefined) { somevar = false; }
	self.my_property = somevar;
 
	// A public method!
	self.alert_loop = function(count) {
		if(count === undefined) { count = 1; }
		for(var i = 0; i < count; i++) {
			alert(self.my_property);
		}
	};
 
	return self;
}

You’ll see that we have two things exposed here: my_property and alert_loop. To access them, do this:

var x = new MyClass("Hi!");
alert(x.my_property); // Alerts "Hi!"
x.alert_loop(2); // Alerts "Hi!" twice

Step 3: Private Methods and Variables

Creating private methods and variables is very easy using this method Javascript OOP. All you do is not attach the variable or method to the self object.

function MyClass(somevar) {
	var self = {};
 
	/* Private */
	var loop_max = 2;
	function add_numbers(x,y) {
		return x + y;
	}
 
	/* Public stuff */
 
	if(somevar === undefined) { somevar = false; }
	self.my_property = somevar;
 
	self.alert_loop = function(count) {
		if(count === undefined) { count = 1; }
		for(var i = 0; i < count; i++) {
			alert(self.my_property);
		}
	};
 
	self.alert_numbers = function() {
		for(var i = 0; i < loop_max; i++) {
			alert(add_numbers(i,i+1));
		}
	};
 
	return self;
}

And to use it:

var x = new MyClass("Hi!");
x.add_numbers(1,2); // Will fail.
alert(x.loop_max);  // Will fail.
x.alert_numbers();  // works!

Conclusion

That’s it! Using this style of OOP in Javascript is easy to learn and easy to understand. If you have any questions, drop a comment and I’ll get back to you.