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.
Closures are an advanced topic.
Make sure you understand JavaScript functions and scope first.
JavaScript Functions
JavaScript Scope
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.
Undeclared variables (created without a keyword var , let , const ), are always global , even if they are created inside a function.
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 3Warning !
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 0It 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.
Solution
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.
plus()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.
Solution
We need a closure.
JavaScript Closures
Example
function myCounter() {
let counter = 0;
return function() {
counter++;
return counter;
};
}
const add = myCounter();
add();
add();
add();
// the counter is now 3Example 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
Old JavaScript code often contains closures, but modern JavaScript will not use closures as frequently.
ECMAScript 2015 and subsequent JavaScript versions have introduced new language features that provide alternatives to closures.
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.