Javascript Closure

JavaScript variables can belong to the local or global scope.

Global variables can be made local (private) with closures.

Global Variables
A function can access all variables defined inside the function, like this:

Example
function myFunction() {
  var a = 4;
  return a * a;
}
But a function can also access variables defined outside the function, like this:

Example
var a = 4;
function myFunction() {
  return a * a;
}
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 a declaration keyword (var, let, or const) are always global, even if they are created inside a function.

Variable Lifetime
Global variables live until the page is is discarded, like when you navigate to another page or close the window.

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:

Example
// Initiate counter
var counter = 0;

// Function to increment counter
function add() {
  counter += 1;
}

// Call add() 3 times
add();
add();
add();

// The counter should now be 3
There is a problem with the solution above: Any code on the page can change the counter, without calling add().

The counter should be local to the add() function, to prevent other code from changing it:

Example
// Initiate counter
var counter = 0;

// Function to increment counter
function add() {
  var counter = 0;
  counter += 1;
}

// Call add() 3 times
add();
add();
add();

//The counter should now be 3. But it is 0
It did not work because we display the global counter instead of the local counter.

We can remove the global counter and access the local counter by letting the function return it:

Example
// Function to increment counter
function add() {
  var counter = 0;
  counter += 1;
  return counter;
}

// Call add() 3 times
add();
add();
add();

//The counter should now be 3. But it is 1.
It did not work because we reset the local counter every time we call the function.

 A JavaScript inner function can solve this.

JavaScript Nested Functions
All functions have access to the global scope.  

In fact, in JavaScript, all functions have access to the scope "above" them.

JavaScript supports nested functions. Nested functions have access to the scope "above" them.

In this example, the inner function plus() has access to the counter variable in the parent function:

Example
function add() {
  var counter = 0;
  function plus() {counter += 1;}
  plus();   
  return counter;
}
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.

JavaScript Closures
Remember self-invoking functions? What does this function do?

Example
var add = (function () {
  var counter = 0;
  return function () {counter += 1; return counter}
})();

add();
add();
add();

// the counter is now 3
Example Explained
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.

This is called a JavaScript closure. It makes it possible for a function to have "private" variables.

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.