Underscore symbol ("_") becomes undefined in nodeJS command line interface

$ node
> _=1
1
> _
1
> _
1
> 
undefined

That is, the _ variable seems to become undefined after a single command which doesn't reference it. By comparison:

> ubuntu@tm-dev /usr/share/tilemill $ node
> a=1
1
> 
undefined
> a
1
> 

Is this normal? Why does it do this?

(It's making it a bit hard to debug some code using the Underscore library.)