Javascript Object Property logged in Google Chrome Console before Declaration

Javascript Object Property logged in Google Chrome Console before Declaration

I've been testing some JavaScript using the Google Chrome version
28.0.1500.95 m console and am looking for a bit more understanding of how
it works: Have a look at the code below:
var obj = {
a: 99,
f: function() { }
}
console.log(obj.a)
console.log(obj.z)
console.log(obj)
o.z = 100;
Demo
This outputs the following results:
99
undefined
Object {a: 99, f: function}
a: 99
f: function () { }
z: 100
__proto__: Object
My question is, why is z visible in the results when it wasn't declared
until after the log?
I'm assuming this is something with how the console works and not some
weird scoping rule in JavaScript, that I'm unaware of?
Can anyone tell me what's happening here please?