With or without var keyword

Question:

What exactly is the function of the var keyword in Javascript, and what is the difference between:

var someNumber =2;
var someFunction =function(){ doSomething;}
var someObject ={}
var someObject.someProperty =5;

　　

and:

someNumber =2;
someFunction =function(){ doSomething;}
someObject ={}
someObject.someProperty =5;

　　

When would you use either one, and why/what does it do?

Answer:

If you're in the global scope then there's no difference.

If you're in a function then "var" will create a local variable, "no var" will look up the scope chain until it finds the variable or hits the global scope (at which point it will create it):

// These are both globals
var foo = 1;
bar = 2;

function()
{
    var foo = 1; // Local
    bar = 2;     // Global

    // Execute an anonymous function
    (function()
    {
        var wibble = 1; // Local
        foo = 2; // Inherits from scope above (creating a closure)
        moo = 3; // Global
    }())
}

　　If you're not doing an assignment then you need to use var:

var x; // Declare x