Setters are no longer called when setting properties in object initializers: what does this mean?

Question

On this JS MDN page it says this:

JavaScript 1.8.1 note

Starting in JavaScript 1.8.1, setters are no longer called when setting p

Answer 1

This code-snippet:

var o = {};
o.seven = 7;

and this code-snippet:

var o = { seven: 7 };

are normally equivalent; but if they're preceded by this code-snippet:

Object.prototype.__defineSetter__('seven', function(x) { alert(x); });

then only the former will alert 7 (because the setter is called by o.seven = 7, but not by o = { seven: 7 }), and only the latter will actually set o.seven to 7.