JavaScript Closures
JavaScript variables can belong to:
The local scope or The global scope
Global variables can be made local (private) with closures.
Closures makes it possible for a function to have "private" variables.
Local Variables
A local variable is a "private" variable defined inside a function.
A function can access all variables in the local scope.
Example
a is a local variable defined inside the function:
function myFunction() {
let a = 4;
return a * a;
}
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Global Variables
A global variable is a "public" variable defined outside a function.
A function can access all variables in the global scope:
Example
a is global variable defined outside the function:
let a = 4;
function myFunction() {
return a * a;
}
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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.
Note
Undeclared variables (created without a keyword var,
let, const),
are always global, even if they are created inside a function.
Example
The variable a is a global variable because it is undeclared:
function myFunction() {
a = 4;
}
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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
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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
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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.
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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.
Example
The inner function plus() has access
to the counter variable in the parent function:
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.
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 3
Try it Yourself »
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
Note
Old JavaScript code will often contain closures, but modern JavaScript will not use closures as frequently as before.
ECMAScript 2015 and subsequent JavaScript versions have introduced new language features that provide alternatives to closures.
While closures are a powerful concept in JavaScript, new JavaScript features make some use cases for closures less necessary.
