This one is kind of misleading, as var t does not actually return the value of t but rather of the whole expression.

> var t
undefined

> t == undefined
true

> t = "Defined"
'Defined'
> var t
undefined // <-- not actually the value of t
> t
'Defined' // <-- t remains defined

As var t not resetting t may still be a WTF moment for some, this post should be renamed (either var t does not reset t or the repl is intuitive for some).

Also, the linked source was talking about undefined being mutable, which it is not anymore.