Float.toString () does work, but integer.toString () doesn't work why?

Question

var a = 2;

a.toString(); // "2"  Works 

2.20.toString(); // "2.2" Works

2..toString(); // "2" Also works  as '2.' is a valid float number 
a.toString(); // "2" Works

But

2.toString(); // Throw error

I am curious to know why javascript works as it works?