codecademy: Introduction to Functions in JS Tutorial 10#
== Global and local Variable in JS ==
Let's talk about an important concept: scope. Scope can be global or local.

Variables defined outside a function are accessible anywhere once they have been declared. They are called global variables and their scope is global.
For example:

var globalVar = "hello";

var foo = function() {
    console.log(globalVar);  // prints "hello"
};

// The variable globalVar can be accessed anywhere, 

even inside the function foo.

Variables defined inside a function are local variables. 

They cannot be accessed outside of that function.

var bar = function() {
    var localVar = "howdy";
}

console.log(localVar);  // error 

The var keyword creates a new variable in the current scope. 

That means if var is used outside a function, that variable has a global scope. 

If var is used inside a function, that variable has a local scope.

 var my_number = 7; //this has global scope

var timesTwo = function(number) {
    var my_number = number * 2;
    console.log("Inside the function my_number is: ");
    console.log(my_number);
};

timesTwo(7);

console.log("Outside the function my_number is: ")
console.log(my_number);
 Out put:
Inside the function my_number is 14
Outside the function my_number is 7