JavaScript Closures

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JavaScript Closures


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() {
  let a = 4;
  return a * a;
}

Try it Yourself »

But a function can also access variables defined outside the function, like this:

Example

let a = 4;function myFunction() {
  return a * a;
}

Try it Yourself »

In the last example, a is a global
variable.
In a web page, global variables belong to the page.
Global variables can be used (and changed) by all other scripts in the page.
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.

Note
Variables created without a declaration keyword (var,
let, or const)
are always
global, even if they are created inside a function.


Example

function myFunction() {
  a = 4;
}

Try it Yourself »








Variable Lifetime
Global variables live until the page 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 counterlet counter = 0;// Function to increment
counterfunction add() {
  counter += 1;
}// Call add() 3 times
add();
add();
add();// The counter should now be 3
Try it Yourself »

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 counterlet counter = 0;// Function to increment
counterfunction add() {  let counter = 0;  
counter += 1;}//
Call add() 3 timesadd();add();add();//The counter should
now be 3. But it is 0
Try it Yourself »

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
counterfunction add() {  let counter = 0;  
counter += 1;  return counter;}//
Call add() 3 timesadd();add();add();//The counter should
now be 3. But it is 1.
Try it Yourself »

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() {
  let counter = 0;
  function plus() {counter += 1;}  plus();   
  return counter; }
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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

const add = (function () {  let counter = 0;  return function () {counter += 1; return counter}})();add();
add();
add();
// the counter is now 3

Try it Yourself »

Example Explained
The variable add is assigned to 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.












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Reference: https://www.w3schools.com/js/js_function_closures.asp