Answer 1

Put this line after definition for <a>

console.log( JSON.stringify( a ) )

You'll see that <a> doesn't have any member named 'foo1'.

(Edit)
In case you're wondering, why it has 'foo0', 'foo2' and 'foo3' members, but not 'foo1' - then know that the difference was due to the use of post and pre increment operator, which modifies value of <i> in different manner.