Invalid underscore variable name created in NodeJS shell

Question

I found the following behavior in a NodeJS shell:

$ node > function foo () { _ } undefined > _ undefined 

Why is the variable _ defined? I expected to get ReferenceError: _ is not defined .

If I create an arrow function, _ will be a reference to the function:

 $ node > () => _ [Function] > _ [Function] > _.toString() '() => _' 

Why is this happening?

After calling toString() in the _ _ variable, it is converted to a string:

 > _ '() => _' 

I tried using _ => () , and we have the same problem:

 $ node > _ => {} [Function] > _ [Function] > _.toString() '_ => {}' > _ '_ => {}' 

I am using node v5.6.0 . This does not play in the file, but only in the NodeJS shell.