what's the result of 1 + undefined

Question

1 + undefined = ?  

1. first, String(undefined) get string \"undefined\"
2. second, 1 + \"undefined\" = \"1undefined\"

what

Answer 1

You expect a string concatenation, but this will only happen if you have at least one string. And in your example nothing is a string. 1 is not a string and undefined is not a string.