Javascript function

1. What is it? 

It has Function Expression

//anonymous function expression

var a = function() {

    return 3;

}

a();

 

//self invoking function expression

(function sayHello() {

    alert("hello!");

})();

 

and  Function Declaration

function bar() {

    return 3;

}

 

 

2.  JavaScript Closures

 JavaScript variables can belong to the local or global scope.Private variables can be made possible with closures.

// Inner class
 function add() {
    var counter = 0;
    function plus() {counter += 1;}
    plus();   
    return counter;
}




REF
 W3C 
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