Scope in Javascript

Variable scope describes when a piece of information is accessible.
When a variable is created, the value it stores is only available for a certain part of the code execution.
Memory is set aside to store that piece of information and then freed when it is no longer needed.


JavaScript, a variable declared outside of a function is available inside a function:


var name = "Jonathan";
function showme()
{
alert(name); // will display "Jonathan"
}
showme();


A variable declared within a function will not be available outside of a function.


function showme()
{
var name = "Jonathan";
}
showme();
alert(name); // name will be undefined


#1 Do not use global variables from within a function
Instead, pass in any required values as parameters to the function.

Using our JavaScript example from before we can do this:

var name = "Jonathan";
function showme(myname)
{
alert(myname); // will display "Jonathan"
}
showme(name);
alert(name); // will display "Jonathan"


#2 Use object-oriented programming (OOP)
This mantra is similar to the first point, but on a larger scale. Instead of a collection of functions that perform related tasks, the functions become methods of an object. (A method is a function that belongs to an object.) Values can then be passed into the object in a variety of useful ways:

By setting the properties of an object
By passing values as parameters of a method
By passing values as parameters of the constructor (the method that gets executed when an object is first instantiated)
Here’s a quick JavaScript OOP example:

function Person(myname)
{
this.name = myname;
}
var me = new Person("Jonathan");
alert(me.name);