Because although 0 is a typeof integer but when you return 0 . It will not been include in the array as it is a false value in javascript.
Basically javascript considered 0 as false.
But in the same code if you write this

if(typeof(a)=='number'){
return true;
}

It will include 0 too.

In the case of typeof a == "number" ,it statisfy the condition as it is a interger.

Hope it would be helpful to you.