node.js displays "undefined" on the console
The JavaScript functions always return something. If you don't specify something to return in the function, 'undefined' is returned by default (you can check this out in Firebug too).
Don't worry though, this doesn't affect anything, you can ignore it.
Just write "hello world"; and hit enter... it will return "hello world" instead of undefined, thus no undefined is displayed. console.log returns undefined and also logs arguments to console so you get multiple messages.
As pointed out by others, javascript function will always return undefined if you do not specify any return value. You can just ignore it. It's not going to cause any harm.But if it's annoying you too much then you can turn it off in repl. Repl has this property ignoreUndefined which is set to false by default. You can set it to true.Try this:
module.exports.repl.ignoreUndefined = true;