JavaScript - Variables

javascript

// JavaScript - Variables

New variables in JavaScript are declared using the var keyword:

var xyz;
var xyz = "hello";

If we declare a variable without assigning it any value, its value is undefined.

If we do not use the var keyword when declaring a variable, or assigning a value
to a variable, JavaScript creates that variable in the global space which may lead
to namespace collision.

The var keyword is only applied to one variable.

(function(){
  var a = b = 3;
})();

In the above code, only a is local to that function, and b is a global variable, 
and therefore:

console.log("a defined? " + (typeof a !== 'undefined')); //logs "a defined? false"
console.log("b defined? " + (typeof b !== 'undefined')); //logs "b defined? true"

If we forget to declare a variable with a var keyword, JavaScript assume that 
we use a global property of the window object