A JavaScript variable is simply a name of storage location or identifier, associated with a value. The variable can be seen as storing, or holding the value, and also a variable contains a value, such as "hello" or 5.

There are two types of variables used in JavaScript :

local variable
global variable.

Some rules which can be declare in JavaScript variable ( it is called identifiers ).

x and X are different variables in Javascript so javaScript variables are case sensitive,
Name start with a letter (a to z or A to Z), and underscore( _ ), or dollar( $ ) sign.
After first letter we can also use digits (0 to 9), (for example value1)

NOTE :-

" Before you use a variable in a your JavaScript program you have to must use var keyword to declare Variable "

Example

<html>
<body>
<script> 
var x = 50; 
var y = "prasant"; 
var z=x+y; 
document.write(z); 
</script> 
</body>
</html>

Output

50 prasant

Local variables : Variables that exist only inside a function are called Local variables. They have no presence outside the function. The values of such Local variables cannot be changed by the main code or other functions.

Example :

<html>
<head> 
 <script> 
function abc()
{ 
 var a= 200;//local variable 
 var b= 300;//local variable 
 var c= 400;//local variable 
 var x = a+b+c;
document.write(x); 
 }
abc();//calling JavaScript function 
 </script> 
<head>
<html>

Output :

900

(追記) (追記ここまで)

Global Variables : A global variable has global scope which means it is defined everywhere in your JavaScript code, Their values can be changed anytime in the code and even by other functions.

Example :

<html>
<head>
<script> 
var x=200;//gloabal variable
var y=400;//gloabal variable
var z=500;//gloabal variable
var w= x+y+z;
function a()
{ 
document.writeln(x); 
} 
function b()
{ 
document.writeln(y);
} 
function c()
{
document.writeln(z);
}
function d()
{ 
document.write("<br>sum of numbers is:"+w);
}
a();//calling JavaScript function 
b(); 
c();
d();
</script>
<head>
<html>

Output :