JavaScript Closures

JavaScript variables can belong to:

• The local scope
• The global scope

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

Closures make it possible for a function to have "private" variables.

A closure is created when a function remembers the variables from its outer scope, even after the outer function has finished executing.

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Local Variables

A local variable is a "private" variable defined inside a function.

A function can access all variables in the local scope.

Example

function myFunction() {

    let a = 4;

    return a * a;

}

Global Variables

A global variable is a "public" variable defined outside a function.

A function can access all variables in the global scope:

Example

let a = 4;
function myFunction() {

    return a * a;

}

In a web page, global variables belong to the page.

Global variables can be used (or changed) by all scripts in the page.

A local variable can only be used inside the function where it is defined. It is private and 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.

Example

function myFunction() {

  a = 4;

}

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 everyone (all functions).

You could use a global variable, and a function to increase the counter:

Example

// Initiate counter
let counter = 0;

// Function to increment
  counter
function add() {

  counter += 1;

}

// Call add() 3 times

add();

add();

add();

// The counter should now be 3

Warning !

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
let counter = 0;

// Function to increment
  counter
function add() {
  let 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() {

  let counter = 0;

  counter += 1;

  return counter;

}

let x= 0;

// Call add() 3 times

x = add();

x = add();

x = 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.

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.

Example

function add() {

    let 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.

JavaScript Closures

Example

function myCounter() {

  let counter = 0;

  return function() {

    counter++;

    return counter;

  };

}

const add = myCounter();

add();

add();

add();

// the counter is now 3

Example Explained

The variable add is assigned to the return value of a function.

The 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 its parent scope.

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

The counter is protected by the scope of the myCounter function, and can only be changed using the add function.

Conclusion

A closure is a function that has access to the parent scope, after the parent function has closed.

Closures has historically been used to:

• Create private variables
• Preserve state between function calls
• Simulate block-scoping before let and const existed
• Implement certain design patterns like currying and memoization

Modern Alternative: Private Class Fields

Since ECMAScript 2022, JavaScript supports real private class fields using the # syntax.

Private fields are enforced by the language and cannot be accessed outside the class.

All modern browsers support private class fields.

Example

class Counter {

  #count = 0;

  increment() {

    this.#count++;

    return this.#count;

  }

}

const myCounter = new Counter();

myCounter.increment();

Private class fields are usually preferred when working with classes.

• They provide true privacy.
• They are simpler than closure-based patterns inside classes.
• They are part of the official ECMAScript standard.

Closures are still important and useful outside of classes.

Closures are also widely used in older codebases and functional programming patterns.