Closure in Javascript

Closure are function inside function.  They help to create isolated stateful function.
var add = (function () {
    var counter = 0;
    return function () {return counter += 1;}
})();

add();
add();
add();

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.


And help to access private variable of a function.