Using 'undefined' as a key in an object causes the use of <undefined `as a key. What for?

Question

As in, {'undefined': 'foo'}leads to {undefined: 'foo'}.

In Chrome:

In Firefox:

In Safari:

Why? I tried other reserved words (such as nulland true) and they are also converted.

Answer 1

This has nothing to do with reserved words. Object keys are displayed in this way, regardless of which line you use for the key. This is just how it is interpreted / presented to you on the console.

var o = {'cheese': 'foo'};
// Object {cheese: "foo"}

o.cheese; // "foo"
o['cheese']; // "foo"

Quotation marks are not needed for access, so they are removed.