What's the best way to initialize empty variables in JS?

If I know the variable will be object later, I use:

var obj;

but it doesn't really matter if I initialize it as null or undefined:

var obj = null;

or

var obj = undefined;

For strings I personally use:

var str = '';

as later on I can do

str += 'some text';

and if I use null for example I get "nullsome text".

null, undefined, empty string or 0 are all falsy values so to check if they are defined. which is the correct way to initialize variables if my variable will be used as object, dom node, etc.. ?