A function can access all variables defined inside the function, like this:
But a function can also access variables defined outside the function, like this:
In the last example, a is a global variable.
In a web page, global variables belong to the window object.
Global variables can be used (and changed) by all scripts in the page (and in the window).
In the first example, a is a local variable.
A local variable can only be used inside the function where it is defined. It is hidden from other functions and other scripting code.
Global and local variables with the same name are different variables. Modifying one, does not modify the other.
Variables created without the keyword var, are always global, even if they are created inside a function.
Global variables live as long as your application (your window / your web page) lives.
Local variables have short lives. They are created when the function is invoked, and deleted when the function is finished.
A Counter Dilemma
Suppose you want to use a variable for counting something, and you want this counter to be available to all functions.
You could use a global variable, and a function to increase the counter:
The counter should only be changed by the add() function.
The problem is, that any script on the page can change the counter, without calling add().
If I declare the counter inside the function, nobody will be able to change it without calling add():
It did not work! Every time I call the add() function, the counter is set to 1.
All functions have access to the global scope.
In this example, the inner function plus() has access to the counter variable in the parent function:
This could have solved the counter dilemma, if we could reach the plus() function from the outside.
We also need to find a way to execute counter = 0 only once.
We need a closure.
Remember self-invoking functions? What does this function do?
The variable add is assigned the return value of a self-invoking function.
The self-invoking function only runs once. It sets the counter to zero (0), and returns a function expression.
This way add becomes a function. The "wonderful" part is that it can access the counter in the parent scope.
The counter is protected by the scope of the anonymous function, and can only be changed using the add function.
A closure is a function having access to the parent scope, even after the parent function has closed.