JavaScript Learning# 7 - Static variables and methods

Static variables and methods

A function is an object. That provides us with a nifty way to create static variables or, in other words, the variables which persist along multiple calls.

For example, we want a variable which counts function calls.

Static variables

There are languages which allow to put a static keyword before a variable, and then such variable is not cleared in next calls.

Example of a static variable in Java language:

1	static int count = 0;void f() {

2

3

count++;

5

}

6

7	new f(); new f(); new f(); // 1 2 3

In JavaScript, there is no term or keyword static, but we can put such data directly into function object (like in any other object).

   

1	function f() {

2	f.count = ++f.count || 1 // f.count is undefined at first

3

4	alert("Call No " + f.count)

5

}

6

7	f(); // Call No 1

8	f(); // Call No 2

Of course, a global variable can keep the counter, but static variables lead to a better architecture.

We could make the code more universal by replacing f with arguments.callee.

1	function f() {

2	arguments.callee.count = ++arguments.callee.count || 1

3

4	alert("Called " + arguments.callee.count + " times")

5

}

Now you can safely rename the function if needed 

Static methods

Static methods, just like variables, are attached to functions. They are used mostly for objects:

   

01	function Animal(name) {

02	arguments.callee.count = ++arguments.callee.count || 1

03

04	this.name = name

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}

06

07	Animal.showCount = function() {

08	alert( Animal.count )

09

}

10

11	var mouse = new Animal("Mouse")

12	var elephant = new Animal("elephant")

13

14	Animal.showCount()  // 2

source: http://javascript.info/