What is the function of the var keyword and when to use it (or omit it)?

1645

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?

Javascript