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JavaScript Closures

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:

var counter = 0;

function add() {

counter += 1;

}

add();

add();

add();

The counter should only be changed by the add() function.The problem is, that any script on the page can change the counter, without calling add().

If I declare the counter inside the function, nobody will be able to change it without calling add():

function add() {

var counter = 0;

counter += 1;

}

add();

add();

add();

t did not work! Every time I call the add() function, the counter is set to 1.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:

function add() {

var 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.

We need a closure.

JavaScript Closures Remember self-invoking functions? What does this function do?

var add = (function () {

var counter = 0;

return function () {return counter += 1;}

})();

add();

add();

add();

Example Explained The variable add is assigned 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.