Question

How can we differentiate between var foo; and var foo=undefined;?

typeof foo will return for both “undefined” and foo in window will return for both true;

Answer 1

var foo implies var foo=undefined, unless of course undefined is set to something other than undefined.

This can be seen in the Browser Console, if you initialise a variable but don’t give it a value, it will have the value undefined (and the type "undefined")