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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.
A
local variable is a "private" variable defined inside a function.A function can access all variables in the local scope.
is a local variable defined inside the function:
function myFunction() {
let a = 4;
return a * a;
}A
global variable is a "public" variable defined outside a function.A function can access all variables in the global scope
is global variable defined outside the function:
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.
(created without a keyword var, let, const
), are always global, even if they are created inside a function.is a global variable because it is undeclaredfunction myFunction() {
a = 4;
}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.
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:
// 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:
// 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:
// 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.
A JavaScript inner function can solve this.
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.has access to the counter variable in the parent function:
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.Formula
We also need to find a way to execute counter = 0 only once.We need a closure.
Example function myCounter() {
let counter = 0;
return function() {
counter++;
return counter;
};
}
const add = myCounter();
add();
add();
add();// the counter is now 3
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.