JS note more than a undefined what is the ghost?

When you practice outputting a row B column table, you output a undefined!

<! DOCTYPE html>
The problem is to write more of the outermost document.write ();
After testing document.write (document.write (2)), there is also a 2 line break Undefined;alert.
Maybe the syntax is wrong ~ ~ The output parameter is the output function itself, which seems to be unreasonable ~
JS note more than a undefined what is the ghost?