This test demonstrates that undefined values from different frames match exactly in a strict equality operator. So, there is no window context specific difference in the undefined values.

In the test, a variable exists in a host document that has an undefined value. In an iframe, another variable exists that has an undefined value. When a link in the iframe is clicked, the value of the variable from the iframe is passed as a parameter value in a call to a function that exists in the host document. In this function, the passed parameter is compared in a strict equality expression with the host document's undefined variable

What this test establishes is that it is always safe to use a strict comparison against a variable that is known to be undefined (instead of using the typeof operator) in order to determine if values are undefined, even if those values originate from a different frame. This is an important thing to establish, because there is an issue with the instanceof operator where using the operator fails to produce the expected result if an instance of an object is passed from a different frame or window.